Optimize reflow by changing enums to traits and inlining more

44 files changed, 2635 insertions, 2272 deletions

diff --git a/Makefile.in b/Makefile.in
index 3d372fd0500..9689d5fd121 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -32,7 +32,7 @@ MKFILE_DEPS := config.stamp $(call rwildcard,$(S)mk/,*)
 # Enable debug!() etc even without configure --enable-debug
 # The evaluation of these prints & their arguments is controlled
 # at runtime by the environment variable RUST_LOG.
-CFG_RUSTC_FLAGS += --cfg debug -Z debug-info
+CFG_RUSTC_FLAGS += -Z debug-info -Z no-debug-borrows
 
 ifdef CFG_DISABLE_OPTIMIZE
   $(info cfg: disabling rustc optimization (CFG_DISABLE_OPTIMIZE))
diff --git a/src/components/gfx/font.rs b/src/components/gfx/font.rs
index 7ec40f29071..faecef29904 100644
--- a/src/components/gfx/font.rs
+++ b/src/components/gfx/font.rs
@@ -2,33 +2,29 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use azure::{AzFloat, AzScaledFontRef};
+use azure::azure_hl::{BackendType, ColorPattern};
+use azure::scaled_font::ScaledFont;
+use extra::arc::Arc;
+use geom::{Point2D, Rect, Size2D};
+use std::cast;
+use std::ptr;
+use std::str;
+use std::vec;
+use servo_util::cache::{Cache, HashCache};
+use servo_util::range::Range;
+use servo_util::time::ProfilerChan;
+use style::computed_values::text_decoration;
+
 use color::Color;
 use font_context::FontContext;
 use servo_util::geometry::Au;
 use platform::font_context::FontContextHandle;
 use platform::font::{FontHandle, FontTable};
 use render_context::RenderContext;
-use servo_util::range::Range;
-use std::cast;
-use std::ptr;
-use std::str;
-use std::vec;
-use servo_util::cache::{Cache, HashCache};
 use text::glyph::{GlyphStore, GlyphIndex};
 use text::shaping::ShaperMethods;
 use text::{Shaper, TextRun};
-use extra::arc::Arc;
-
-use azure::{AzFloat, AzScaledFontRef};
-use azure::scaled_font::ScaledFont;
-use azure::azure_hl::{BackendType, ColorPattern};
-use geom::{Point2D, Rect, Size2D};
-
-use servo_util::time;
-use servo_util::time::profile;
-use servo_util::time::ProfilerChan;
-
-use style::computed_values::text_decoration;
 
 // FontHandle encapsulates access to the platform's font API,
 // e.g. quartz, FreeType. It provides access to metrics and tables
@@ -459,13 +455,11 @@ impl Font {
     }
 
     pub fn shape_text(@mut self, text: ~str, is_whitespace: bool) -> Arc<GlyphStore> {
-        do profile(time::LayoutShapingCategory, self.profiler_chan.clone()) {
-            let shaper = self.get_shaper();
-            do self.shape_cache.find_or_create(&text) |txt| {
-                let mut glyphs = GlyphStore::new(text.char_len(), is_whitespace);
-                shaper.shape_text(*txt, &mut glyphs);
-                Arc::new(glyphs)
-            }
+        let shaper = self.get_shaper();
+        do self.shape_cache.find_or_create(&text) |txt| {
+            let mut glyphs = GlyphStore::new(text.char_len(), is_whitespace);
+            shaper.shape_text(*txt, &mut glyphs);
+            Arc::new(glyphs)
         }
     }
 
diff --git a/src/components/gfx/platform/linux/font.rs b/src/components/gfx/platform/linux/font.rs
index f17ca694813..0f33720d420 100644
--- a/src/components/gfx/platform/linux/font.rs
+++ b/src/components/gfx/platform/linux/font.rs
@@ -276,7 +276,7 @@ impl<'self> FontHandle {
     }
 
     #[fixed_stack_segment]
-    pub fn new_from_file(fctx: &FontContextHandle, file: ~str,
+    pub fn new_from_file(fctx: &FontContextHandle, file: &str,
                          style: &SpecifiedFontStyle) -> Result<FontHandle, ()> {
         unsafe {
             let ft_ctx: FT_Library = fctx.ctx.ctx;
@@ -293,7 +293,7 @@ impl<'self> FontHandle {
             }
             if FontHandle::set_char_size(face, style.pt_size).is_ok() {
                 Ok(FontHandle {
-                    source: FontSourceFile(file),
+                    source: FontSourceFile(file.to_str()),
                     face: face,
                     handle: *fctx
                 })
diff --git a/src/components/gfx/text/glyph.rs b/src/components/gfx/text/glyph.rs
index 0baa11dff51..077a393d9c3 100644
--- a/src/components/gfx/text/glyph.rs
+++ b/src/components/gfx/text/glyph.rs
@@ -303,7 +303,11 @@ impl Ord for DetailedGlyphRecord {
 // usage pattern of setting/appending all the detailed glyphs, and
 // then querying without setting.
 struct DetailedGlyphStore {
+    // TODO(pcwalton): Allocation of this buffer is expensive. Consider a small-vector
+    // optimization.
     detail_buffer: ~[DetailedGlyph],
+    // TODO(pcwalton): Allocation of this buffer is expensive. Consider a small-vector
+    // optimization.
     detail_lookup: ~[DetailedGlyphRecord],
     lookup_is_sorted: bool,
 }
@@ -506,8 +510,12 @@ impl<'self> GlyphInfo<'self> {
 
 // Public data structure and API for storing and retrieving glyph data
 pub struct GlyphStore {
+    // TODO(pcwalton): Allocation of this buffer is expensive. Consider a small-vector
+    // optimization.
     entry_buffer: ~[GlyphEntry],
+
     detail_store: DetailedGlyphStore,
+
     is_whitespace: bool,
 }
 
@@ -602,6 +610,7 @@ impl<'self> GlyphStore {
         self.iter_glyphs_for_char_range(&Range::new(i, 1))
     }
 
+    #[inline]
     pub fn iter_glyphs_for_char_range(&'self self, rang: &Range) -> GlyphIterator<'self> {
         if rang.begin() >= self.entry_buffer.len() {
             fail!("iter_glyphs_for_range: range.begin beyond length!");
@@ -682,23 +691,44 @@ pub struct GlyphIterator<'self> {
     priv glyph_range: Option<iter::Range<uint>>,
 }
 
+impl<'self> GlyphIterator<'self> {
+    // Slow path when there is a glyph range.
+    #[inline(never)]
+    fn next_glyph_range(&mut self) -> Option<(uint, GlyphInfo<'self>)> {
+        match self.glyph_range.get_mut_ref().next() {
+            Some(j) => Some((self.char_index,
+                DetailGlyphInfo(self.store, self.char_index, j as u16))),
+            None => {
+                // No more glyphs for current character.  Try to get another.
+                self.glyph_range = None;
+                self.next()
+            }
+        }
+    }
+
+    // Slow path when there is a complex glyph.
+    #[inline(never)]
+    fn next_complex_glyph(&mut self, entry: &GlyphEntry, i: uint)
+                          -> Option<(uint, GlyphInfo<'self>)> {
+        let glyphs = self.store.detail_store.get_detailed_glyphs_for_entry(i, entry.glyph_count());
+        self.glyph_range = Some(range(0, glyphs.len()));
+        self.next()
+    }
+}
+
 impl<'self> Iterator<(uint, GlyphInfo<'self>)> for GlyphIterator<'self> {
     // I tried to start with something simpler and apply FlatMap, but the
     // inability to store free variables in the FlatMap struct was problematic.
-
+    //
+    // This function consists of the fast path and is designed to be inlined into its caller. The
+    // slow paths, which should not be inlined, are `next_glyph_range()` and
+    // `next_complex_glyph()`.
+    #[inline(always)]
     fn next(&mut self) -> Option<(uint, GlyphInfo<'self>)> {
         // Would use 'match' here but it borrows contents in a way that
         // interferes with mutation.
         if self.glyph_range.is_some() {
-            match self.glyph_range.get_mut_ref().next() {
-                Some(j) => Some((self.char_index,
-                    DetailGlyphInfo(self.store, self.char_index, j as u16))),
-                None => {
-                    // No more glyphs for current character.  Try to get another.
-                    self.glyph_range = None;
-                    self.next()
-                }
-            }
+            self.next_glyph_range()
         } else {
             // No glyph range.  Look at next character.
             match self.char_range.next() {
@@ -709,10 +739,8 @@ impl<'self> Iterator<(uint, GlyphInfo<'self>)> for GlyphIterator<'self> {
                     if entry.is_simple() {
                         Some((self.char_index, SimpleGlyphInfo(self.store, i)))
                     } else {
-                        let glyphs = self.store.detail_store
-                                     .get_detailed_glyphs_for_entry(i, entry.glyph_count());
-                        self.glyph_range = Some(range(0, glyphs.len()));
-                        self.next()
+                        // Fall back to the slow path.
+                        self.next_complex_glyph(entry, i)
                     }
                 },
                 None => None
diff --git a/src/components/gfx/text/text_run.rs b/src/components/gfx/text/text_run.rs
index bcc8d3c876e..81df6f0ee3a 100644
--- a/src/components/gfx/text/text_run.rs
+++ b/src/components/gfx/text/text_run.rs
@@ -14,18 +14,19 @@ use style::computed_values::text_decoration;
 
 /// A text run.
 pub struct TextRun {
-    text: ~str,
+    text: Arc<~str>,
     font: @mut Font,
     decoration: text_decoration::T,
-    glyphs: ~[Arc<GlyphStore>],
+    glyphs: Arc<~[Arc<GlyphStore>]>,
 }
 
-/// This is a hack until TextRuns are normally sendable, or we instead use Arc<TextRun> everywhere.
+/// The same as a text run, but with a font descriptor instead of a font. This makes them thread
+/// safe.
 pub struct SendableTextRun {
-    text: ~str,
+    text: Arc<~str>,
     font: FontDescriptor,
     decoration: text_decoration::T,
-    priv glyphs: ~[Arc<GlyphStore>],
+    priv glyphs: Arc<~[Arc<GlyphStore>]>,
 }
 
 impl SendableTextRun {
@@ -51,6 +52,8 @@ pub struct SliceIterator<'self> {
 }
 
 impl<'self> Iterator<(&'self GlyphStore, uint, Range)> for SliceIterator<'self> {
+    // inline(always) due to the inefficient rt failures messing up inline heuristics, I think.
+    #[inline(always)]
     fn next(&mut self) -> Option<(&'self GlyphStore, uint, Range)> {
         loop {
             let slice_glyphs = self.glyph_iter.next();
@@ -121,10 +124,10 @@ impl<'self> TextRun {
         let glyphs = TextRun::break_and_shape(font, text);
 
         let run = TextRun {
-            text: text,
+            text: Arc::new(text),
             font: font,
             decoration: decoration,
-            glyphs: glyphs,
+            glyphs: Arc::new(glyphs),
         };
         return run;
     }
@@ -198,12 +201,14 @@ impl<'self> TextRun {
     }
 
     pub fn char_len(&self) -> uint {
-        do self.glyphs.iter().fold(0u) |len, slice_glyphs| {
+        do self.glyphs.get().iter().fold(0u) |len, slice_glyphs| {
             len + slice_glyphs.get().char_len()
         }
     }
 
-    pub fn glyphs(&'self self) -> &'self ~[Arc<GlyphStore>] { &self.glyphs }
+    pub fn glyphs(&'self self) -> &'self ~[Arc<GlyphStore>] {
+        self.glyphs.get()
+    }
 
     pub fn range_is_trimmable_whitespace(&self, range: &Range) -> bool {
         for (slice_glyphs, _, _) in self.iter_slices_for_range(range) {
@@ -233,7 +238,7 @@ impl<'self> TextRun {
 
     pub fn iter_slices_for_range(&'self self, range: &Range) -> SliceIterator<'self> {
         SliceIterator {
-            glyph_iter: self.glyphs.iter(),
+            glyph_iter: self.glyphs.get().iter(),
             range:      *range,
             offset:     0,
         }
diff --git a/src/components/main/compositing/compositor_layer.rs b/src/components/main/compositing/compositor_layer.rs
index 13d22f6602a..29b79ef06d4 100644
--- a/src/components/main/compositing/compositor_layer.rs
+++ b/src/components/main/compositing/compositor_layer.rs
@@ -19,7 +19,8 @@ use script::script_task::SendEventMsg;
 use servo_msg::compositor_msg::{LayerBuffer, LayerBufferSet, Epoch, Tile};
 use servo_msg::constellation_msg::PipelineId;
 use std::cell::Cell;
-use windowing::{MouseWindowEvent, MouseWindowClickEvent, MouseWindowMouseDownEvent, MouseWindowMouseUpEvent};
+use windowing::{MouseWindowEvent, MouseWindowClickEvent, MouseWindowMouseDownEvent};
+use windowing::{MouseWindowMouseUpEvent};
 
 #[cfg(not(target_os="macos"))]
 use layers::texturegl::TextureTarget2D;
@@ -372,7 +373,7 @@ impl CompositorLayer {
         }
     }
 
-    // Returns whether the layer should be vertically flipped. and
+    // Returns whether the layer should be vertically flipped.
     #[cfg(target_os="macos")]
     fn texture_flip_and_target(cpu_painting: bool, size: Size2D<uint>) -> (Flip, TextureTarget) {
         let flip = if cpu_painting {
diff --git a/src/components/main/css/node_style.rs b/src/components/main/css/node_style.rs
index 49ab10710e8..e4c40e1e3b4 100644
--- a/src/components/main/css/node_style.rs
+++ b/src/components/main/css/node_style.rs
@@ -9,8 +9,6 @@ use layout::incremental::RestyleDamage;
 
 use style::ComputedValues;
 use script::dom::node::{AbstractNode, LayoutView};
-use servo_util::tree::TreeNodeRef;
-
 
 /// Node mixin providing `style` method that returns a `NodeStyle`
 pub trait StyledNode {
@@ -20,7 +18,8 @@ pub trait StyledNode {
 
 impl StyledNode for AbstractNode<LayoutView> {
     fn style(&self) -> &ComputedValues {
-        assert!(self.is_element()); // Only elements can have styles
+        // FIXME(pcwalton): Is this assertion needed for memory safety? It's slow.
+        //assert!(self.is_element()); // Only elements can have styles
         let results = self.get_css_select_results();
         results
     }
diff --git a/src/components/main/layout/block.rs b/src/components/main/layout/block.rs
index 45044caa341..ba3b4746a63 100644
--- a/src/components/main/layout/block.rs
+++ b/src/components/main/layout/block.rs
@@ -4,11 +4,11 @@
 
 //! CSS block layout.
 
-use layout::box::{RenderBox};
+use layout::box::{RenderBox, RenderBoxUtils};
 use layout::context::LayoutContext;
 use layout::display_list_builder::{DisplayListBuilder, ExtraDisplayListData};
-use layout::flow::{BlockFlow, FlowContext, FlowData, InlineBlockFlow, FloatFlow, InlineFlow};
-use layout::inline::InlineLayout;
+use layout::flow::{BlockFlowClass, FlowClass, FlowContext, FlowData, ImmutableFlowUtils};
+use layout::flow;
 use layout::model::{MaybeAuto, Specified, Auto};
 use layout::float_context::{FloatContext, Invalid};
 
@@ -20,29 +20,29 @@ use gfx::display_list::DisplayList;
 use servo_util::geometry::{Au, to_frac_px};
 use servo_util::geometry;
 
-pub struct BlockFlowData {
+pub struct BlockFlow {
     /// Data common to all flows.
-    common: FlowData,
+    base: FlowData,
 
     /// The associated render box.
-    box: Option<RenderBox>,
+    box: Option<@RenderBox>,
 
     /// Whether this block flow is the root flow.
     is_root: bool
 }
 
-impl BlockFlowData {
-    pub fn new(common: FlowData) -> BlockFlowData {
-        BlockFlowData {
-            common: common,
+impl BlockFlow {
+    pub fn new(base: FlowData) -> BlockFlow {
+        BlockFlow {
+            base: base,
             box: None,
             is_root: false
         }
     }
 
-    pub fn new_root(common: FlowData) -> BlockFlowData {
-        BlockFlowData {
-            common: common,
+    pub fn new_root(base: FlowData) -> BlockFlow {
+        BlockFlow {
+            base: base,
             box: None,
             is_root: true
         }
@@ -54,78 +54,18 @@ impl BlockFlowData {
         }
         self.box = None;
     }
-}
-
-pub trait BlockLayout {
-    fn starts_root_flow(&self) -> bool;
-    fn starts_block_flow(&self) -> bool;
-}
-
-impl BlockLayout for FlowContext {
-    fn starts_root_flow(&self) -> bool {
-        match *self {
-            BlockFlow(ref info) => info.is_root,
-            _ => false
-        }
-    }
-
-    fn starts_block_flow(&self) -> bool {
-        match *self {
-            BlockFlow(*) | InlineBlockFlow(*) | FloatFlow(*) => true,
-            _ => false 
-        }
-    }
-}
-
-impl BlockFlowData {
-    /* Recursively (bottom-up) determine the context's preferred and
-    minimum widths.  When called on this context, all child contexts
-    have had their min/pref widths set. This function must decide
-    min/pref widths based on child context widths and dimensions of
-    any boxes it is responsible for flowing.  */
-
-    /* TODO: floats */
-    /* TODO: absolute contexts */
-    /* TODO: inline-blocks */
-    pub fn bubble_widths_block(&mut self, ctx: &LayoutContext) {
-        let mut min_width = Au(0);
-        let mut pref_width = Au(0);
-        let mut num_floats = 0;
-
-        /* find max width from child block contexts */
-        for child_ctx in self.common.child_iter() {
-            assert!(child_ctx.starts_block_flow() || child_ctx.starts_inline_flow());
-
-            do child_ctx.with_mut_base |child_node| {
-                min_width = geometry::max(min_width, child_node.min_width);
-                pref_width = geometry::max(pref_width, child_node.pref_width);
-
-                num_floats = num_floats + child_node.num_floats;
-            }
-        }
-
-        /* if not an anonymous block context, add in block box's widths.
-           these widths will not include child elements, just padding etc. */
-        for box in self.box.iter() {
-            min_width = min_width.add(&box.get_min_width(ctx));
-            pref_width = pref_width.add(&box.get_pref_width(ctx));
-        }
 
-        self.common.min_width = min_width;
-        self.common.pref_width = pref_width;
-        self.common.num_floats = num_floats;
-    }
- 
-    /// Computes left and right margins and width based on CSS 2.1 secion 10.3.3.
-    /// Requires borders and padding to already be computed
-    fn compute_horiz( &self,
-                            width: MaybeAuto, 
-                            left_margin: MaybeAuto, 
-                            right_margin: MaybeAuto, 
-                            available_width: Au) -> (Au, Au, Au) {
-
-        //If width is not 'auto', and width + margins > available_width, all 'auto' margins are treated as '0'
-        let (left_margin, right_margin) = match width{
+    /// Computes left and right margins and width based on CSS 2.1 section 10.3.3.
+    /// Requires borders and padding to already be computed.
+    fn compute_horiz(&self,
+                     width: MaybeAuto, 
+                     left_margin: MaybeAuto, 
+                     right_margin: MaybeAuto, 
+                     available_width: Au)
+                     -> (Au, Au, Au) {
+        // If width is not 'auto', and width + margins > available_width, all 'auto' margins are
+        // treated as 0.
+        let (left_margin, right_margin) = match width {
             Auto => (left_margin, right_margin),
             Specified(width) => {
                 let left = left_margin.specified_or_zero();
@@ -152,9 +92,9 @@ impl BlockFlowData {
             (Specified(margin_l), Specified(width), Auto) => (margin_l, width, available_width - (margin_l + width)),
 
             //If width is set to 'auto', any other 'auto' value becomes '0', and width is solved for
-            (Auto, Auto, Specified(margin_r)) => (Au(0), available_width - margin_r, margin_r),
-            (Specified(margin_l), Auto, Auto) => (margin_l, available_width - margin_l, Au(0)),
-            (Auto, Auto, Auto) => (Au(0), available_width, Au(0)),
+            (Auto, Auto, Specified(margin_r)) => (Au::new(0), available_width - margin_r, margin_r),
+            (Specified(margin_l), Auto, Auto) => (margin_l, available_width - margin_l, Au::new(0)),
+            (Auto, Auto, Auto) => (Au::new(0), available_width, Au::new(0)),
 
             //If left and right margins are auto, they become equal
             (Auto, Specified(width), Auto) => {
@@ -167,126 +107,32 @@ impl BlockFlowData {
         (width_Au, left_margin_Au, right_margin_Au)
     }
 
-    /// Recursively (top-down) determines the actual width of child contexts and boxes. When called
-    /// on this context, the context has had its width set by the parent context.
-    ///
-    /// Dual boxes consume some width first, and the remainder is assigned to all child (block)
-    /// contexts.
-    pub fn assign_widths_block(&mut self, ctx: &LayoutContext) { 
-        debug!("assign_widths_block: assigning width for flow %?",  self.common.id);
-        if self.is_root {
-            debug!("Setting root position");
-            self.common.position.origin = Au::zero_point();
-            self.common.position.size.width = ctx.screen_size.size.width;
-            self.common.floats_in = FloatContext::new(self.common.num_floats);
-            self.common.is_inorder = false;
-        }
-
-        //position was set to the containing block by the flow's parent
-        let mut remaining_width = self.common.position.size.width;
-        let mut x_offset = Au(0);
-
-        for &box in self.box.iter() {
-            let style = box.style();
-            do box.with_model |model| {
-                //Can compute border width here since it doesn't depend on anything
-                model.compute_borders(style);
-
-                // Can compute padding here since we know containing block width.
-                model.compute_padding(style, remaining_width);
-
-                // Margins are 0 right now so model.noncontent_width() is just borders + padding.
-                let available_width = remaining_width - model.noncontent_width();
-
-                // Top and bottom margins for blocks are 0 if auto.
-                let margin_top = MaybeAuto::from_style(style.Margin.margin_top,
-                                                       remaining_width).specified_or_zero();
-                let margin_bottom = MaybeAuto::from_style(style.Margin.margin_bottom,
-                                                          remaining_width).specified_or_zero();
-
-                let (width, margin_left, margin_right) =
-                    (MaybeAuto::from_style(style.Box.width, remaining_width),
-                     MaybeAuto::from_style(style.Margin.margin_left, remaining_width),
-                     MaybeAuto::from_style(style.Margin.margin_right, remaining_width));
-
-                let (width, margin_left, margin_right) = self.compute_horiz(width,
-                                                                            margin_left,
-                                                                            margin_right,
-                                                                            available_width);
-
-                model.margin.top = margin_top;
-                model.margin.right = margin_right;
-                model.margin.bottom = margin_bottom;
-                model.margin.left = margin_left;
-
-                x_offset = model.offset();
-                remaining_width = width;
-            }
-
-            do box.with_mut_base |base| {
-                //The associated box is the border box of this flow
-                base.position.origin.x = base.model.margin.left;
-
-                let pb = base.model.padding.left + base.model.padding.right +
-                    base.model.border.left + base.model.border.right;
-                base.position.size.width = remaining_width + pb;
-            }
-        }
-
-        let has_inorder_children = self.common.is_inorder || self.common.num_floats > 0;
-        for kid in self.common.child_iter() {
-            assert!(kid.starts_block_flow() || kid.starts_inline_flow());
-
-            do kid.with_mut_base |child_node| {
-                child_node.position.origin.x = x_offset;
-                child_node.position.size.width = remaining_width;
-                child_node.is_inorder = has_inorder_children;
-
-                if !child_node.is_inorder {
-                    child_node.floats_in = FloatContext::new(0);
-                }
-            }
-        }
-    }
-
-    pub fn assign_height_inorder_block(&mut self, ctx: &mut LayoutContext) {
-        debug!("assign_height_inorder_block: assigning height for block %?", self.common.id);
-        self.assign_height_block_base(ctx, true);
-    }
-
-    pub fn assign_height_block(&mut self, ctx: &mut LayoutContext) {
-        debug!("assign_height_block: assigning height for block %?", self.common.id);
-        // This is the only case in which a block flow can start an inorder
-        // subtraversal.
-        if self.is_root && self.common.num_floats > 0 {
-            self.assign_height_inorder_block(ctx);
-            return;
-        }
-        self.assign_height_block_base(ctx, false);
-    }
-
+    // inline(always) because this is only ever called by in-order or non-in-order top-level
+    // methods
+    #[inline(always)]
     fn assign_height_block_base(&mut self, ctx: &mut LayoutContext, inorder: bool) {
-        let mut cur_y = Au(0);
-        let mut clearance = Au(0);
-        let mut top_offset = Au(0);
-        let mut bottom_offset = Au(0);
-        let mut left_offset = Au(0);
+        let mut cur_y = Au::new(0);
+        let mut clearance = Au::new(0);
+        let mut top_offset = Au::new(0);
+        let mut bottom_offset = Au::new(0);
+        let mut left_offset = Au::new(0);
         let mut float_ctx = Invalid;
 
         for &box in self.box.iter() {
-            clearance = match box.clear() {
-                None => Au(0),
+            let base = box.base();
+            clearance = match base.clear() {
+                None => Au::new(0),
                 Some(clear) => {
-                    self.common.floats_in.clearance(clear)
+                    self.base.floats_in.clearance(clear)
                 }
             };
 
-            do box.with_model |model| {
-                top_offset = clearance + model.margin.top + model.border.top + model.padding.top;
-                cur_y = cur_y + top_offset;
-                bottom_offset = model.margin.bottom + model.border.bottom + model.padding.bottom;
-                left_offset = model.offset();
-            };
+            let mut model_ref = base.model.mutate();
+            let model = &mut model_ref.ptr;
+            top_offset = clearance + model.margin.top + model.border.top + model.padding.top;
+            cur_y = cur_y + top_offset;
+            bottom_offset = model.margin.bottom + model.border.bottom + model.padding.bottom;
+            left_offset = model.offset();
         }
 
         if inorder {
@@ -297,84 +143,48 @@ impl BlockFlowData {
             // visit child[i]
             // repeat until all children are visited.
             // last_child.floats_out -> self.floats_out (done at the end of this method)
-            float_ctx = self.common.floats_in.translate(Point2D(-left_offset, -top_offset));
-            for kid in self.common.child_iter() {
-                do kid.with_mut_base |child_node| {
-                    child_node.floats_in = float_ctx.clone();
-                }
+            float_ctx = self.base.floats_in.translate(Point2D(-left_offset, -top_offset));
+            for kid in self.base.child_iter() {
+                flow::mut_base(*kid).floats_in = float_ctx.clone();
                 kid.assign_height_inorder(ctx);
-                do kid.with_mut_base |child_node| {
-                    float_ctx = child_node.floats_out.clone();
-                }
+                float_ctx = flow::mut_base(*kid).floats_out.clone();
             }
         }
 
-        let mut collapsible = Au(0);
-        let mut collapsing = Au(0);
-        let mut margin_top = Au(0);
-        let mut margin_bottom = Au(0);
+        let mut collapsible = Au::new(0);
+        let mut collapsing = Au::new(0);
+        let mut margin_top = Au::new(0);
+        let mut margin_bottom = Au::new(0);
         let mut top_margin_collapsible = false;
         let mut bottom_margin_collapsible = false;
-        let mut first_inflow = true;
+        let mut first_in_flow = true;
         for &box in self.box.iter() {
-            do box.with_model |model| {
-                if !self.is_root && model.border.top == Au(0) && model.padding.top == Au(0) {
-                    collapsible = model.margin.top;
-                    top_margin_collapsible = true;
-                }
-                if !self.is_root && model.border.bottom == Au(0) && model.padding.bottom == Au(0) {
-                    bottom_margin_collapsible = true;
-                }
-                margin_top = model.margin.top;
-                margin_bottom = model.margin.bottom;
+            let base = box.base();
+            let mut model_ref = base.model.mutate();
+            let model = &mut model_ref.ptr;
+            if !self.is_root && model.border.top == Au::new(0) && model.padding.top == Au::new(0) {
+                collapsible = model.margin.top;
+                top_margin_collapsible = true;
             }
-        }
-
-        for kid in self.common.child_iter() {
-            match *kid {
-                BlockFlow(ref info) => {
-                    for &box in info.box.iter() {
-                        do box.with_model |model| {
-                            // The top margin collapses with its first in-flow block-level child's
-                            // top margin if the parent has no top boder, no top padding.
-                            if first_inflow && top_margin_collapsible {
-                                // If top-margin of parent is less than top-margin of its first child, 
-                                // the parent box goes down until its top is aligned with the child.
-                                if margin_top < model.margin.top {
-                                    // TODO: The position of child floats should be updated and this
-                                    // would influence clearance as well. See #725
-                                    let extra_margin = model.margin.top - margin_top;
-                                    top_offset = top_offset + extra_margin;
-                                    margin_top = model.margin.top;
-                                }
-                            }
-                            // The bottom margin of an in-flow block-level element collapses 
-                            // with the top margin of its next in-flow block-level sibling.
-                            collapsing = geometry::min(model.margin.top, collapsible);
-                            collapsible = model.margin.bottom;
-                        }
-                    }
-                    first_inflow = false;
-                }
-                InlineFlow(ref info) => {
-                    collapsing = Au(0);
-                    // Non-empty inline flows prevent collapsing between the previous margion and the next.
-                    if info.common.position.size.height > Au(0) {
-                        collapsible = Au(0);
-                    }
-                }
-                // Margins between a floated box and any other box do not collapse.
-                _ => {
-                    collapsing = Au(0);
-                }
-                // TODO: Handling for AbsoluteFlow, InlineBlockFlow and TableFlow?
+            if !self.is_root && model.border.bottom == Au::new(0) && model.padding.bottom == Au::new(0) {
+                bottom_margin_collapsible = true;
             }
+            margin_top = model.margin.top;
+            margin_bottom = model.margin.bottom;
+        }
 
-            do kid.with_mut_base |child_node| {
-                cur_y = cur_y - collapsing;
-                child_node.position.origin.y = cur_y;
-                cur_y = cur_y + child_node.position.size.height;
-            };
+        for kid in self.base.child_iter() {
+            kid.collapse_margins(top_margin_collapsible,
+                                 &mut first_in_flow,
+                                 &mut margin_top,
+                                 &mut top_offset,
+                                 &mut collapsing,
+                                 &mut collapsible);
+
+            let child_node = flow::mut_base(*kid);
+            cur_y = cur_y - collapsing;
+            child_node.position.origin.y = cur_y;
+            cur_y = cur_y + child_node.position.size.height;
         }
 
         // The bottom margin collapses with its last in-flow block-level child's bottom margin
@@ -385,7 +195,7 @@ impl BlockFlowData {
             }
             collapsible
         } else {
-            Au(0)
+            Au::new(0)
         };
         
         // TODO: A box's own margins collapse if the 'min-height' property is zero, and it has neither
@@ -400,61 +210,67 @@ impl BlockFlowData {
         };
 
         for &box in self.box.iter() {
-            let style = box.style();
-            let maybe_height = MaybeAuto::from_style(style.Box.height, Au(0));
+            let base = box.base();
+            let style = base.style();
+            let maybe_height = MaybeAuto::from_style(style.Box.height, Au::new(0));
             let maybe_height = maybe_height.specified_or_zero();
             height = geometry::max(height, maybe_height);
         }
 
-        let mut noncontent_height = Au(0);
-        for box in self.box.mut_iter() {
-            do box.with_mut_base |base| {
-                //The associated box is the border box of this flow
-                base.model.margin.top = margin_top;
-                base.model.margin.bottom = margin_bottom;
+        let mut noncontent_height = Au::new(0);
+        for box in self.box.iter() {
+            let base = box.base();
+            let mut model_ref = base.model.mutate();
+            let mut position_ref = base.position.mutate();
+            let (model, position) = (&mut model_ref.ptr, &mut position_ref.ptr);
+
+            // The associated box is the border box of this flow.
+            model.margin.top = margin_top;
+            model.margin.bottom = margin_bottom;
 
-                base.position.origin.y = clearance + base.model.margin.top;
+            position.origin.y = clearance + model.margin.top;
 
-                noncontent_height = base.model.padding.top + base.model.padding.bottom +
-                    base.model.border.top + base.model.border.bottom;
-                base.position.size.height = height + noncontent_height;
+            noncontent_height = model.padding.top + model.padding.bottom + model.border.top +
+                model.border.bottom;
+            position.size.height = height + noncontent_height;
 
-                noncontent_height = noncontent_height + clearance +
-                    base.model.margin.top + base.model.margin.bottom;
-            }
+            noncontent_height = noncontent_height + clearance + model.margin.top +
+                model.margin.bottom;
         }
 
         //TODO(eatkinson): compute heights using the 'height' property.
-        self.common.position.size.height = height + noncontent_height;
+        self.base.position.size.height = height + noncontent_height;
 
         if inorder {
             let extra_height = height - (cur_y - top_offset) + bottom_offset; 
-            self.common.floats_out = float_ctx.translate(Point2D(left_offset, -extra_height));
+            self.base.floats_out = float_ctx.translate(Point2D(left_offset, -extra_height));
         } else {
-            self.common.floats_out = self.common.floats_in.clone();
+            self.base.floats_out = self.base.floats_in.clone();
         }
     }
 
-    pub fn build_display_list_block<E:ExtraDisplayListData>(&mut self,
-                                                            builder: &DisplayListBuilder,
-                                                            dirty: &Rect<Au>, 
-                                                            list: &Cell<DisplayList<E>>) 
-                                                            -> bool {
-
-        if self.common.node.is_iframe_element() {
-            let x = self.common.abs_position.x + do self.box.as_ref().map_default(Au(0)) |box| {
-                box.with_model(|model| model.margin.left + model.border.left + model.padding.left)
+    pub fn build_display_list_block<E:ExtraDisplayListData>(
+                                    &mut self,
+                                    builder: &DisplayListBuilder,
+                                    dirty: &Rect<Au>, 
+                                    list: &Cell<DisplayList<E>>) 
+                                    -> bool {
+        if self.base.node.is_iframe_element() {
+            let x = self.base.abs_position.x + do self.box.map_default(Au::new(0)) |box| {
+                let model = box.base().model.get();
+                model.margin.left + model.border.left + model.padding.left
             };
-            let y = self.common.abs_position.y + do self.box.as_ref().map_default(Au(0)) |box| {
-                box.with_model(|model| model.margin.top + model.border.top + model.padding.top)
+            let y = self.base.abs_position.y + do self.box.map_default(Au::new(0)) |box| {
+                let model = box.base().model.get();
+                model.margin.top + model.border.top + model.padding.top
             };
-            let w = self.common.position.size.width - do self.box.as_ref().map_default(Au(0)) |box| {
-                box.with_model(|model| model.noncontent_width())
+            let w = self.base.position.size.width - do self.box.map_default(Au::new(0)) |box| {
+                box.base().model.get().noncontent_width()
             };
-            let h = self.common.position.size.height - do self.box.as_ref().map_default(Au(0)) |box| {
-                box.with_model(|model| model.noncontent_height())
+            let h = self.base.position.size.height - do self.box.map_default(Au::new(0)) |box| {
+                box.base().model.get().noncontent_height()
             };
-            do self.common.node.with_mut_iframe_element |iframe_element| {
+            do self.base.node.with_mut_iframe_element |iframe_element| {
                 iframe_element.size.get_mut_ref().set_rect(Rect(Point2D(to_frac_px(x) as f32,
                                                                         to_frac_px(y) as f32),
                                                                 Size2D(to_frac_px(w) as f32,
@@ -462,7 +278,7 @@ impl BlockFlowData {
             }
         }
 
-        let abs_rect = Rect(self.common.abs_position, self.common.position.size);
+        let abs_rect = Rect(self.base.abs_position, self.base.position.size);
         if !abs_rect.intersects(dirty) {
             return true;
         }
@@ -471,18 +287,199 @@ impl BlockFlowData {
 
         // add box that starts block context
         for box in self.box.iter() {
-            box.build_display_list(builder, dirty, &self.common.abs_position, list)
+            box.build_display_list(builder, dirty, &self.base.abs_position, list)
         }
 
         // TODO: handle any out-of-flow elements
-        let this_position = self.common.abs_position;
-        for child in self.common.child_iter() {
-            do child.with_mut_base |base| {
-                base.abs_position = this_position + base.position.origin;
-            }
+        let this_position = self.base.abs_position;
+        for child in self.base.child_iter() {
+            let child_base = flow::mut_base(*child);
+            child_base.abs_position = this_position + child_base.position.origin;
         }
 
         false
     }
 }
 
+impl FlowContext for BlockFlow {
+    fn class(&self) -> FlowClass {
+        BlockFlowClass
+    }
+
+    fn as_block<'a>(&'a mut self) -> &'a mut BlockFlow {
+        self
+    }
+
+    /* Recursively (bottom-up) determine the context's preferred and
+    minimum widths.  When called on this context, all child contexts
+    have had their min/pref widths set. This function must decide
+    min/pref widths based on child context widths and dimensions of
+    any boxes it is responsible for flowing.  */
+
+    /* TODO: floats */
+    /* TODO: absolute contexts */
+    /* TODO: inline-blocks */
+    fn bubble_widths(&mut self, _: &mut LayoutContext) {
+        let mut min_width = Au::new(0);
+        let mut pref_width = Au::new(0);
+        let mut num_floats = 0;
+
+        /* find max width from child block contexts */
+        for child_ctx in self.base.child_iter() {
+            assert!(child_ctx.starts_block_flow() || child_ctx.starts_inline_flow());
+
+            let child_base = flow::mut_base(*child_ctx);
+            min_width = geometry::max(min_width, child_base.min_width);
+            pref_width = geometry::max(pref_width, child_base.pref_width);
+
+            num_floats = num_floats + child_base.num_floats;
+        }
+
+        /* if not an anonymous block context, add in block box's widths.
+           these widths will not include child elements, just padding etc. */
+        for box in self.box.iter() {
+            {
+                // Can compute border width here since it doesn't depend on anything.
+                let base = box.base();
+                base.model.mutate().ptr.compute_borders(base.style())
+            }
+
+            let (this_minimum_width, this_preferred_width) = box.minimum_and_preferred_widths();
+            min_width = min_width + this_minimum_width;
+            pref_width = pref_width + this_preferred_width;
+        }
+
+        self.base.min_width = min_width;
+        self.base.pref_width = pref_width;
+        self.base.num_floats = num_floats;
+    }
+ 
+    /// Recursively (top-down) determines the actual width of child contexts and boxes. When called
+    /// on this context, the context has had its width set by the parent context.
+    ///
+    /// Dual boxes consume some width first, and the remainder is assigned to all child (block)
+    /// contexts.
+    fn assign_widths(&mut self, ctx: &mut LayoutContext) { 
+        debug!("assign_widths_block: assigning width for flow %?",  self.base.id);
+        if self.is_root {
+            debug!("Setting root position");
+            self.base.position.origin = Au::zero_point();
+            self.base.position.size.width = ctx.screen_size.size.width;
+            self.base.floats_in = FloatContext::new(self.base.num_floats);
+            self.base.is_inorder = false;
+        }
+
+        //position was set to the containing block by the flow's parent
+        let mut remaining_width = self.base.position.size.width;
+        let mut x_offset = Au::new(0);
+
+        for &box in self.box.iter() {
+            let base = box.base();
+            let style = base.style();
+            let mut model_ref = base.model.mutate();
+            let model = &mut model_ref.ptr;
+
+            // Can compute padding here since we know containing block width.
+            model.compute_padding(style, remaining_width);
+
+            // Margins are 0 right now so model.noncontent_width() is just borders + padding.
+            let available_width = remaining_width - model.noncontent_width();
+
+            // Top and bottom margins for blocks are 0 if auto.
+            let margin_top = MaybeAuto::from_style(style.Margin.margin_top,
+                                                   remaining_width).specified_or_zero();
+            let margin_bottom = MaybeAuto::from_style(style.Margin.margin_bottom,
+                                                      remaining_width).specified_or_zero();
+
+            let (width, margin_left, margin_right) =
+                (MaybeAuto::from_style(style.Box.width, remaining_width),
+                 MaybeAuto::from_style(style.Margin.margin_left, remaining_width),
+                 MaybeAuto::from_style(style.Margin.margin_right, remaining_width));
+
+            let (width, margin_left, margin_right) = self.compute_horiz(width,
+                                                                        margin_left,
+                                                                        margin_right,
+                                                                        available_width);
+
+            model.margin.top = margin_top;
+            model.margin.right = margin_right;
+            model.margin.bottom = margin_bottom;
+            model.margin.left = margin_left;
+
+            x_offset = model.offset();
+            remaining_width = width;
+
+            //The associated box is the border box of this flow
+            let position_ref = base.position.mutate();
+            position_ref.ptr.origin.x = model.margin.left;
+            let padding_and_borders = model.padding.left + model.padding.right +
+                model.border.left + model.border.right;
+            position_ref.ptr.size.width = remaining_width + padding_and_borders;
+        }
+
+        let has_inorder_children = self.base.is_inorder || self.base.num_floats > 0;
+        for kid in self.base.child_iter() {
+            assert!(kid.starts_block_flow() || kid.starts_inline_flow());
+
+            let child_base = flow::mut_base(*kid);
+            child_base.position.origin.x = x_offset;
+            child_base.position.size.width = remaining_width;
+            child_base.is_inorder = has_inorder_children;
+
+            if !child_base.is_inorder {
+                child_base.floats_in = FloatContext::new(0);
+            }
+        }
+    }
+
+    fn assign_height_inorder(&mut self, ctx: &mut LayoutContext) {
+        debug!("assign_height_inorder: assigning height for block %?", self.base.id);
+        self.assign_height_block_base(ctx, true);
+    }
+
+    fn assign_height(&mut self, ctx: &mut LayoutContext) {
+        debug!("assign_height: assigning height for block %?", self.base.id);
+        // This is the only case in which a block flow can start an inorder
+        // subtraversal.
+        if self.is_root && self.base.num_floats > 0 {
+            self.assign_height_inorder(ctx);
+            return;
+        }
+        self.assign_height_block_base(ctx, false);
+    }
+
+    fn collapse_margins(&mut self,
+                        top_margin_collapsible: bool,
+                        first_in_flow: &mut bool,
+                        margin_top: &mut Au,
+                        top_offset: &mut Au,
+                        collapsing: &mut Au,
+                        collapsible: &mut Au) {
+        for &box in self.box.iter() {
+            let base = box.base();
+            let mut model_ref = base.model.mutate();
+            let model = &mut model_ref.ptr;
+
+            // The top margin collapses with its first in-flow block-level child's
+            // top margin if the parent has no top border, no top padding.
+            if *first_in_flow && top_margin_collapsible {
+                // If top-margin of parent is less than top-margin of its first child, 
+                // the parent box goes down until its top is aligned with the child.
+                if *margin_top < model.margin.top {
+                    // TODO: The position of child floats should be updated and this
+                    // would influence clearance as well. See #725
+                    let extra_margin = model.margin.top - *margin_top;
+                    *top_offset = *top_offset + extra_margin;
+                    *margin_top = model.margin.top;
+                }
+            }
+            // The bottom margin of an in-flow block-level element collapses 
+            // with the top margin of its next in-flow block-level sibling.
+            *collapsing = geometry::min(model.margin.top, *collapsible);
+            *collapsible = model.margin.bottom;
+        }
+
+        *first_in_flow = false;
+    }
+}
+
diff --git a/src/components/main/layout/box.rs b/src/components/main/layout/box.rs
index 012fd5b681f..364b6c03f70 100644
--- a/src/components/main/layout/box.rs
+++ b/src/components/main/layout/box.rs
@@ -4,26 +4,27 @@
 
 //! The `RenderBox` type, which represents the leaves of the layout tree.
 
-use css::node_style::StyledNode;
-use layout::context::LayoutContext;
-use layout::display_list_builder::{DisplayListBuilder, ExtraDisplayListData, ToGfxColor};
-use layout::float_context::{ClearType, ClearLeft, ClearRight, ClearBoth};
-use layout::model::{BoxModel, MaybeAuto};
-use layout::text;
-
-use std::cell::Cell;
-use std::cmp::ApproxEq;
-use std::managed;
-use std::num::Zero;
+use extra::url::Url;
 use geom::{Point2D, Rect, Size2D, SideOffsets2D};
 use gfx::display_list::{BaseDisplayItem, BorderDisplayItem, BorderDisplayItemClass};
 use gfx::display_list::{DisplayList, ImageDisplayItem, ImageDisplayItemClass};
 use gfx::display_list::{SolidColorDisplayItem, SolidColorDisplayItemClass, TextDisplayItem};
 use gfx::display_list::{TextDisplayItemClass};
 use gfx::font::{FontStyle, FontWeight300};
-use servo_util::geometry::Au;
 use gfx::text::text_run::TextRun;
 use gfx::color::rgb;
+use script::dom::node::{AbstractNode, LayoutView};
+use servo_net::image::holder::ImageHolder;
+use servo_net::local_image_cache::LocalImageCache;
+use servo_util::geometry::Au;
+use servo_util::range::*;
+use servo_util::slot::Slot;
+use servo_util::tree::{TreeNodeRef, ElementLike};
+use std::cast;
+use std::cell::Cell;
+use std::cmp::ApproxEq;
+use std::num::Zero;
+use std::unstable::raw::Box;
 use style::ComputedValues;
 use style::computed_values::border_style;
 use style::computed_values::clear;
@@ -35,19 +36,17 @@ use style::computed_values::position;
 use style::computed_values::text_align;
 use style::computed_values::text_decoration;
 use style::computed_values::vertical_align;
-use script::dom::node::{AbstractNode, LayoutView};
-use servo_net::image::holder::ImageHolder;
-use servo_net::local_image_cache::LocalImageCache;
-use servo_util::range::*;
-use servo_util::tree::{TreeNodeRef, ElementLike};
-use extra::url::Url;
 
-/// Render boxes (`struct RenderBox`) are the leaves of the layout tree. They cannot position
-/// themselves. In general, render boxes do not have a simple correspondence with CSS boxes as in
-/// the specification:
+use css::node_style::StyledNode;
+use layout::display_list_builder::{DisplayListBuilder, ExtraDisplayListData, ToGfxColor};
+use layout::float_context::{ClearType, ClearLeft, ClearRight, ClearBoth};
+use layout::model::{BoxModel, MaybeAuto};
+
+/// Boxes (`struct Box`) are the leaves of the layout tree. They cannot position themselves. In
+/// general, boxes do not have a simple correspondence with CSS boxes in the specification:
 ///
-/// * Several render boxes may correspond to the same CSS box or DOM node. For example, a CSS text
-///   box broken across two lines is represented by two render boxes.
+/// * Several boxes may correspond to the same CSS box or DOM node. For example, a CSS text box
+/// broken across two lines is represented by two boxes.
 ///
 /// * Some CSS boxes are not created at all, such as some anonymous block boxes induced by inline
 ///   boxes with block-level sibling boxes. In that case, Servo uses an `InlineFlow` with
@@ -61,27 +60,168 @@ use extra::url::Url;
 /// A box's type influences how its styles are interpreted during layout. For example, replaced
 /// content such as images are resized differently from tables, text, or other content. Different
 /// types of boxes may also contain custom data; for example, text boxes contain text.
-#[deriving(Clone)]
-pub enum RenderBox {
-    GenericRenderBoxClass(@mut RenderBoxBase),
-    ImageRenderBoxClass(@mut ImageRenderBox),
-    TextRenderBoxClass(@mut TextRenderBox),
-    UnscannedTextRenderBoxClass(@mut UnscannedTextRenderBox),
+pub trait RenderBox {
+    /// Returns the class of render box that this is.
+    fn class(&self) -> RenderBoxClass;
+
+    /// If this is an image render box, returns the underlying object. Fails otherwise.
+    ///
+    /// FIXME(pcwalton): Ugly. Replace with a real downcast operation.
+    fn as_image_render_box(@self) -> @ImageRenderBox {
+        fail!("as_text_render_box() called on a non-text-render-box")
+    }
+
+    /// If this is a text render box, returns the underlying object. Fails otherwise.
+    ///
+    /// FIXME(pcwalton): Ugly. Replace with a real downcast operation.
+    fn as_text_render_box(@self) -> @TextRenderBox {
+        fail!("as_text_render_box() called on a non-text-render-box")
+    }
+
+    /// If this is an unscanned text render box, returns the underlying object. Fails otherwise.
+    ///
+    /// FIXME(pcwalton): Ugly. Replace with a real downcast operation.
+    fn as_unscanned_text_render_box(@self) -> @UnscannedTextRenderBox {
+        fail!("as_unscanned_text_render_box() called on a non-unscanned-text-render-box")
+    }
+
+    /// Cleans up all memory associated with this render box.
+    fn teardown(&self) {}
+
+    /// Returns true if this element is an unscanned text box that consists entirely of whitespace.
+    fn is_whitespace_only(&self) -> bool {
+        false
+    }
+
+    /// Attempts to split this box so that its width is no more than `max_width`. Fails if this box
+    /// is an unscanned text box.
+    fn split_to_width(@self, _: Au, _: bool) -> SplitBoxResult;
+
+    /// Determines whether this box can merge with another box.
+    fn can_merge_with_box(&self, _: &RenderBox) -> bool {
+        false
+    }
+
+    /// Returns the *minimum width* and *preferred width* of this render box as defined by CSS 2.1.
+    fn minimum_and_preferred_widths(&self) -> (Au, Au);
+
+    fn box_height(&self) -> Au;
+
+    /// Assigns the appropriate width.
+    fn assign_width(&self);
+
+    fn debug_str(&self) -> ~str {
+        ~"???"
+    }
+}
+
+impl Clone for @RenderBox {
+    fn clone(&self) -> @RenderBox {
+        *self
+    }
 }
 
-impl RenderBox {
-    pub fn teardown(&self) {
-        match *self {
-            TextRenderBoxClass(box) => box.teardown(),
-            _ => ()
+// FIXME(pcwalton): These are botches and can be removed once Rust gets trait fields.
+
+pub trait RenderBoxUtils {
+    fn base<'a>(&'a self) -> &'a RenderBoxBase;
+
+    /// Returns true if this element is replaced content. This is true for images, form elements,
+    /// and so on.
+    fn is_replaced(&self) -> bool;
+    
+    /// Returns true if this element can be split. This is true for text boxes.
+    fn can_split(&self) -> bool;
+    
+    /// Returns the amount of left and right "fringe" used by this box. This is based on margins,
+    /// borders, padding, and width.
+    fn get_used_width(&self) -> (Au, Au);
+    
+    /// Returns the amount of left and right "fringe" used by this box. This should be based on
+    /// margins, borders, padding, and width.
+    fn get_used_height(&self) -> (Au, Au);
+
+    /// Adds the display items necessary to paint the background of this render box to the display
+    /// list if necessary.
+    fn paint_background_if_applicable<E:ExtraDisplayListData>(
+                                      &self,
+                                      list: &Cell<DisplayList<E>>,
+                                      absolute_bounds: &Rect<Au>);
+
+    /// Adds the display items necessary to paint the borders of this render box to a display list
+    /// if necessary.
+    fn paint_borders_if_applicable<E:ExtraDisplayListData>(
+                                   &self,
+                                   list: &Cell<DisplayList<E>>,
+                                   abs_bounds: &Rect<Au>);
+
+    /// Adds the display items for this render box to the given display list.
+    ///
+    /// Arguments:
+    /// * `builder`: The display list builder, which manages the coordinate system and options.
+    /// * `dirty`: The dirty rectangle in the coordinate system of the owning flow.
+    /// * `origin`: The total offset from the display list root flow to the owning flow of this
+    ///   box.
+    /// * `list`: The display list to which items should be appended.
+    ///
+    /// TODO: To implement stacking contexts correctly, we need to create a set of display lists,
+    /// one per layer of the stacking context (CSS 2.1 § 9.9.1). Each box is passed the list set
+    /// representing the box's stacking context. When asked to construct its constituent display
+    /// items, each box puts its display items into the correct stack layer according to CSS 2.1
+    /// Appendix E. Finally, the builder flattens the list.
+    fn build_display_list<E:ExtraDisplayListData>(
+                          &self,
+                          _: &DisplayListBuilder,
+                          dirty: &Rect<Au>,
+                          offset: &Point2D<Au>,
+                          list: &Cell<DisplayList<E>>);
+}
+
+pub trait RenderBoxRefUtils<'self> {
+    fn base(self) -> &'self RenderBoxBase;
+}
+
+/// A box that represents a generic render box.
+pub struct GenericRenderBox {
+    base: RenderBoxBase,
+}
+
+impl GenericRenderBox {
+    pub fn new(base: RenderBoxBase) -> GenericRenderBox {
+        GenericRenderBox {
+            base: base,
         }
     }
 }
 
+impl RenderBox for GenericRenderBox {
+    fn class(&self) -> RenderBoxClass {
+        GenericRenderBoxClass
+    }
+
+    fn minimum_and_preferred_widths(&self) -> (Au, Au) {
+        let guessed_width = self.base.guess_width();
+        (guessed_width, guessed_width)
+    }
+
+    fn split_to_width(@self, _: Au, _: bool) -> SplitBoxResult {
+        CannotSplit(self as @RenderBox)
+    }
+
+    fn box_height(&self) -> Au {
+        Au::new(0)
+    }
+
+    fn assign_width(&self) {
+        // FIXME(pcwalton): This seems clownshoes; can we remove?
+        self.base.position.mutate().ptr.size.width = Au::from_px(45)
+    }
+}
+
 /// A box that represents a (replaced content) image and its accompanying borders, shadows, etc.
 pub struct ImageRenderBox {
     base: RenderBoxBase,
-    image: ImageHolder,
+    image: Slot<ImageHolder>,
 }
 
 impl ImageRenderBox {
@@ -91,13 +231,104 @@ impl ImageRenderBox {
 
         ImageRenderBox {
             base: base,
-            image: ImageHolder::new(image_url, local_image_cache),
+            image: Slot::init(ImageHolder::new(image_url, local_image_cache)),
         }
     }
+
+    // Calculate the width of an image, accounting for the width attribute
+    // TODO: This could probably go somewhere else
+    pub fn image_width(&self) -> Au {
+        let attr_width: Option<int> = do self.base.node.with_imm_element |elt| {
+            match elt.get_attr("width") {
+                Some(width) => {
+                    FromStr::from_str(width)
+                }
+                None => {
+                    None
+                }
+            }
+        };
+
+        // TODO: Consult margins and borders?
+        let px_width = if attr_width.is_some() {
+            attr_width.unwrap()
+        } else {
+            self.image.mutate().ptr.get_size().unwrap_or(Size2D(0, 0)).width
+        };
+
+        Au::from_px(px_width)
+    }
+
+    // Calculate the height of an image, accounting for the height attribute
+    // TODO: This could probably go somewhere else
+    pub fn image_height(&self) -> Au {
+        let attr_height: Option<int> = do self.base.node.with_imm_element |elt| {
+            match elt.get_attr("height") {
+                Some(height) => {
+                    FromStr::from_str(height)
+                }
+                None => {
+                    None
+                }
+            }
+        };
+
+        // TODO: Consult margins and borders?
+        let px_height = if attr_height.is_some() {
+            attr_height.unwrap()
+        } else {
+            self.image.mutate().ptr.get_size().unwrap_or(Size2D(0, 0)).height
+        };
+
+        Au::from_px(px_height)
+    }
+
+    /// If this is an image render box, returns the underlying object. Fails otherwise.
+    ///
+    /// FIXME(pcwalton): Ugly. Replace with a real downcast operation.
+    fn as_image_render_box(@self) -> @ImageRenderBox {
+        self
+    }
+}
+
+impl RenderBox for ImageRenderBox {
+    fn class(&self) -> RenderBoxClass {
+        ImageRenderBoxClass
+    }
+
+    fn split_to_width(@self, _: Au, _: bool) -> SplitBoxResult {
+        CannotSplit(self as @RenderBox)
+    }
+
+    fn minimum_and_preferred_widths(&self) -> (Au, Au) {
+        let guessed_width = self.base.guess_width();
+        let image_width = self.image_width();
+        (guessed_width + image_width, guessed_width + image_width)
+    }
+
+    fn box_height(&self) -> Au {
+        let size = self.image.mutate().ptr.get_size();
+        let height = Au::from_px(size.unwrap_or(Size2D(0, 0)).height);
+        self.base.position.mutate().ptr.size.height = height;
+        debug!("box_height: found image height: %?", height);
+        height
+    }
+
+    fn assign_width(&self) {
+        let width = self.image_width();
+        self.base.position.mutate().ptr.size.width = width;
+    }
+
+    /// If this is an image render box, returns the underlying object. Fails otherwise.
+    ///
+    /// FIXME(pcwalton): Ugly. Replace with a real downcast operation.
+    fn as_image_render_box(@self) -> @ImageRenderBox {
+        self
+    }
 }
 
 /// A box representing a single run of text with a distinct style. A `TextRenderBox` may be split
-/// into two or more render boxes across line breaks. Several `TextBox`es may correspond to a
+/// into two or more boxes across line breaks. Several `TextBox`es may correspond to a
 /// single DOM text node. Split text boxes are implemented by referring to subsets of a master
 /// `TextRun` object.
 pub struct TextRenderBox {
@@ -107,9 +338,168 @@ pub struct TextRenderBox {
 }
 
 impl TextRenderBox {
+    /// Creates a TextRenderBox from a base render box, a range, and a text run. The size of the
+    /// the base render box is ignored and becomes the size of the text run.
+    ///
+    /// FIXME(pcwalton): This API is confusing.
+    pub fn new(base: RenderBoxBase, run: @TextRun, range: Range) -> TextRenderBox {
+        debug!("Creating textbox with span: (strlen=%u, off=%u, len=%u) of textrun (%s) (len=%u)",
+               run.char_len(),
+               range.begin(),
+               range.length(),
+               *run.text.get(),
+               run.char_len());
+
+        assert!(range.begin() < run.char_len());
+        assert!(range.end() <= run.char_len());
+        assert!(range.length() > 0);
+
+        let metrics = run.metrics_for_range(&range);
+
+        // FIXME(pcwalton): This block is necessary due to Rust #6248. If we don't have it, then
+        // the "currently borrowed" flag will be moved before the destructor runs, causing a
+        // (harmless) undefined memory write and a (very harmful) sticking of `position` in the
+        // "mutably borrowed" state, which will cause failures later.
+        {
+            base.position.mutate().ptr.size = metrics.bounding_box.size;
+        }
+
+        TextRenderBox {
+            base: base,
+            run: run,
+            range: range,
+        }
+    }
+}
+
+impl RenderBox for TextRenderBox {
+    fn class(&self) -> RenderBoxClass {
+        TextRenderBoxClass
+    }
+
+    fn as_text_render_box(@self) -> @TextRenderBox {
+        self
+    }
+
     fn teardown(&self) {
         self.run.teardown();
     }
+
+    fn minimum_and_preferred_widths(&self) -> (Au, Au) {
+        let guessed_width = self.base.guess_width();
+        let min_width = self.run.min_width_for_range(&self.range);
+
+        let mut max_line_width = Au::new(0);
+        for line_range in self.run.iter_natural_lines_for_range(&self.range) {
+            let line_metrics = self.run.metrics_for_range(&line_range);
+            max_line_width = Au::max(max_line_width, line_metrics.advance_width);
+        }
+
+        (guessed_width + min_width, guessed_width + max_line_width)
+    }
+
+    fn box_height(&self) -> Au {
+        let range = &self.range;
+        let run = &self.run;
+
+        // Compute the height based on the line-height and font size
+        let text_bounds = run.metrics_for_range(range).bounding_box;
+        let em_size = text_bounds.size.height;
+        let line_height = self.base.calculate_line_height(em_size);
+
+        line_height
+    }
+
+    fn assign_width(&self) {
+        // Text boxes are preinitialized.
+    }
+
+    /// Attempts to split this box so that its width is no more than `max_width`. Fails if this box
+    /// is an unscanned text box.
+    fn split_to_width(@self, max_width: Au, starts_line: bool) -> SplitBoxResult {
+        let mut pieces_processed_count: uint = 0;
+        let mut remaining_width: Au = max_width;
+        let mut left_range = Range::new(self.range.begin(), 0);
+        let mut right_range: Option<Range> = None;
+
+        debug!("split_to_width: splitting text box (strlen=%u, range=%?, avail_width=%?)",
+               self.run.text.get().len(),
+               self.range,
+               max_width);
+
+        for (glyphs, offset, slice_range) in self.run.iter_slices_for_range(&self.range) {
+            debug!("split_to_width: considering slice (offset=%?, range=%?, remain_width=%?)",
+                   offset,
+                   slice_range,
+                   remaining_width);
+
+            let metrics = self.run.metrics_for_slice(glyphs, &slice_range);
+            let advance = metrics.advance_width;
+            let should_continue: bool;
+
+            if advance <= remaining_width {
+                should_continue = true;
+
+                if starts_line && pieces_processed_count == 0 && glyphs.is_whitespace() {
+                    debug!("split_to_width: case=skipping leading trimmable whitespace");
+                    left_range.shift_by(slice_range.length() as int);
+                } else {
+                    debug!("split_to_width: case=enlarging span");
+                    remaining_width = remaining_width - advance;
+                    left_range.extend_by(slice_range.length() as int);
+                }
+            } else {    // The advance is more than the remaining width.
+                should_continue = false;
+                let slice_begin = offset + slice_range.begin();
+                let slice_end = offset + slice_range.end();
+
+                if glyphs.is_whitespace() {
+                    // If there are still things after the trimmable whitespace, create the
+                    // right chunk.
+                    if slice_end < self.range.end() {
+                        debug!("split_to_width: case=skipping trimmable trailing \
+                                whitespace, then split remainder");
+                        let right_range_end = self.range.end() - slice_end;
+                        right_range = Some(Range::new(slice_end, right_range_end));
+                    } else {
+                        debug!("split_to_width: case=skipping trimmable trailing \
+                                whitespace");
+                    }
+                } else if slice_begin < self.range.end() {
+                    // There are still some things left over at the end of the line. Create
+                    // the right chunk.
+                    let right_range_end = self.range.end() - slice_begin;
+                    right_range = Some(Range::new(slice_begin, right_range_end));
+                    debug!("split_to_width: case=splitting remainder with right range=%?",
+                           right_range);
+                }
+            }
+
+            pieces_processed_count += 1;
+
+            if !should_continue {
+                break
+            }
+        }
+
+        let left_box = if left_range.length() > 0 {
+            let new_text_box = @TextRenderBox::new(self.base.clone(), self.run, left_range);
+            Some(new_text_box as @RenderBox)
+        } else {
+            None
+        };
+
+        let right_box = do right_range.map_default(None) |range: Range| {
+            let new_text_box = @TextRenderBox::new(self.base.clone(), self.run, range);
+            Some(new_text_box as @RenderBox)
+        };
+
+        if pieces_processed_count == 1 || left_box.is_none() {
+            SplitDidNotFit(left_box, right_box)
+        } else {
+            SplitDidFit(left_box, right_box)
+        }
+    }
 }
 
 /// The data for an unscanned text box.
@@ -133,40 +523,94 @@ impl UnscannedTextRenderBox {
             // FIXME(pcwalton): If we're just looking at node data, do we have to ensure this is
             // a text node?
             UnscannedTextRenderBox {
-                base: base,
+                base: base.clone(),
                 text: text_node.element.data.to_str(),
                 font_style: None,
                 text_decoration: None,
             }
         }
     }
+
+    /// Copies out the text from an unscanned text box.
+    pub fn raw_text(&self) -> ~str {
+        self.text.clone()
+    }
 }
 
-pub enum RenderBoxType {
-    RenderBox_Generic,
-    RenderBox_Image,
-    RenderBox_Text,
+impl RenderBox for UnscannedTextRenderBox {
+    fn class(&self) -> RenderBoxClass {
+        UnscannedTextRenderBoxClass
+    }
+
+    fn is_whitespace_only(&self) -> bool {
+        self.text.is_whitespace()
+    }
+
+    fn can_merge_with_box(&self, other: &RenderBox) -> bool {
+        if other.class() == UnscannedTextRenderBoxClass {
+            let this_base = &self.base;
+            let other_base = other.base();
+            return this_base.font_style() == other_base.font_style() &&
+                this_base.text_decoration() == other_base.text_decoration()
+        }
+        false
+    }
+
+    fn box_height(&self) -> Au {
+        fail!("can't get height of unscanned text box")
+    }
+
+    /// Attempts to split this box so that its width is no more than `max_width`. Fails if this box
+    /// is an unscanned text box.
+    fn split_to_width(@self, _: Au, _: bool) -> SplitBoxResult {
+        fail!("WAT: shouldn't be an unscanned text box here.")
+    }
+
+    /// Returns the *minimum width* and *preferred width* of this render box as defined by CSS 2.1.
+    fn minimum_and_preferred_widths(&self) -> (Au, Au) {
+        fail!("WAT: shouldn't be an unscanned text box here.")
+    }
+
+    fn assign_width(&self) {
+        fail!("WAT: shouldn't be an unscanned text box here.")
+    }
+
+    /// If this is an unscanned text render box, returns the underlying object. Fails otherwise.
+    ///
+    /// FIXME(pcwalton): Ugly. Replace with a real downcast operation.
+    fn as_unscanned_text_render_box(@self) -> @UnscannedTextRenderBox {
+        self
+    }
 }
 
-/// Represents the outcome of attempting to split a render box.
+#[deriving(Eq)]
+pub enum RenderBoxClass {
+    GenericRenderBoxClass,
+    ImageRenderBoxClass,
+    TextRenderBoxClass,
+    UnscannedTextRenderBoxClass,
+}
+
+/// Represents the outcome of attempting to split a box.
 pub enum SplitBoxResult {
-    CannotSplit(RenderBox),
+    CannotSplit(@RenderBox),
     // in general, when splitting the left or right side can
     // be zero length, due to leading/trailing trimmable whitespace
-    SplitDidFit(Option<RenderBox>, Option<RenderBox>),
-    SplitDidNotFit(Option<RenderBox>, Option<RenderBox>)
+    SplitDidFit(Option<@RenderBox>, Option<@RenderBox>),
+    SplitDidNotFit(Option<@RenderBox>, Option<@RenderBox>)
 }
 
-/// Data common to all render boxes.
+/// Data common to all boxes.
+#[deriving(Clone)]
 pub struct RenderBoxBase {
     /// The DOM node that this `RenderBox` originates from.
     node: AbstractNode<LayoutView>,
 
     /// The position of this box relative to its owning flow.
-    position: Rect<Au>,
+    position: Slot<Rect<Au>>,
 
     /// The core parameters (border, padding, margin) used by the box model.
-    model: BoxModel,
+    model: Slot<BoxModel>,
 
     /// A debug ID.
     ///
@@ -180,442 +624,307 @@ impl RenderBoxBase {
                -> RenderBoxBase {
         RenderBoxBase {
             node: node,
-            position: Au::zero_rect(),
-            model: Zero::zero(),
+            position: Slot::init(Au::zero_rect()),
+            model: Slot::init(Zero::zero()),
             id: id,
         }
     }
-}
 
-impl RenderBox {
-    /// Borrows this render box immutably in order to work with its common data.
-    #[inline(always)]
-    pub fn with_base<R>(&self, callback: &fn(&RenderBoxBase) -> R) -> R {
-        match *self {
-            GenericRenderBoxClass(generic_box) => callback(generic_box),
-            ImageRenderBoxClass(image_box) => {
-                callback(&image_box.base)
-            }
-            TextRenderBoxClass(text_box) => {
-                callback(&text_box.base)
-            }
-            UnscannedTextRenderBoxClass(unscanned_text_box) => {
-                callback(&unscanned_text_box.base)
-            }
-        }
+    pub fn id(&self) -> int {
+        0
     }
 
-    /// Borrows this render box mutably in order to work with its common data.
-    #[inline(always)]
-    pub fn with_mut_base<R>(&self, callback: &fn(&mut RenderBoxBase) -> R) -> R {
-        match *self {
-            GenericRenderBoxClass(generic_box) => callback(generic_box),
-            ImageRenderBoxClass(image_box) => {
-                callback(&mut image_box.base)
-            }
-            TextRenderBoxClass(text_box) => {
-                callback(&mut text_box.base)
-            }
-            UnscannedTextRenderBoxClass(unscanned_text_box) => {
-                callback(&mut unscanned_text_box.base)
-            }
+    fn guess_width(&self) -> Au {
+        if !self.node.is_element() {
+            return Au(0)
         }
+
+        let style = self.style();
+        let width = MaybeAuto::from_style(style.Box.width, Au::new(0)).specified_or_zero();
+        let margin_left = MaybeAuto::from_style(style.Margin.margin_left,
+                                                Au::new(0)).specified_or_zero();
+        let margin_right = MaybeAuto::from_style(style.Margin.margin_right,
+                                                 Au::new(0)).specified_or_zero();
+
+        let mut model_ref = self.model.mutate();
+        let model = &mut model_ref.ptr;
+        let padding_left = model.compute_padding_length(style.Padding.padding_left, Au::new(0));
+        let padding_right = model.compute_padding_length(style.Padding.padding_right, Au::new(0));
+
+        width + margin_left + margin_right + padding_left + padding_right + model.border.left +
+            model.border.right
     }
 
-    /// A convenience function to return the position of this box.
-    pub fn position(&self) -> Rect<Au> {
-        do self.with_base |base| {
-            base.position
+    pub fn calculate_line_height(&self, font_size: Au) -> Au { 
+        match self.line_height() {
+            line_height::Normal => font_size.scale_by(1.14),
+            line_height::Number(l) => font_size.scale_by(l),
+            line_height::Length(l) => l
         }
     }
 
-    /// A convenience function to return the debugging ID of this box.
-    pub fn id(&self) -> int {
-        do self.with_mut_base |base| {
-            base.id
-        }
+    pub fn compute_padding(&self, containing_block_width: Au) {
+        self.model.mutate().ptr.compute_padding(self.node.style(), containing_block_width);
     }
 
-    /// Returns true if this element is replaced content. This is true for images, form elements,
-    /// and so on.
-    pub fn is_replaced(&self) -> bool {
-        match *self {
-            ImageRenderBoxClass(*) => true,
-            _ => false
-        }
+    pub fn get_noncontent_width(&self) -> Au {
+        let model_ref = self.model.mutate();
+        model_ref.ptr.border.left + model_ref.ptr.padding.left + model_ref.ptr.border.right +
+            model_ref.ptr.padding.right
     }
 
-    /// Returns true if this element can be split. This is true for text boxes.
-    pub fn can_split(&self) -> bool {
-        match *self {
-            TextRenderBoxClass(*) => true,
-            _ => false
-        }
+    /// The box formed by the content edge as defined in CSS 2.1 § 8.1. Coordinates are relative to
+    /// the owning flow.
+    pub fn content_box(&self) -> Rect<Au> {
+        let (position, model) = (self.position.get(), self.model.get());
+        let origin = Point2D(position.origin.x + model.border.left + model.padding.left,
+                             position.origin.y);
+        let size = Size2D(position.size.width - self.get_noncontent_width(), position.size.height);
+        Rect(origin, size)
     }
 
-    /// Returns true if this element is an unscanned text box that consists entirely of whitespace.
-    pub fn is_whitespace_only(&self) -> bool {
-        match *self {
-            UnscannedTextRenderBoxClass(unscanned_text_box) => {
-                unscanned_text_box.text.is_whitespace()
-            }
-            _ => false
-        }
+    /// The box formed by the border edge as defined in CSS 2.1 § 8.1. Coordinates are relative to
+    /// the owning flow.
+    pub fn border_box(&self) -> Rect<Au> {
+        // TODO: Actually compute the content box, padding, and border.
+        self.content_box()
     }
 
-    /// Determines whether this box can merge with another render box.
-    pub fn can_merge_with_box(&self, other: RenderBox) -> bool {
-        match (self, &other) {
-            (&UnscannedTextRenderBoxClass(*), &UnscannedTextRenderBoxClass(*)) => {
-                self.font_style() == other.font_style() && self.text_decoration() == other.text_decoration()
-            },
-            (&TextRenderBoxClass(text_box_a), &TextRenderBoxClass(text_box_b)) => {
-                managed::ptr_eq(text_box_a.run, text_box_b.run)
+    /// The box formed by the margin edge as defined in CSS 2.1 § 8.1. Coordinates are relative to
+    /// the owning flow.
+    pub fn margin_box(&self) -> Rect<Au> {
+        // TODO: Actually compute the content_box, padding, border, and margin.
+        self.content_box()
+    }
+
+    /// Returns the nearest ancestor-or-self `Element` to the DOM node that this render box
+    /// represents.
+    ///
+    /// If there is no ancestor-or-self `Element` node, fails.
+    pub fn nearest_ancestor_element(&self) -> AbstractNode<LayoutView> {
+        let mut node = self.node;
+        while !node.is_element() {
+            match node.parent_node() {
+                None => fail!("no nearest element?!"),
+                Some(parent) => node = parent,
             }
-            (_, _) => false,
         }
+        node
     }
 
-    /// Attempts to split this box so that its width is no more than `max_width`. Fails if this box
-    /// is an unscanned text box.
-    pub fn split_to_width(&self, max_width: Au, starts_line: bool)
-                      -> SplitBoxResult {
-        match *self {
-            GenericRenderBoxClass(*) | ImageRenderBoxClass(*) => CannotSplit(*self),
-            UnscannedTextRenderBoxClass(*) => {
-                fail!(~"WAT: shouldn't be an unscanned text box here.")
-            }
+    // Always inline for SCCP.
+    #[inline(always)]
+    pub fn clear(&self) -> Option<ClearType> {
+        let style = self.node.style();
+        match style.Box.clear {
+            clear::none => None,
+            clear::left => Some(ClearLeft),
+            clear::right => Some(ClearRight),
+            clear::both => Some(ClearBoth),
+        }
+    }
 
-            TextRenderBoxClass(text_box) => {
-                let mut pieces_processed_count: uint = 0;
-                let mut remaining_width: Au = max_width;
-                let mut left_range = Range::new(text_box.range.begin(), 0);
-                let mut right_range: Option<Range> = None;
-
-                debug!("split_to_width: splitting text box (strlen=%u, range=%?, avail_width=%?)",
-                       text_box.run.text.len(),
-                       text_box.range,
-                       max_width);
-
-                for (glyphs, offset, slice_range) in text_box.run.iter_slices_for_range(&text_box.range) {
-                    debug!("split_to_width: considering slice (offset=%?, range=%?, remain_width=%?)",
-                           offset,
-                           slice_range,
-                           remaining_width);
-
-                    let metrics = text_box.run.metrics_for_slice(glyphs, &slice_range);
-                    let advance = metrics.advance_width;
-                    let should_continue: bool;
-
-                    if advance <= remaining_width {
-                        should_continue = true;
-
-                        if starts_line && pieces_processed_count == 0 && glyphs.is_whitespace() {
-                            debug!("split_to_width: case=skipping leading trimmable whitespace");
-                            left_range.shift_by(slice_range.length() as int);
-                        } else {
-                            debug!("split_to_width: case=enlarging span");
-                            remaining_width = remaining_width - advance;
-                            left_range.extend_by(slice_range.length() as int);
-                        }
-                    } else {    // The advance is more than the remaining width.
-                        should_continue = false;
-                        let slice_begin = offset + slice_range.begin();
-                        let slice_end = offset + slice_range.end();
-
-                        if glyphs.is_whitespace() {
-                            // If there are still things after the trimmable whitespace, create the
-                            // right chunk.
-                            if slice_end < text_box.range.end() {
-                                debug!("split_to_width: case=skipping trimmable trailing \
-                                        whitespace, then split remainder");
-                                let right_range_end =
-                                    text_box.range.end() - slice_end;
-                                right_range = Some(Range::new(slice_end, right_range_end));
-                            } else {
-                                debug!("split_to_width: case=skipping trimmable trailing \
-                                        whitespace");
-                            }
-                        } else if slice_begin < text_box.range.end() {
-                            // There are still some things left over at the end of the line. Create
-                            // the right chunk.
-                            let right_range_end =
-                                text_box.range.end() - slice_begin;
-                            right_range = Some(Range::new(slice_begin, right_range_end));
-                            debug!("split_to_width: case=splitting remainder with right range=%?",
-                                   right_range);
-                        }
-                    }
+    /// Converts this node's computed style to a font style used for rendering.
+    pub fn font_style(&self) -> FontStyle {
+        let my_style = self.nearest_ancestor_element().style();
 
-                    pieces_processed_count += 1;
+        debug!("(font style) start: %?", self.nearest_ancestor_element().type_id());
 
-                    if !should_continue {
-                        break
-                    }
-                }
+        // FIXME: Too much allocation here.
+        let font_families = do my_style.Font.font_family.map |family| {
+            match *family {
+                FamilyName(ref name) => (*name).clone(),
+            }
+        };
+        let font_families = font_families.connect(", ");
+        debug!("(font style) font families: `%s`", font_families);
 
-                let left_box = if left_range.length() > 0 {
-                    let new_text_box = @mut text::adapt_textbox_with_range(text_box.base,
-                                                                           text_box.run,
-                                                                           left_range);
-                    Some(TextRenderBoxClass(new_text_box))
-                } else {
-                    None
-                };
+        let font_size = my_style.Font.font_size.to_f64().unwrap() / 60.0;
+        debug!("(font style) font size: `%fpx`", font_size);
 
-                let right_box = do right_range.map_default(None) |range: Range| {
-                    let new_text_box = @mut text::adapt_textbox_with_range(text_box.base,
-                                                                           text_box.run,
-                                                                           range);
-                    Some(TextRenderBoxClass(new_text_box))
-                };
+        let (italic, oblique) = match my_style.Font.font_style {
+            font_style::normal => (false, false),
+            font_style::italic => (true, false),
+            font_style::oblique => (false, true),
+        };
 
-                if pieces_processed_count == 1 || left_box.is_none() {
-                    SplitDidNotFit(left_box, right_box)
-                } else {
-                    SplitDidFit(left_box, right_box)
-                }
-            }
+        FontStyle {
+            pt_size: font_size,
+            weight: FontWeight300,
+            italic: italic,
+            oblique: oblique,
+            families: font_families,
         }
     }
 
-    /// Guess the intrinsic width of this box for
-    /// computation of min and preferred widths.
-    //
-    // TODO(eatkinson): this is unspecified in
-    // CSS 2.1, but we need to do something reasonable
-    // here. What this function does currently is
-    // NOT reasonable.
-    //
-    // TODO(eatkinson): integrate with
-    // get_min_width and get_pref_width?
-    fn guess_width (&self) -> Au {
-        do self.with_base |base| {
-            if(!base.node.is_element()) {
-                Au(0)
-            } else {
-                let style = self.style();
-                let width = MaybeAuto::from_style(style.Box.width,
-                                                  Au(0)).specified_or_zero();
-                let margin_left = MaybeAuto::from_style(style.Margin.margin_left,
-                                                        Au(0)).specified_or_zero();
-                let margin_right = MaybeAuto::from_style(style.Margin.margin_right,
-                                                         Au(0)).specified_or_zero();
-                let padding_left = base.model.compute_padding_length(style.Padding.padding_left,
-                                                                     Au(0));
-                let padding_right = base.model.compute_padding_length(style.Padding.padding_right,
-                                                                      Au(0));
-                let border_left = style.Border.border_left_width;
-                let border_right = style.Border.border_right_width;
-
-                width + margin_left + margin_right + padding_left + padding_right +
-                    border_left + border_right
-            }
-        }
+    // FIXME(pcwalton): Why &'static??? Isn't that wildly unsafe?
+    #[inline(always)]
+    pub fn style(&self) -> &'static ComputedValues {
+        self.node.style()
     }
 
-    /// Returns the *minimum width* of this render box as defined by the CSS specification.
-    pub fn get_min_width(&self, _: &LayoutContext) -> Au {
-        // FIXME(pcwalton): I think we only need to calculate this if the damage says that CSS
-        // needs to be restyled.
+    /// Returns the text alignment of the computed style of the nearest ancestor-or-self `Element`
+    /// node.
+    pub fn text_align(&self) -> text_align::T {
+        self.nearest_ancestor_element().style().Text.text_align
+    }
 
-        self.guess_width() + match *self {
-            // TODO: This should account for the minimum width of the box element in isolation.
-            // That includes borders, margins, and padding, but not child widths. The block
-            // `FlowContext` will combine the width of this element and that of its children to
-            // arrive at the context width.
-            GenericRenderBoxClass(*) => Au(0),
+    pub fn line_height(&self) -> line_height::T {
+        self.nearest_ancestor_element().style().Box.line_height
+    }
 
-            ImageRenderBoxClass(image_box) => {
-                self.image_width(image_box)
-            }
+    pub fn vertical_align(&self) -> vertical_align::T {
+        self.nearest_ancestor_element().style().Box.vertical_align
+    }
 
-            TextRenderBoxClass(text_box) => {
-                text_box.run.min_width_for_range(&text_box.range)
+    /// Returns the text decoration of the computed style of the nearest `Element` node
+    pub fn text_decoration(&self) -> text_decoration::T {
+        /// Computes the propagated value of text-decoration, as specified in CSS 2.1 § 16.3.1
+        /// TODO: make sure this works with anonymous box generation.
+        fn get_propagated_text_decoration(element: AbstractNode<LayoutView>)
+                                          -> text_decoration::T {
+            //Skip over non-element nodes in the DOM
+            if !element.is_element() {
+                return match element.parent_node() {
+                    None => text_decoration::none,
+                    Some(parent) => get_propagated_text_decoration(parent),
+                };
             }
 
-            UnscannedTextRenderBoxClass(*) => fail!(~"Shouldn't see unscanned boxes here.")
-        }
-    }
+            // FIXME: Implement correctly.
+            let display_in_flow = true;
 
-    /// Returns the *preferred width* of this render box as defined by the CSS specification.
-    pub fn get_pref_width(&self, _: &LayoutContext) -> Au {
-        self.guess_width() + match *self {
-            // TODO: This should account for the preferred width of the box element in isolation.
-            // That includes borders, margins, and padding, but not child widths. The block
-            // `FlowContext` will combine the width of this element and that of its children to
-            // arrive at the context width.
-            GenericRenderBoxClass(*) => Au(0),
+            let position = element.style().Box.position;
+            let float = element.style().Box.float;
 
-            ImageRenderBoxClass(image_box) => {
-                self.image_width(image_box)
-            }
+            let in_flow = (position == position::static_) && (float == float::none) &&
+                display_in_flow;
 
-            TextRenderBoxClass(text_box) => {
-                // A text box cannot span lines, so assume that this is an unsplit text box.
-                //
-                // TODO: If text boxes have been split to wrap lines, then they could report a
-                // smaller preferred width during incremental reflow. Maybe text boxes should
-                // report nothing and the parent flow can factor in minimum/preferred widths of any
-                // text runs that it owns.
-                let mut max_line_width = Au(0);
-                for line_range in text_box.run.iter_natural_lines_for_range(&text_box.range) {
-                    let line_metrics = text_box.run.metrics_for_range(&line_range);
-                    max_line_width = Au::max(max_line_width, line_metrics.advance_width);
-                }
+            let text_decoration = element.style().Text.text_decoration;
 
-                max_line_width
+            if text_decoration == text_decoration::none && in_flow {
+                match element.parent_node() {
+                    None => text_decoration::none,
+                    Some(parent) => get_propagated_text_decoration(parent),
+                }
+            }
+            else {
+                text_decoration
             }
-
-            UnscannedTextRenderBoxClass(*) => fail!(~"Shouldn't see unscanned boxes here."),
         }
+        get_propagated_text_decoration(self.nearest_ancestor_element())
     }
 
-    // Calculate the width of an image, accounting for the width attribute
-    // TODO: This could probably go somewhere else
-    pub fn image_width(&self, image_box: @mut ImageRenderBox) -> Au {
-        let attr_width: Option<int> = do self.with_base |base| {
-            do base.node.with_imm_element |elt| {
-                match elt.get_attr("width") {
-                    Some(width) => {
-                        FromStr::from_str(width)
-                    }
-                    None => {
-                        None
-                    }
-                }
-            }
-        };
-
-        // TODO: Consult margins and borders?
-        let px_width = if attr_width.is_some() {
-            attr_width.unwrap()
-        } else {
-            image_box.image.get_size().unwrap_or(Size2D(0, 0)).width
-        };
+}
 
-        Au::from_px(px_width)
+impl RenderBoxUtils for @RenderBox {
+    #[inline(always)]
+    fn base<'a>(&'a self) -> &'a RenderBoxBase {
+        unsafe {
+            let (_, box_ptr): (uint, *Box<RenderBoxBase>) = cast::transmute(*self);
+            cast::transmute(&(*box_ptr).data)
+        }
     }
 
-    // Calculate the height of an image, accounting for the height attribute
-    // TODO: This could probably go somewhere else
-    pub fn image_height(&self, image_box: @mut ImageRenderBox) -> Au {
-        let attr_height: Option<int> = do self.with_base |base| {
-            do base.node.with_imm_element |elt| {
-                match elt.get_attr("height") {
-                    Some(height) => {
-                        FromStr::from_str(height)
-                    }
-                    None => {
-                        None
-                    }
-                }
-            }
-        };
-
-        // TODO: Consult margins and borders?
-        let px_height = if attr_height.is_some() {
-            attr_height.unwrap()
-        } else {
-            image_box.image.get_size().unwrap_or(Size2D(0, 0)).height
-        };
+    fn is_replaced(&self) -> bool {
+        self.class() == ImageRenderBoxClass
+    }
 
-        Au::from_px(px_height)
+    fn can_split(&self) -> bool {
+        self.class() == TextRenderBoxClass
     }
 
     /// Returns the amount of left and right "fringe" used by this box. This is based on margins,
     /// borders, padding, and width.
-    pub fn get_used_width(&self) -> (Au, Au) {
+    fn get_used_width(&self) -> (Au, Au) {
         // TODO: This should actually do some computation! See CSS 2.1, Sections 10.3 and 10.4.
-        (Au(0), Au(0))
+        (Au::new(0), Au::new(0))
     }
 
     /// Returns the amount of left and right "fringe" used by this box. This should be based on
     /// margins, borders, padding, and width.
-    pub fn get_used_height(&self) -> (Au, Au) {
+    fn get_used_height(&self) -> (Au, Au) {
         // TODO: This should actually do some computation! See CSS 2.1, Sections 10.5 and 10.6.
-        (Au(0), Au(0))
+        (Au::new(0), Au::new(0))
     }
 
-    pub fn compute_padding(&self, cb_width: Au) {
-        do self.with_mut_base |base| {
-            base.model.compute_padding(base.node.style(), cb_width);
-        }
-    }
+    /// Adds the display items necessary to paint the background of this render box to the display
+    /// list if necessary.
+    fn paint_background_if_applicable<E:ExtraDisplayListData>(
+                                      &self,
+                                      list: &Cell<DisplayList<E>>,
+                                      absolute_bounds: &Rect<Au>) {
+        // FIXME: This causes a lot of background colors to be displayed when they are clearly not
+        // needed. We could use display list optimization to clean this up, but it still seems
+        // inefficient. What we really want is something like "nearest ancestor element that
+        // doesn't have a render box".
+        let nearest_ancestor_element = self.base().nearest_ancestor_element();
 
-    pub fn get_noncontent_width(&self) -> Au {
-        do self.with_base |base| {
-            base.model.border.left + base.model.padding.left +
-            base.model.border.right + base.model.padding.right
-        }
-    }
+        let style = nearest_ancestor_element.style();
+        let background_color = style.resolve_color(style.Background.background_color);
+        if !background_color.alpha.approx_eq(&0.0) {
+            do list.with_mut_ref |list| {
+                let solid_color_display_item = ~SolidColorDisplayItem {
+                    base: BaseDisplayItem {
+                        bounds: *absolute_bounds,
+                        extra: ExtraDisplayListData::new(self),
+                    },
+                    color: background_color.to_gfx_color(),
+                };
 
-    pub fn with_model<R>(&self, callback: &fn(&mut BoxModel) -> R) ->  R {
-        do self.with_mut_base |base| {
-            callback(&mut base.model)
+                list.append_item(SolidColorDisplayItemClass(solid_color_display_item))
+            }
         }
     }
 
-    /// The box formed by the content edge as defined in CSS 2.1 § 8.1. Coordinates are relative to
-    /// the owning flow.
-    pub fn content_box(&self) -> Rect<Au> {
-        do self.with_base |base| {
-            let origin = Point2D(base.position.origin.x +
-                                 base.model.border.left +
-                                 base.model.padding.left,
-                                 base.position.origin.y);
-            let size = Size2D(base.position.size.width - self.get_noncontent_width(),
-                              base.position.size.height);
-            Rect(origin, size)
+    /// Adds the display items necessary to paint the borders of this render box to a display list
+    /// if necessary.
+    fn paint_borders_if_applicable<E:ExtraDisplayListData>(
+                                   &self,
+                                   list: &Cell<DisplayList<E>>,
+                                   abs_bounds: &Rect<Au>) {
+        // Fast path.
+        let base = self.base();
+        let border = base.model.get().border;
+        if border.is_zero() {
+            return
         }
-    }
 
-    /// The box formed by the border edge as defined in CSS 2.1 § 8.1. Coordinates are relative to
-    /// the owning flow.
-    pub fn border_box(&self) -> Rect<Au> {
-        // TODO: Actually compute the content box, padding, and border.
-        self.content_box()
-    }
-
-    /// The box formed by the margin edge as defined in CSS 2.1 § 8.1. Coordinates are relative to
-    /// the owning flow.
-    pub fn margin_box(&self) -> Rect<Au> {
-        // TODO: Actually compute the content_box, padding, border, and margin.
-        self.content_box()
-    }
-
-    /// A convenience function to access the computed style of the DOM node that this render box
-    /// represents.
-    pub fn style(&self) -> &ComputedValues {
-        self.with_base(|base| base.node.style())
-    }
+        let style = base.style();
+        let top_color = style.resolve_color(style.Border.border_top_color);
+        let right_color = style.resolve_color(style.Border.border_right_color);
+        let bottom_color = style.resolve_color(style.Border.border_bottom_color);
+        let left_color = style.resolve_color(style.Border.border_left_color);
+        let top_style = style.Border.border_top_style;
+        let right_style = style.Border.border_right_style;
+        let bottom_style = style.Border.border_bottom_style;
+        let left_style = style.Border.border_left_style;
 
-    /// A convenience function to access the DOM node that this render box represents.
-    pub fn node(&self) -> AbstractNode<LayoutView> {
-        self.with_base(|base| base.node)
-    }
+        // Append the border to the display list.
+        do list.with_mut_ref |list| {
+            let border_display_item = ~BorderDisplayItem {
+                base: BaseDisplayItem {
+                    bounds: *abs_bounds,
+                    extra: ExtraDisplayListData::new(self),
+                },
+                border: SideOffsets2D::new(border.top,
+                                           border.right,
+                                           border.bottom,
+                                           border.left),
+                color: SideOffsets2D::new(top_color.to_gfx_color(),
+                                          right_color.to_gfx_color(),
+                                          bottom_color.to_gfx_color(),
+                                          left_color.to_gfx_color()),
+                style: SideOffsets2D::new(top_style,
+                                          right_style,
+                                          bottom_style,
+                                          left_style)
+            };
 
-    /// Returns the nearest ancestor-or-self `Element` to the DOM node that this render box
-    /// represents.
-    ///
-    /// If there is no ancestor-or-self `Element` node, fails.
-    pub fn nearest_ancestor_element(&self) -> AbstractNode<LayoutView> {
-        do self.with_base |base| {
-            let mut node = base.node;
-            while !node.is_element() {
-                match node.parent_node() {
-                    None => fail!(~"no nearest element?!"),
-                    Some(parent) => node = parent,
-                }
-            }
-            node
+            list.append_item(BorderDisplayItemClass(border_display_item))
         }
     }
 
-    //
-    // Painting
-    //
-
     /// Adds the display items for this render box to the given display list.
     ///
     /// Arguments:
@@ -630,12 +939,14 @@ impl RenderBox {
     /// representing the box's stacking context. When asked to construct its constituent display
     /// items, each box puts its display items into the correct stack layer according to CSS 2.1
     /// Appendix E. Finally, the builder flattens the list.
-    pub fn build_display_list<E:ExtraDisplayListData>(&self,
-                                                  _: &DisplayListBuilder,
-                                                  dirty: &Rect<Au>,
-                                                  offset: &Point2D<Au>,
-                                                  list: &Cell<DisplayList<E>>) {
-        let box_bounds = self.position();
+    fn build_display_list<E:ExtraDisplayListData>(
+                          &self,
+                          _: &DisplayListBuilder,
+                          dirty: &Rect<Au>,
+                          offset: &Point2D<Au>,
+                          list: &Cell<DisplayList<E>>) {
+        let base = self.base();
+        let box_bounds = base.position.get();
         let absolute_box_bounds = box_bounds.translate(offset);
         debug!("RenderBox::build_display_list at rel=%?, abs=%?: %s",
                box_bounds, absolute_box_bounds, self.debug_str());
@@ -648,14 +959,15 @@ impl RenderBox {
             return;
         }
 
-        match *self {
-            UnscannedTextRenderBoxClass(*) => fail!(~"Shouldn't see unscanned boxes here."),
-            TextRenderBoxClass(text_box) => {
+        match self.class() {
+            UnscannedTextRenderBoxClass => fail!("Shouldn't see unscanned boxes here."),
+            TextRenderBoxClass => {
+                let text_box = self.as_text_render_box();
 
                 // Add the background to the list, if applicable.
                 self.paint_background_if_applicable(list, &absolute_box_bounds);
 
-                let nearest_ancestor_element = self.nearest_ancestor_element();
+                let nearest_ancestor_element = base.nearest_ancestor_element();
                 let color = nearest_ancestor_element.style().Color.color.to_gfx_color();
 
                 // Create the text box.
@@ -663,7 +975,7 @@ impl RenderBox {
                     let text_display_item = ~TextDisplayItem {
                         base: BaseDisplayItem {
                             bounds: absolute_box_bounds,
-                            extra: ExtraDisplayListData::new(*self),
+                            extra: ExtraDisplayListData::new(self),
                         },
                         // FIXME(pcwalton): Allocation? Why?!
                         text_run: ~text_box.run.serialize(),
@@ -686,7 +998,7 @@ impl RenderBox {
                         let border_display_item = ~BorderDisplayItem {
                             base: BaseDisplayItem {
                                 bounds: absolute_box_bounds,
-                                extra: ExtraDisplayListData::new(*self),
+                                extra: ExtraDisplayListData::new(self),
                             },
                             border: debug_border,
                             color: SideOffsets2D::new_all_same(rgb(0, 0, 200)),
@@ -708,7 +1020,7 @@ impl RenderBox {
                         let border_display_item = ~BorderDisplayItem {
                             base: BaseDisplayItem {
                                 bounds: baseline,
-                                extra: ExtraDisplayListData::new(*self),
+                                extra: ExtraDisplayListData::new(self),
                             },
                             border: debug_border,
                             color: SideOffsets2D::new_all_same(rgb(0, 200, 0)),
@@ -721,8 +1033,7 @@ impl RenderBox {
                     ()
                 });
             },
-            GenericRenderBoxClass(_) => {
-
+            GenericRenderBoxClass => {
                 // Add the background to the list, if applicable.
                 self.paint_background_if_applicable(list, &absolute_box_bounds);
 
@@ -735,7 +1046,7 @@ impl RenderBox {
                         let border_display_item = ~BorderDisplayItem {
                             base: BaseDisplayItem {
                                 bounds: absolute_box_bounds,
-                                extra: ExtraDisplayListData::new(*self),
+                                extra: ExtraDisplayListData::new(self),
                             },
                             border: debug_border,
                             color: SideOffsets2D::new_all_same(rgb(0, 0, 200)),
@@ -747,14 +1058,14 @@ impl RenderBox {
 
                     ()
                 });
-
             },
-            ImageRenderBoxClass(image_box) => {
+            ImageRenderBoxClass => {
+                let image_box = self.as_image_render_box();
 
                 // Add the background to the list, if applicable.
                 self.paint_background_if_applicable(list, &absolute_box_bounds);
 
-                match image_box.image.get_image() {
+                match image_box.image.mutate().ptr.get_image() {
                     Some(image) => {
                         debug!("(building display list) building image box");
 
@@ -763,7 +1074,7 @@ impl RenderBox {
                             let image_display_item = ~ImageDisplayItem {
                                 base: BaseDisplayItem {
                                     bounds: absolute_box_bounds,
-                                    extra: ExtraDisplayListData::new(*self),
+                                    extra: ExtraDisplayListData::new(self),
                                 },
                                 image: image.clone(),
                             };
@@ -785,257 +1096,15 @@ impl RenderBox {
         // TODO: Outlines.
         self.paint_borders_if_applicable(list, &absolute_box_bounds);
     }
+}
 
-    /// Adds the display items necessary to paint the background of this render box to the display
-    /// list if necessary.
-    pub fn paint_background_if_applicable<E:ExtraDisplayListData>(&self,
-                                                              list: &Cell<DisplayList<E>>,
-                                                              absolute_bounds: &Rect<Au>) {
-        // FIXME: This causes a lot of background colors to be displayed when they are clearly not
-        // needed. We could use display list optimization to clean this up, but it still seems
-        // inefficient. What we really want is something like "nearest ancestor element that
-        // doesn't have a render box".
-        let nearest_ancestor_element = self.nearest_ancestor_element();
-
-        let style = nearest_ancestor_element.style();
-        let background_color = style.resolve_color(style.Background.background_color);
-        if !background_color.alpha.approx_eq(&0.0) {
-            do list.with_mut_ref |list| {
-                let solid_color_display_item = ~SolidColorDisplayItem {
-                    base: BaseDisplayItem {
-                        bounds: *absolute_bounds,
-                        extra: ExtraDisplayListData::new(*self),
-                    },
-                    color: background_color.to_gfx_color(),
-                };
-
-                list.append_item(SolidColorDisplayItemClass(solid_color_display_item))
-            }
-        }
-    }
-
-    pub fn clear(&self) -> Option<ClearType> {
-        let style = self.style();
-        match style.Box.clear {
-            clear::none => None,
-            clear::left => Some(ClearLeft),
-            clear::right => Some(ClearRight),
-            clear::both => Some(ClearBoth)
-        }
-    }
-
-    /// Converts this node's computed style to a font style used for rendering.
-    pub fn font_style(&self) -> FontStyle {
-        fn get_font_style(element: AbstractNode<LayoutView>) -> FontStyle {
-            let my_style = element.style();
-
-            debug!("(font style) start: %?", element.type_id());
-
-            // FIXME: Too much allocation here.
-            let font_families = do my_style.Font.font_family.map |family| {
-                match *family {
-                    FamilyName(ref name) => (*name).clone()
-                }
-            };
-            let font_families = font_families.connect(", ");
-            debug!("(font style) font families: `%s`", font_families);
-
-            let font_size = my_style.Font.font_size.to_f64().unwrap() / 60.0;
-            debug!("(font style) font size: `%fpx`", font_size);
-
-            let (italic, oblique) = match my_style.Font.font_style {
-                font_style::normal => (false, false),
-                font_style::italic => (true, false),
-                font_style::oblique => (false, true),
-            };
-
-            FontStyle {
-                pt_size: font_size,
-                weight: FontWeight300,
-                italic: italic,
-                oblique: oblique,
-                families: font_families,
-            }
-        }
-
-        let font_style_cached = match *self {
-            UnscannedTextRenderBoxClass(ref box) => {
-                match box.font_style {
-                    Some(ref style) => Some(style.clone()),
-                    None => None
-                }
-            }
-            _ => None
-        };
-
-        if font_style_cached.is_some() {
-            return font_style_cached.unwrap();
-        } else {
-            let font_style = get_font_style(self.nearest_ancestor_element());
-            match *self {
-                UnscannedTextRenderBoxClass(ref box) => {
-                    box.font_style = Some(font_style.clone());
-                }
-                _ => ()
-            }
-            return font_style;
-        }
-    }
-
-    /// Returns the text alignment of the computed style of the nearest ancestor-or-self `Element`
-    /// node.
-    pub fn text_align(&self) -> text_align::T {
-        self.nearest_ancestor_element().style().Text.text_align
-    }
-
-    pub fn line_height(&self) -> line_height::T {
-        self.nearest_ancestor_element().style().Box.line_height
-    }
-
-    pub fn vertical_align(&self) -> vertical_align::T {
-        self.nearest_ancestor_element().style().Box.vertical_align
-    }
-
-    /// Returns the text decoration of the computed style of the nearest `Element` node
-    pub fn text_decoration(&self) -> text_decoration::T {
-        /// Computes the propagated value of text-decoration, as specified in CSS 2.1 § 16.3.1
-        /// TODO: make sure this works with anonymous box generation.
-        fn get_propagated_text_decoration(element: AbstractNode<LayoutView>) -> text_decoration::T {
-            //Skip over non-element nodes in the DOM
-            if(!element.is_element()){
-                return match element.parent_node() {
-                    None => text_decoration::none,
-                    Some(parent) => get_propagated_text_decoration(parent),
-                };
-            }
-
-            //FIXME: Implement correctly
-            let display_in_flow = true;
-
-            let position = element.style().Box.position;
-            let float = element.style().Box.float;
-
-            let in_flow = (position == position::static_) && (float == float::none) &&
-                display_in_flow;
-
-            let text_decoration = element.style().Text.text_decoration;
-
-            if text_decoration == text_decoration::none && in_flow {
-                match element.parent_node() {
-                    None => text_decoration::none,
-                    Some(parent) => get_propagated_text_decoration(parent),
-                }
-            }
-            else {
-                text_decoration
-            }
-        }
-
-        let text_decoration_cached = match *self {
-            UnscannedTextRenderBoxClass(ref box) => {
-                match box.text_decoration {
-                    Some(ref decoration) => Some(decoration.clone()),
-                    None => None
-                }
-            }
-            _ => None
-        };
-
-        if text_decoration_cached.is_some() {
-            return text_decoration_cached.unwrap();
-        } else {
-            let text_decoration = get_propagated_text_decoration(self.nearest_ancestor_element());
-            match *self {
-                UnscannedTextRenderBoxClass(ref box) => {
-                    box.text_decoration = Some(text_decoration.clone());
-                }
-                _ => ()
-            }
-            return text_decoration;
-        }
-    }
-
-    /// Dumps this node, for debugging.
-    pub fn dump(&self) {
-        self.dump_indent(0);
-    }
-
-    /// Dumps a render box for debugging, with indentation.
-    pub fn dump_indent(&self, indent: uint) {
-        let mut string = ~"";
-        for _ in range(0u, indent) {
-            string.push_str("    ");
-        }
-
-        string.push_str(self.debug_str());
-        debug!("%s", string);
-    }
-
-    /// Returns a debugging string describing this box.
-    pub fn debug_str(&self) -> ~str {
-        let representation = match *self {
-            GenericRenderBoxClass(*) => ~"GenericRenderBox",
-            ImageRenderBoxClass(*) => ~"ImageRenderBox",
-            TextRenderBoxClass(text_box) => {
-                fmt!("TextRenderBox(text=%s)", text_box.run.text.slice_chars(text_box.range.begin(),
-                                                                             text_box.range.end()))
-            }
-            UnscannedTextRenderBoxClass(text_box) => {
-                fmt!("UnscannedTextRenderBox(%s)", text_box.text)
-            }
-        };
-
-        fmt!("box b%?: %s", self.id(), representation)
-    }
-
-    //
-    // Painting
-    //
-
-    /// Adds the display items necessary to paint the borders of this render box to a display list
-    /// if necessary.
-    pub fn paint_borders_if_applicable<E:ExtraDisplayListData>(&self,
-                                                               list: &Cell<DisplayList<E>>,
-                                                               abs_bounds: &Rect<Au>) {
-        // Fast path.
-        let border = do self.with_base |base| {
-            base.model.border
-        };
-        if border.is_zero() {
-            return
-        }
-
-        let style = self.style();
-        let top_color = style.resolve_color(style.Border.border_top_color);
-        let right_color = style.resolve_color(style.Border.border_right_color);
-        let bottom_color = style.resolve_color(style.Border.border_bottom_color);
-        let left_color = style.resolve_color(style.Border.border_left_color);
-        let top_style = style.Border.border_top_style;
-        let right_style = style.Border.border_right_style;
-        let bottom_style = style.Border.border_bottom_style;
-        let left_style = style.Border.border_left_style;
-        // Append the border to the display list.
-        do list.with_mut_ref |list| {
-            let border_display_item = ~BorderDisplayItem {
-                base: BaseDisplayItem {
-                    bounds: *abs_bounds,
-                    extra: ExtraDisplayListData::new(*self),
-                },
-                border: SideOffsets2D::new(border.top,
-                                           border.right,
-                                           border.bottom,
-                                           border.left),
-                color: SideOffsets2D::new(top_color.to_gfx_color(),
-                                          right_color.to_gfx_color(),
-                                          bottom_color.to_gfx_color(),
-                                          left_color.to_gfx_color()),
-                style: SideOffsets2D::new(top_style,
-                                          right_style,
-                                          bottom_style,
-                                          left_style)
-            };
-
-            list.append_item(BorderDisplayItemClass(border_display_item))
+impl<'self> RenderBoxRefUtils<'self> for &'self RenderBox {
+    #[inline(always)]
+    fn base(self) -> &'self RenderBoxBase {
+        unsafe {
+            let (_, box_ptr): (uint, *RenderBoxBase) = cast::transmute(self);
+            cast::transmute(box_ptr)
         }
     }
 }
+
diff --git a/src/components/main/layout/box_builder.rs b/src/components/main/layout/box_builder.rs
index 9686ddcb6cb..7afff453df0 100644
--- a/src/components/main/layout/box_builder.rs
+++ b/src/components/main/layout/box_builder.rs
@@ -4,16 +4,18 @@
 
 //! Creates CSS boxes from a DOM tree.
 
-use layout::block::BlockFlowData;
-use layout::float::FloatFlowData;
-use layout::box::{GenericRenderBoxClass, ImageRenderBox, ImageRenderBoxClass, RenderBox};
-use layout::box::{RenderBoxBase, RenderBoxType, RenderBox_Generic, RenderBox_Image};
-use layout::box::{RenderBox_Text, UnscannedTextRenderBox, UnscannedTextRenderBoxClass};
+use layout::block::BlockFlow;
+use layout::float::FloatFlow;
+use layout::box::{GenericRenderBox, GenericRenderBoxClass, ImageRenderBox, ImageRenderBoxClass};
+use layout::box::{RenderBox, RenderBoxBase, RenderBoxClass, RenderBoxUtils, TextRenderBoxClass};
+use layout::box::{UnscannedTextRenderBox, UnscannedTextRenderBoxClass};
 use layout::context::LayoutContext;
-use layout::flow::{AbsoluteFlow, BlockFlow, FloatFlow, Flow_Absolute, Flow_Block, Flow_Float};
-use layout::flow::{Flow_Inline, Flow_InlineBlock, Flow_Root, Flow_Table, FlowContext};
-use layout::flow::{FlowContextType, FlowData, InlineBlockFlow, InlineFlow, TableFlow};
-use layout::inline::{InlineFlowData, InlineLayout};
+use layout::float_context::FloatType;
+use layout::flow::{AbsoluteFlow, BlockFlowClass, FloatFlowClass, FlowContext, FlowData};
+use layout::flow::{ImmutableFlowUtils, InlineBlockFlow, InlineBlockFlowClass, InlineFlowClass};
+use layout::flow::{MutableFlowUtils, TableFlow};
+use layout::flow;
+use layout::inline::{InlineFlow};
 use layout::text::TextRunScanner;
 use css::node_style::StyledNode;
 
@@ -25,8 +27,19 @@ use script::dom::node::{ElementNodeTypeId, LayoutView, TextNodeTypeId};
 use script::dom::node::DocumentFragmentNodeTypeId;
 use servo_util::range::Range;
 use servo_util::tree::{TreeNodeRef, TreeNode};
+use std::cast;
 use std::cell::Cell;
 
+enum FlowType {
+    AbsoluteFlowType,
+    BlockFlowType,
+    FloatFlowType(FloatType),
+    InlineBlockFlowType,
+    InlineFlowType,
+    RootFlowType,
+    TableFlowType,
+}
+
 pub struct LayoutTreeBuilder {
     next_cid: int,
     next_bid: int,
@@ -64,12 +77,15 @@ impl<'self> BoxGenerator<'self> {
         }
     }
 
-    fn with_clone<R> (&mut self, cb: &fn(BoxGenerator<'self>) -> R) -> R {
-        let gen = BoxGenerator {
-            flow: &mut *self.flow,
-            range_stack: self.range_stack
-        };
-        cb(gen)
+    fn with_clone<R>(&mut self, cb: &fn(BoxGenerator<'self>) -> R) -> R {
+        // FIXME(pcwalton): This is a hack; it can be done safely with linearity.
+        unsafe {
+            let gen = BoxGenerator {
+                flow: cast::transmute_copy(&self.flow),
+                range_stack: self.range_stack
+            };
+            cb(gen)
+        }
     }
 
     /* Whether "spacer" boxes are needed to stand in for this DOM node */
@@ -81,7 +97,7 @@ impl<'self> BoxGenerator<'self> {
     fn make_inline_spacer_for_node_side(_: &LayoutContext,
                                         _: AbstractNode<LayoutView>,
                                         _: InlineSpacerSide)
-                                        -> Option<RenderBox> {
+                                        -> Option<@RenderBox> {
         None
     }
 
@@ -89,17 +105,18 @@ impl<'self> BoxGenerator<'self> {
                      ctx: &LayoutContext,
                      node: AbstractNode<LayoutView>,
                      builder: &mut LayoutTreeBuilder) {
-        debug!("BoxGenerator[f%d]: pushing node: %s", self.flow.id(), node.debug_str());
+        debug!("BoxGenerator[f%d]: pushing node: %s", flow::base(self.flow).id, node.debug_str());
 
         // TODO: remove this once UA styles work
         let box_type = self.decide_box_type(node);
 
-        debug!("BoxGenerator[f%d]: point a", self.flow.id());
+        debug!("BoxGenerator[f%d]: point a", flow::base(self.flow).id);
 
         let range_stack = &mut self.range_stack;
         // depending on flow, make a box for this node.
-        match *self.flow {
-            InlineFlow(ref mut inline) => {
+        match self.flow.class() {
+            InlineFlowClass => {
+                let inline = self.flow.as_inline();
                 let node_range_start = inline.boxes.len();
                 range_stack.push(node_range_start);
 
@@ -116,42 +133,43 @@ impl<'self> BoxGenerator<'self> {
                 }
                 // TODO: cases for inline-block, etc.
             },
-            BlockFlow(ref mut block) => {
-                debug!("BoxGenerator[f%d]: point b", block.common.id);
+            BlockFlowClass => {
+                let block = self.flow.as_block();
+                debug!("BoxGenerator[f%d]: point b", block.base.id);
                 let new_box = BoxGenerator::make_box(ctx, box_type, node, builder);
 
                 debug!("BoxGenerator[f%d]: attaching box[b%d] to block flow (node: %s)",
-                       block.common.id,
-                       new_box.id(),
+                       block.base.id,
+                       new_box.base().id(),
                        node.debug_str());
 
                 assert!(block.box.is_none());
                 block.box = Some(new_box);
             }
-            FloatFlow(ref mut float) => {
-                debug!("BoxGenerator[f%d]: point b", float.common.id);
+            FloatFlowClass => {
+                let float = self.flow.as_float();
+                debug!("BoxGenerator[f%d]: point b", float.base.id);
 
                 let new_box = BoxGenerator::make_box(ctx, box_type, node, builder);
 
                 debug!("BoxGenerator[f%d]: attaching box[b%d] to float flow (node: %s)",
-                        float.common.id,
-                        new_box.id(),
+                        float.base.id,
+                        new_box.base().id(),
                         node.debug_str());
 
                 assert!(float.box.is_none() && float.index.is_none());
                 float.box = Some(new_box);
             }
-            _ => warn!("push_node() not implemented for flow f%d", self.flow.id()),
+            _ => warn!("push_node() not implemented for flow f%d", flow::base(self.flow).id),
         }
     }
 
-    pub fn pop_node(&mut self,
-                    ctx: &LayoutContext,
-                    node: AbstractNode<LayoutView>) {
-        debug!("BoxGenerator[f%d]: popping node: %s", self.flow.id(), node.debug_str());
+    pub fn pop_node(&mut self, ctx: &LayoutContext, node: AbstractNode<LayoutView>) {
+        debug!("BoxGenerator[f%d]: popping node: %s", flow::base(self.flow).id, node.debug_str());
 
-        match *self.flow {
-            InlineFlow(ref mut inline) => {
+        match self.flow.class() {
+            InlineFlowClass => {
+                let inline = self.flow.as_inline();
                 let inline = &mut *inline;
 
                 if BoxGenerator::inline_spacers_needed_for_node(node) {
@@ -173,24 +191,26 @@ impl<'self> BoxGenerator<'self> {
                 debug!("BoxGenerator: adding element range=%?", node_range);
                 inline.elems.add_mapping(node, &node_range);
             },
-            BlockFlow(*) => assert!(self.range_stack.len() == 0),
-            FloatFlow(*) => assert!(self.range_stack.len() == 0),
-            _ => warn!("pop_node() not implemented for flow %?", self.flow.id()),
+            BlockFlowClass => assert!(self.range_stack.len() == 0),
+            FloatFlowClass => assert!(self.range_stack.len() == 0),
+            _ => warn!("pop_node() not implemented for flow %?", flow::base(self.flow).id),
         }
     }
 
     /// Disambiguate between different methods here instead of inlining, since each case has very
     /// different complexity.
     fn make_box(layout_ctx: &LayoutContext,
-                ty: RenderBoxType,
+                ty: RenderBoxClass,
                 node: AbstractNode<LayoutView>,
                 builder: &mut LayoutTreeBuilder)
-                -> RenderBox {
+                -> @RenderBox {
         let base = RenderBoxBase::new(node, builder.next_box_id());
         let result = match ty {
-            RenderBox_Generic => GenericRenderBoxClass(@mut base),
-            RenderBox_Text => UnscannedTextRenderBoxClass(@mut UnscannedTextRenderBox::new(base)),
-            RenderBox_Image => BoxGenerator::make_image_box(layout_ctx, node, base),
+            GenericRenderBoxClass => @GenericRenderBox::new(base) as @RenderBox,
+            TextRenderBoxClass | UnscannedTextRenderBoxClass => {
+                @UnscannedTextRenderBox::new(base) as @RenderBox
+            }
+            ImageRenderBoxClass => BoxGenerator::make_image_box(layout_ctx, node, base),
         };
         debug!("BoxGenerator: created box: %s", result.debug_str());
         result
@@ -199,36 +219,36 @@ impl<'self> BoxGenerator<'self> {
     fn make_image_box(layout_ctx: &LayoutContext,
                       node: AbstractNode<LayoutView>,
                       base: RenderBoxBase)
-                      -> RenderBox {
+                      -> @RenderBox {
         assert!(node.is_image_element());
 
         do node.with_imm_image_element |image_element| {
             if image_element.image.is_some() {
                 // FIXME(pcwalton): Don't copy URLs.
                 let url = (*image_element.image.get_ref()).clone();
-                ImageRenderBoxClass(@mut ImageRenderBox::new(base, url, layout_ctx.image_cache))
+                @ImageRenderBox::new(base.clone(), url, layout_ctx.image_cache) as @RenderBox
             } else {
                 info!("Tried to make image box, but couldn't find image. Made generic box \
                        instead.");
-                GenericRenderBoxClass(@mut base)
+                @GenericRenderBox::new(base.clone()) as @RenderBox
             }
         }
     }
 
-    fn decide_box_type(&self, node: AbstractNode<LayoutView>) -> RenderBoxType {
+    fn decide_box_type(&self, node: AbstractNode<LayoutView>) -> RenderBoxClass {
         if node.is_text() {
-            RenderBox_Text
+            TextRenderBoxClass
         } else if node.is_image_element() {
             do node.with_imm_image_element |image_element| {
                 match image_element.image {
-                    Some(_) => RenderBox_Image,
-                    None => RenderBox_Generic,
+                    Some(_) => ImageRenderBoxClass,
+                    None => GenericRenderBoxClass,
                 }
             }
         } else if node.is_element() {
-            RenderBox_Generic
+            GenericRenderBoxClass
         } else {
-            fail!(~"Hey, doctypes and comments shouldn't get here! They are display:none!")
+            fail!("Hey, doctypes and comments shouldn't get here! They are display:none!")
         }
     }
 
@@ -261,13 +281,14 @@ impl LayoutTreeBuilder {
 
     /// Creates necessary box(es) and flow context(s) for the current DOM node,
     /// and recurses on its children.
-    pub fn construct_recursively<'a>(&mut self,
-                                     layout_ctx: &LayoutContext,
-                                     cur_node: AbstractNode<LayoutView>,
-                                     mut grandparent_generator: Option<BoxGenerator<'a>>,
-                                     mut parent_generator: BoxGenerator<'a>,
-                                     mut prev_sibling_generator: Option<BoxGenerator<'a>>)
-                                     -> BoxConstructResult<'a> {
+    pub fn construct_recursively<'a>(
+                                 &mut self,
+                                 layout_ctx: &LayoutContext,
+                                 cur_node: AbstractNode<LayoutView>,
+                                 mut grandparent_generator: Option<BoxGenerator<'a>>,
+                                 mut parent_generator: BoxGenerator<'a>,
+                                 mut prev_sibling_generator: Option<BoxGenerator<'a>>)
+                                 -> BoxConstructResult<'a> {
         debug!("Considering node: %s", cur_node.debug_str());
         let box_gen_result = {
             let grandparent_gen_ref = match grandparent_generator {
@@ -285,32 +306,29 @@ impl LayoutTreeBuilder {
 
         debug!("result from generator_for_node: %?", &box_gen_result);
         // Skip over nodes that don't belong in the flow tree
-        let (this_generator, next_generator) = 
-            match box_gen_result {
-                NoGenerator => return Normal(prev_sibling_generator),
+        let (this_generator, next_generator) = match box_gen_result {
+            NoGenerator => return Normal(prev_sibling_generator),
+            ParentGenerator => {
+                do parent_generator.with_clone |clone| {
+                    (clone, None)
+                }
+            }
+            SiblingGenerator => (prev_sibling_generator.take_unwrap(), None),
+            NewGenerator(gen) => (gen, None),
+            ReparentingGenerator(gen) => {
+                reparent = true;
+                (gen, None)
+            }
+            Mixed(gen, next_gen) => (gen, Some(match *next_gen {
                 ParentGenerator => {
                     do parent_generator.with_clone |clone| {
-                        (clone, None)
+                        clone
                     }
                 }
-                SiblingGenerator => (prev_sibling_generator.take_unwrap(), None),
-                NewGenerator(gen) => (gen, None),
-                ReparentingGenerator(gen) => {
-                    reparent = true;
-                    (gen, None)
-                }
-                Mixed(gen, next_gen) => (gen, Some(match *next_gen {
-                    ParentGenerator => {
-                        do parent_generator.with_clone |clone| {
-                            clone
-                        }
-                    }
-                    SiblingGenerator => prev_sibling_generator.take_unwrap(),
-                    _ => fail!("Unexpect BoxGenResult")
-                }))
-            };
-
-
+                SiblingGenerator => prev_sibling_generator.take_unwrap(),
+                _ => fail!("Unexpect BoxGenResult")
+            }))
+        };
 
         let mut this_generator = this_generator;
 
@@ -395,7 +413,12 @@ impl LayoutTreeBuilder {
             }
         };
 
-        let sibling_flow: Option<&mut FlowContext> = sibling_generator.as_mut().map(|gen| &mut *gen.flow);
+        // FIXME(pcwalton): Unsafe.
+        let sibling_flow: Option<&mut FlowContext> = sibling_generator.as_mut().map(|gen| {
+            unsafe {
+                cast::transmute_copy(&gen.flow)
+            }
+        });
 
         let is_float = if (node.is_element()) {
             match node.style().Box.float {
@@ -406,85 +429,100 @@ impl LayoutTreeBuilder {
         } else {
             None
         };
-        
 
-        let new_generator = match (display, &mut parent_generator.flow, sibling_flow) { 
+        let sibling_flow_class = match sibling_flow {
+            None => None,
+            Some(flow) => Some(flow.class()),
+        };
+        
+        let new_generator = match (display, parent_generator.flow.class(), sibling_flow_class) {
             // Floats
-            (display::block, & &BlockFlow(_), _) |
-            (display::block, & &FloatFlow(_), _) if is_float.is_some() => {
-                self.create_child_generator(node, parent_generator, Flow_Float(is_float.unwrap()))
+            (display::block, BlockFlowClass, _) |
+            (display::block, FloatFlowClass, _) if is_float.is_some() => {
+                self.create_child_generator(node,
+                                            parent_generator,
+                                            FloatFlowType(is_float.unwrap()))
             }
             // If we're placing a float after an inline, append the float to the inline flow,
             // then continue building from the inline flow in case there are more inlines
             // afterward.
-            (display::block, _, Some(&InlineFlow(_))) if is_float.is_some() => {
+            (display::block, _, Some(InlineFlowClass)) if is_float.is_some() => {
+                let float_type = FloatFlowType(is_float.unwrap());
                 let float_generator = self.create_child_generator(node, 
                                                                   sibling_generator.unwrap(), 
-                                                                  Flow_Float(is_float.unwrap()));
+                                                                  float_type);
                 return Mixed(float_generator, ~SiblingGenerator);
             }
             // This is a catch-all case for when:
             // a) sibling_flow is None
             // b) sibling_flow is a BlockFlow
-            (display::block, & &InlineFlow(_), _) if is_float.is_some() => {
-                self.create_child_generator(node, parent_generator, Flow_Float(is_float.unwrap()))
+            (display::block, InlineFlowClass, _) if is_float.is_some() => {
+                self.create_child_generator(node,
+                                            parent_generator,
+                                            FloatFlowType(is_float.unwrap()))
             }
 
-            (display::block, & &BlockFlow(ref info), _) => match (info.is_root, node.parent_node().is_some()) {
-                // If this is the root node, then use the root flow's
-                // context. Otherwise, make a child block context.
-                (true, true) => self.create_child_generator(node, parent_generator, Flow_Block),
-                (true, false) => { return ParentGenerator }
-                (false, _)   => {
-                    self.create_child_generator(node, parent_generator, Flow_Block)
+            (display::block, BlockFlowClass, _) => {
+                match (parent_generator.flow.as_block().is_root, node.parent_node()) {
+                    // If this is the root node, then use the root flow's
+                    // context. Otherwise, make a child block context.
+                    (true, Some(_)) => {
+                        self.create_child_generator(node, parent_generator, BlockFlowType)
+                    }
+                    (true, None) => return ParentGenerator,
+                    (false, _) => {
+                        self.create_child_generator(node, parent_generator, BlockFlowType)
+                    }
                 }
-            },
+            }
 
-            (display::block, & &FloatFlow(*), _) => {
-                self.create_child_generator(node, parent_generator, Flow_Block)
+            (display::block, FloatFlowClass, _) => {
+                self.create_child_generator(node, parent_generator, BlockFlowType)
             }
 
             // Inlines that are children of inlines are part of the same flow
-            (display::inline, & &InlineFlow(*), _) => return ParentGenerator,
-            (display::inline_block, & &InlineFlow(*), _) => return ParentGenerator,
+            (display::inline, InlineFlowClass, _) => return ParentGenerator,
+            (display::inline_block, InlineFlowClass, _) => return ParentGenerator,
 
             // Inlines that are children of blocks create new flows if their
             // previous sibling was a block.
-            (display::inline, & &BlockFlow(*), Some(&BlockFlow(*))) |
-            (display::inline_block, & &BlockFlow(*), Some(&BlockFlow(*))) => {
-                self.create_child_generator(node, parent_generator, Flow_Inline)
+            (display::inline, BlockFlowClass, Some(BlockFlowClass)) |
+            (display::inline_block, BlockFlowClass, Some(BlockFlowClass)) => {
+                self.create_child_generator(node, parent_generator, InlineFlowType)
             }
 
             // The first two cases should only be hit when a FloatFlow
             // is the first child of a BlockFlow. Other times, we will
-            (display::inline, _, Some(&FloatFlow(*))) |
-            (display::inline_block, _, Some(&FloatFlow(*))) |
-            (display::inline, & &FloatFlow(*), _) |
-            (display::inline_block, & &FloatFlow(*), _) => {
-                self.create_child_generator(node, parent_generator, Flow_Inline)
+            (display::inline, _, Some(FloatFlowClass)) |
+            (display::inline_block, _, Some(FloatFlowClass)) |
+            (display::inline, FloatFlowClass, _) |
+            (display::inline_block, FloatFlowClass, _) => {
+                self.create_child_generator(node, parent_generator, InlineFlowType)
             }
 
             // Inlines whose previous sibling was not a block try to use their
             // sibling's flow context.
-            (display::inline, & &BlockFlow(*), _) |
-            (display::inline_block, & &BlockFlow(*), _) => {
+            (display::inline, BlockFlowClass, _) |
+            (display::inline_block, BlockFlowClass, _) => {
                 return match sibling_generator {
                     None => NewGenerator(self.create_child_generator(node, 
                                                                      parent_generator, 
-                                                                     Flow_Inline)),
+                                                                     InlineFlowType)),
                     Some(*) => SiblingGenerator
                 }
             }
 
             // blocks that are children of inlines need to split their parent
             // flows.
-            (display::block, & &InlineFlow(*), _) => {
+            (display::block, InlineFlowClass, _) => {
                 match grandparent_generator {
                     None => fail!("expected to have a grandparent block flow"),
                     Some(grandparent_gen) => {
                         assert!(grandparent_gen.flow.is_block_like());
 
-                        let block_gen = self.create_child_generator(node, grandparent_gen, Flow_Block);
+                        let block_gen = self.create_child_generator(node,
+                                                                    grandparent_gen,
+                                                                    BlockFlowType);
                         return ReparentingGenerator(block_gen);
                     }
                 }
@@ -496,15 +534,16 @@ impl LayoutTreeBuilder {
         NewGenerator(new_generator)
     }
 
-    pub fn create_child_generator<'a>(&mut self,
-                              node: AbstractNode<LayoutView>,
-                              parent_generator: &mut BoxGenerator<'a>,
-                              ty: FlowContextType)
-                              -> BoxGenerator<'a> {
-
+    pub fn create_child_generator<'a>(
+                                  &mut self,
+                                  node: AbstractNode<LayoutView>,
+                                  parent_generator: &mut BoxGenerator<'a>,
+                                  ty: FlowType)
+                                  -> BoxGenerator<'a> {
         let new_flow = self.make_flow(ty, node);
         parent_generator.flow.add_new_child(new_flow);
-        BoxGenerator::new(parent_generator.flow.last_child().unwrap())
+        let flow_ref = flow::last_child(parent_generator.flow).unwrap();
+        BoxGenerator::new(*flow_ref)
     }
 
     /// Fix up any irregularities such as:
@@ -516,15 +555,15 @@ impl LayoutTreeBuilder {
     /// The latter can only be done immediately adjacent to, or at the beginning or end of a block
     /// flow. Otherwise, the whitespace might affect whitespace collapsing with adjacent text.
     pub fn simplify_children_of_flow(&self, ctx: &LayoutContext, parent_flow: &mut FlowContext) {
-        match *parent_flow {
-            InlineFlow(*) => {
+        match parent_flow.class() {
+            InlineFlowClass => {
                 let mut found_child_inline = false;
                 let mut found_child_block = false;
 
-                for child_ctx in parent_flow.child_iter() {
-                    match child_ctx {
-                        &InlineFlow(*) | &InlineBlockFlow(*) => found_child_inline = true,
-                        &BlockFlow(*) => found_child_block = true,
+                for child_ctx in flow::child_iter(parent_flow) {
+                    match child_ctx.class() {
+                        InlineFlowClass | InlineBlockFlowClass => found_child_inline = true,
+                        BlockFlowClass => found_child_block = true,
                         _ => {}
                     }
                 }
@@ -532,23 +571,27 @@ impl LayoutTreeBuilder {
                 if found_child_block && found_child_inline {
                     self.fixup_split_inline(parent_flow)
                 }
-            },
-            BlockFlow(*) | FloatFlow(*) => {
+            }
+            BlockFlowClass | FloatFlowClass => {
                 // check first/last child for whitespace-ness
                 let mut do_remove = false;
-                let p_id = parent_flow.id();
+                let p_id = flow::base(parent_flow).id;
                 do parent_flow.with_first_child |mut first_child| {
                     for first_flow in first_child.mut_iter() {
                         if first_flow.starts_inline_flow() {
                             // FIXME: workaround for rust#6393
                             {
-                                let boxes = &first_flow.imm_inline().boxes;
-                                if boxes.len() == 1 && boxes[0].is_whitespace_only() {
-                                    debug!("LayoutTreeBuilder: pruning whitespace-only first child \
-                                            flow f%d from parent f%d", 
-                                            first_flow.id(),
-                                            p_id);
-                                    do_remove = true;
+                                let first_inline_flow = first_flow.as_inline();
+                                let boxes = &first_inline_flow.boxes;
+                                if boxes.len() == 1 {
+                                    let first_box = boxes[0];   // FIXME(pcwalton): Rust bug
+                                    if first_box.is_whitespace_only() {
+                                        debug!("LayoutTreeBuilder: pruning whitespace-only first \
+                                                child flow f%d from parent f%d", 
+                                               first_inline_flow.base.id,
+                                               p_id);
+                                        do_remove = true;
+                                    }
                                 }
                             }
                         }
@@ -560,19 +603,23 @@ impl LayoutTreeBuilder {
 
 
                 do_remove = false;
-                let p_id = parent_flow.id();
+                let p_id = flow::base(parent_flow).id;
                 do parent_flow.with_last_child |mut last_child| {
                     for last_flow in last_child.mut_iter() {
                         if last_flow.starts_inline_flow() {
                             // FIXME: workaround for rust#6393
                             {
-                                let boxes = &last_flow.imm_inline().boxes;
+                                let last_inline_flow = last_flow.as_inline();
+                                let boxes = &last_inline_flow.boxes;
                                 if boxes.len() == 1 && boxes.last().is_whitespace_only() {
-                                    debug!("LayoutTreeBuilder: pruning whitespace-only last child \
-                                            flow f%d from parent f%d", 
-                                            last_flow.id(),
-                                            p_id);
-                                    do_remove = true;
+                                    let last_box = boxes.last();    // FIXME(pcwalton): Rust bug
+                                    if last_box.is_whitespace_only() {
+                                        debug!("LayoutTreeBuilder: pruning whitespace-only last \
+                                                child flow f%d from parent f%d", 
+                                               last_inline_flow.base.id,
+                                               p_id);
+                                        do_remove = true;
+                                    }
                                 }
                             }
                         }
@@ -584,16 +631,16 @@ impl LayoutTreeBuilder {
 
                 // Issue 543: We only need to do this if there are inline child
                 // flows, but there's not a quick way to check at the moment.
-                for child_flow in (*parent_flow).child_iter() {
-                    match *child_flow {
-                        InlineFlow(*) | InlineBlockFlow(*) => {
+                for child_flow in flow::child_iter(parent_flow) {
+                    match child_flow.class() {
+                        InlineFlowClass | InlineBlockFlowClass => {
                             let mut scanner = TextRunScanner::new();
-                            scanner.scan_for_runs(ctx, child_flow);
+                            scanner.scan_for_runs(ctx, *child_flow);
                         }
                         _ => {}
                     }
                 }
-            },
+            }
             _ => {}
         }
     }
@@ -605,29 +652,30 @@ impl LayoutTreeBuilder {
 
     /// Entry point for box creation. Should only be called on the root DOM element.
     pub fn construct_trees(&mut self, layout_ctx: &LayoutContext, root: AbstractNode<LayoutView>)
-                       -> Result<FlowContext, ()> {
+                       -> Result<~FlowContext:, ()> {
         debug!("Constructing flow tree for DOM: ");
-        root.dump();
+        debug!("%?", root.dump());
 
-        let mut new_flow = self.make_flow(Flow_Root, root);
+        let mut new_flow = self.make_flow(RootFlowType, root);
         {
-            let new_generator = BoxGenerator::new(&mut new_flow);
+            let new_generator = BoxGenerator::new(new_flow);
             self.construct_recursively(layout_ctx, root, None, new_generator, None);
         }
         return Ok(new_flow)
     }
 
     /// Creates a flow of the given type for the supplied node.
-    pub fn make_flow(&mut self, ty: FlowContextType, node: AbstractNode<LayoutView>) -> FlowContext {
+    pub fn make_flow(&mut self, flow_type: FlowType, node: AbstractNode<LayoutView>)
+                     -> ~FlowContext: {
         let info = FlowData::new(self.next_flow_id(), node);
-        let result = match ty {
-            Flow_Absolute       => AbsoluteFlow(~info),
-            Flow_Block          => BlockFlow(~BlockFlowData::new(info)),
-            Flow_Float(f_type)  => FloatFlow(~FloatFlowData::new(info, f_type)),
-            Flow_InlineBlock    => InlineBlockFlow(~info),
-            Flow_Inline         => InlineFlow(~InlineFlowData::new(info)),
-            Flow_Root           => BlockFlow(~BlockFlowData::new_root(info)),
-            Flow_Table          => TableFlow(~info),
+        let result = match flow_type {
+            AbsoluteFlowType        => ~AbsoluteFlow::new(info) as ~FlowContext:,
+            BlockFlowType           => ~BlockFlow::new(info) as ~FlowContext:,
+            FloatFlowType(f_type)   => ~FloatFlow::new(info, f_type) as ~FlowContext:,
+            InlineBlockFlowType     => ~InlineBlockFlow::new(info) as ~FlowContext:,
+            InlineFlowType          => ~InlineFlow::new(info) as ~FlowContext:,
+            RootFlowType            => ~BlockFlow::new_root(info) as ~FlowContext:,
+            TableFlowType           => ~TableFlow::new(info) as ~FlowContext:,
         };
         debug!("LayoutTreeBuilder: created flow: %s", result.debug_str());
         result
diff --git a/src/components/main/layout/display_list_builder.rs b/src/components/main/layout/display_list_builder.rs
index 5dc6a7ebec2..ac0cfaf716a 100644
--- a/src/components/main/layout/display_list_builder.rs
+++ b/src/components/main/layout/display_list_builder.rs
@@ -4,7 +4,7 @@
 
 //! Constructs display lists from render boxes.
 
-use layout::box::RenderBox;
+use layout::box::{RenderBox, RenderBoxUtils};
 use layout::context::LayoutContext;
 use std::cast::transmute;
 use script::dom::node::AbstractNode;
@@ -16,28 +16,28 @@ use style;
 /// that nodes in this view shoud not really be touched. The idea is to
 /// store the nodes in the display list and have layout transmute them.
 pub trait ExtraDisplayListData {
-    fn new(box: RenderBox) -> Self;
+    fn new(box: &@RenderBox) -> Self;
 }
 
 pub type Nothing = ();
 
 impl ExtraDisplayListData for AbstractNode<()> {
-    fn new (box: RenderBox) -> AbstractNode<()> {
+    fn new(box: &@RenderBox) -> AbstractNode<()> {
         unsafe { 
-            transmute(box.node())
+            transmute(box.base().node)
         }
     }
 }
 
 impl ExtraDisplayListData for Nothing {
-    fn new(_: RenderBox) -> Nothing {
+    fn new(_: &@RenderBox) -> Nothing {
         ()
     }
 }
 
-impl ExtraDisplayListData for RenderBox {
-    fn new(box: RenderBox) -> RenderBox {
-        box
+impl ExtraDisplayListData for @RenderBox {
+    fn new(box: &@RenderBox) -> @RenderBox {
+        *box
     }
 }
 
diff --git a/src/components/main/layout/float.rs b/src/components/main/layout/float.rs
index 857f03dc4c5..df97ebf9112 100644
--- a/src/components/main/layout/float.rs
+++ b/src/components/main/layout/float.rs
@@ -2,10 +2,11 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use layout::box::{RenderBox};
+use layout::box::{RenderBox, RenderBoxUtils};
 use layout::context::LayoutContext;
 use layout::display_list_builder::{DisplayListBuilder, ExtraDisplayListData};
-use layout::flow::{FlowData};
+use layout::flow::{FloatFlowClass, FlowClass, FlowContext, FlowData};
+use layout::flow;
 use layout::model::{MaybeAuto};
 use layout::float_context::{FloatContext, PlacementInfo, FloatType};
 
@@ -16,12 +17,12 @@ use gfx::display_list::DisplayList;
 use servo_util::geometry::Au;
 use servo_util::geometry;
 
-pub struct FloatFlowData {
+pub struct FloatFlow {
     /// Data common to all flows.
-    common: FlowData,
+    base: FlowData,
 
     /// The associated render box.
-    box: Option<RenderBox>,
+    box: Option<@RenderBox>,
 
     containing_width: Au,
 
@@ -39,10 +40,10 @@ pub struct FloatFlowData {
 
 }
 
-impl FloatFlowData {
-    pub fn new(common: FlowData, float_type: FloatType) -> FloatFlowData {
-        FloatFlowData {
-            common: common,
+impl FloatFlow {
+    pub fn new(base: FlowData, float_type: FloatType) -> FloatFlow {
+        FloatFlow {
+            base: base,
             containing_width: Au(0),
             box: None,
             index: None,
@@ -59,119 +60,157 @@ impl FloatFlowData {
         self.box = None;
         self.index = None;
     }
+
+    pub fn build_display_list_float<E:ExtraDisplayListData>(&mut self,
+                                                            builder: &DisplayListBuilder,
+                                                            dirty: &Rect<Au>, 
+                                                            list: &Cell<DisplayList<E>>) 
+                                                            -> bool {
+        //TODO: implement iframe size messaging
+        if self.base.node.is_iframe_element() {
+            error!("float iframe size messaging not implemented yet");
+        }
+        let abs_rect = Rect(self.base.abs_position, self.base.position.size);
+        if !abs_rect.intersects(dirty) {
+            return true;
+        }
+
+
+        let offset = self.base.abs_position + self.rel_pos;
+        // add box that starts block context
+        for box in self.box.iter() {
+            box.build_display_list(builder, dirty, &offset, list)
+        }
+
+
+        // TODO: handle any out-of-flow elements
+
+        // go deeper into the flow tree
+        for child in self.base.child_iter() {
+            let child_base = flow::mut_base(*child);
+            child_base.abs_position = offset + child_base.position.origin;
+        }
+
+        false
+    }
 }
 
-impl FloatFlowData {
-    pub fn bubble_widths_float(&mut self, ctx: &LayoutContext) {
+impl FlowContext for FloatFlow {
+    fn class(&self) -> FlowClass {
+        FloatFlowClass
+    }
+
+    fn as_float<'a>(&'a mut self) -> &'a mut FloatFlow {
+        self
+    }
+
+    fn bubble_widths(&mut self, _: &mut LayoutContext) {
         let mut min_width = Au(0);
         let mut pref_width = Au(0);
         let mut num_floats = 0;
 
-        for child_ctx in self.common.child_iter() {
+        for child_ctx in self.base.child_iter() {
             //assert!(child_ctx.starts_block_flow() || child_ctx.starts_inline_flow());
 
-            do child_ctx.with_mut_base |child_node| {
-                min_width = geometry::max(min_width, child_node.min_width);
-                pref_width = geometry::max(pref_width, child_node.pref_width);
-                num_floats = num_floats + child_node.num_floats;
-            }
+            let child_base = flow::mut_base(*child_ctx);
+            min_width = geometry::max(min_width, child_base.min_width);
+            pref_width = geometry::max(pref_width, child_base.pref_width);
+            num_floats = num_floats + child_base.num_floats;
         }
 
-        self.common.num_floats = 1;
+        self.base.num_floats = 1;
         self.floated_children = num_floats;
 
-
         for box in self.box.iter() {
-            let style = box.style();
-            do box.with_model |model| {
-                model.compute_borders(style)
+            {
+                let base = box.base();
+                base.model.mutate().ptr.compute_borders(base.style());
             }
 
-            min_width = min_width.add(&box.get_min_width(ctx));
-            pref_width = pref_width.add(&box.get_pref_width(ctx));
+            let (this_minimum_width, this_preferred_width) = box.minimum_and_preferred_widths();
+            min_width = min_width + this_minimum_width;
+            pref_width = pref_width + this_preferred_width;
         }
 
-        self.common.min_width = min_width;
-        self.common.pref_width = pref_width;
+        self.base.min_width = min_width;
+        self.base.pref_width = pref_width;
     }
 
-    pub fn assign_widths_float(&mut self) { 
-        debug!("assign_widths_float: assigning width for flow %?",  self.common.id);
+    fn assign_widths(&mut self, _: &mut LayoutContext) {
+        debug!("assign_widths_float: assigning width for flow %?",  self.base.id);
         // position.size.width is set by parent even though we don't know
         // position.origin yet.
-        let mut remaining_width = self.common.position.size.width;
+        let mut remaining_width = self.base.position.size.width;
         self.containing_width = remaining_width;
         let mut x_offset = Au(0);
         
         // Parent usually sets this, but floats are never inorder
-        self.common.is_inorder = false;
+        self.base.is_inorder = false;
 
         for &box in self.box.iter() {
-            let style = box.style();
-            do box.with_model |model| {
-                // Can compute padding here since we know containing block width.
-                model.compute_padding(style, remaining_width);
+            let base = box.base();
+            let style = base.style();
+            let mut position_ref = base.position.mutate();
+            let mut model_ref = base.model.mutate();
+            let (position, model) = (&mut position_ref.ptr, &mut model_ref.ptr);
 
-                // Margins for floats are 0 if auto.
-                let margin_top = MaybeAuto::from_style(style.Margin.margin_top,
-                                                        remaining_width).specified_or_zero();
-                let margin_bottom = MaybeAuto::from_style(style.Margin.margin_bottom,
-                                                          remaining_width).specified_or_zero();
-                let margin_left = MaybeAuto::from_style(style.Margin.margin_left,
-                                                        remaining_width).specified_or_zero();
-                let margin_right = MaybeAuto::from_style(style.Margin.margin_right,
-                                                         remaining_width).specified_or_zero();
+            // Can compute padding here since we know containing block width.
+            model.compute_padding(style, remaining_width);
 
+            // Margins for floats are 0 if auto.
+            let margin_top = MaybeAuto::from_style(style.Margin.margin_top,
+                                                   remaining_width).specified_or_zero();
+            let margin_bottom = MaybeAuto::from_style(style.Margin.margin_bottom,
+                                                      remaining_width).specified_or_zero();
+            let margin_left = MaybeAuto::from_style(style.Margin.margin_left,
+                                                    remaining_width).specified_or_zero();
+            let margin_right = MaybeAuto::from_style(style.Margin.margin_right,
+                                                     remaining_width).specified_or_zero();
 
 
-                let shrink_to_fit = geometry::min(self.common.pref_width, 
-                                                  geometry::max(self.common.min_width, 
-                                                                remaining_width));
+            let shrink_to_fit = geometry::min(self.base.pref_width, 
+                                              geometry::max(self.base.min_width, remaining_width));
 
 
-                let width = MaybeAuto::from_style(style.Box.width,
-                                                  remaining_width).specified_or_default(shrink_to_fit);
-                debug!("assign_widths_float -- width: %?", width);
+            let width = MaybeAuto::from_style(style.Box.width, 
+                                              remaining_width).specified_or_default(shrink_to_fit);
+            debug!("assign_widths_float -- width: %?", width);
 
-                model.margin.top = margin_top;
-                model.margin.right = margin_right;
-                model.margin.bottom = margin_bottom;
-                model.margin.left = margin_left;
+            model.margin.top = margin_top;
+            model.margin.right = margin_right;
+            model.margin.bottom = margin_bottom;
+            model.margin.left = margin_left;
 
-                x_offset = model.offset();
-                remaining_width = width;
-            }
+            x_offset = model.offset();
+            remaining_width = width;
 
-            do box.with_mut_base |base| {
-                //The associated box is the border box of this flow
-                base.position.origin.x = base.model.margin.left;
+            // The associated box is the border box of this flow.
+            position.origin.x = model.margin.left;
 
-                let pb = base.model.padding.left + base.model.padding.right +
-                    base.model.border.left + base.model.border.right;
-                base.position.size.width = remaining_width + pb;
-            }
+            let padding_and_borders = model.padding.left + model.padding.right +
+                model.border.left + model.border.right;
+            position.size.width = remaining_width + padding_and_borders;
         }
 
-        self.common.position.size.width = remaining_width;
+        self.base.position.size.width = remaining_width;
 
-        let has_inorder_children = self.common.num_floats > 0;
-        for kid in self.common.child_iter() {
+        let has_inorder_children = self.base.num_floats > 0;
+        for kid in self.base.child_iter() {
             //assert!(kid.starts_block_flow() || kid.starts_inline_flow());
 
-            do kid.with_mut_base |child_node| {
-                child_node.position.origin.x = x_offset;
-                child_node.position.size.width = remaining_width;
-                child_node.is_inorder = has_inorder_children;
+            let child_base = flow::mut_base(*kid);
+            child_base.position.origin.x = x_offset;
+            child_base.position.size.width = remaining_width;
+            child_base.is_inorder = has_inorder_children;
 
-                if !child_node.is_inorder {
-                    child_node.floats_in = FloatContext::new(0);
-                }
+            if !child_base.is_inorder {
+                child_base.floats_in = FloatContext::new(0);
             }
         }
     }
 
-    pub fn assign_height_inorder_float(&mut self) {
-        debug!("assign_height_inorder_float: assigning height for float %?", self.common.id);
+    fn assign_height_inorder(&mut self, _: &mut LayoutContext) {
+        debug!("assign_height_inorder_float: assigning height for float %?", self.base.id);
         // assign_height_float was already called by the traversal function
         // so this is well-defined
 
@@ -181,28 +220,23 @@ impl FloatFlowData {
         let mut margin_height = Au(0);
 
         for box in self.box.iter() {
-            height = do box.with_base |base| {
-                base.position.size.height
-            };
-            clearance = match box.clear() {
+            let base = box.base();
+            height = base.position.borrow().ptr.size.height;
+            clearance = match base.clear() {
                 None => Au(0),
-                Some(clear) => {
-                    self.common.floats_in.clearance(clear)
-                }
+                Some(clear) => self.base.floats_in.clearance(clear),
             };
 
-            do box.with_base |base| {
-                let noncontent_width = base.model.padding.left + base.model.padding.right +
-                    base.model.border.left + base.model.border.right;
-
-                full_noncontent_width = noncontent_width + base.model.margin.left + base.model.margin.right;
-                margin_height = base.model.margin.top + base.model.margin.bottom;
-            }
+            let model = base.model.get();
+            let noncontent_width = model.padding.left + model.padding.right + model.border.left +
+                model.border.right;
 
+            full_noncontent_width = noncontent_width + model.margin.left + model.margin.right;
+            margin_height = model.margin.top + model.margin.bottom;
         }
 
         let info = PlacementInfo {
-            width: self.common.position.size.width + full_noncontent_width,
+            width: self.base.position.size.width + full_noncontent_width,
             height: height + margin_height,
             ceiling: clearance,
             max_width: self.containing_width,
@@ -211,23 +245,19 @@ impl FloatFlowData {
 
         // Place the float and return the FloatContext back to the parent flow.
         // After, grab the position and use that to set our position.
-        self.common.floats_out = self.common.floats_in.add_float(&info);
-        self.rel_pos = self.common.floats_out.last_float_pos();
+        self.base.floats_out = self.base.floats_in.add_float(&info);
+        self.rel_pos = self.base.floats_out.last_float_pos();
     }
 
-    pub fn assign_height_float(&mut self, ctx: &mut LayoutContext) {
-        debug!("assign_height_float: assigning height for float %?", self.common.id);
-        let has_inorder_children = self.common.num_floats > 0;
+    fn assign_height(&mut self, ctx: &mut LayoutContext) {
+        debug!("assign_height_float: assigning height for float %?", self.base.id);
+        let has_inorder_children = self.base.num_floats > 0;
         if has_inorder_children {
             let mut float_ctx = FloatContext::new(self.floated_children);
-            for kid in self.common.child_iter() {
-                do kid.with_mut_base |child_node| {
-                    child_node.floats_in = float_ctx.clone();
-                }
+            for kid in self.base.child_iter() {
+                flow::mut_base(*kid).floats_in = float_ctx.clone();
                 kid.assign_height_inorder(ctx);
-                do kid.with_mut_base |child_node| {
-                    float_ctx = child_node.floats_out.clone();
-                }
+                float_ctx = flow::mut_base(*kid).floats_out.clone();
             }
         }
 
@@ -235,83 +265,54 @@ impl FloatFlowData {
         let mut top_offset = Au(0);
 
         for &box in self.box.iter() {
-            do box.with_model |model| {
-                top_offset = model.margin.top + model.border.top + model.padding.top;
-                cur_y = cur_y + top_offset;
-            }
+            let base = box.base();
+            let model_ref = base.model.borrow();
+            top_offset = model_ref.ptr.margin.top + model_ref.ptr.border.top +
+                model_ref.ptr.padding.top;
+            cur_y = cur_y + top_offset;
         }
 
-        for kid in self.common.child_iter() {
-            do kid.with_mut_base |child_node| {
-                child_node.position.origin.y = cur_y;
-                cur_y = cur_y + child_node.position.size.height;
-            };
+        for kid in self.base.child_iter() {
+            let child_base = flow::mut_base(*kid);
+            child_base.position.origin.y = cur_y;
+            cur_y = cur_y + child_base.position.size.height;
         }
 
         let mut height = cur_y - top_offset;
 
-        let mut noncontent_width = Au(0);
-        let mut noncontent_height = Au(0);
-        for box in self.box.mut_iter() {
-            do box.with_mut_base |base| {
-                //The associated box is the border box of this flow
-                base.position.origin.y = base.model.margin.top;
+        let mut noncontent_height;
+        for box in self.box.iter() {
+            let base = box.base();
+            let mut model_ref = base.model.mutate();
+            let mut position_ref = base.position.mutate();
+            let (model, position) = (&mut model_ref.ptr, &mut position_ref.ptr);
 
-                noncontent_width = base.model.padding.left + base.model.padding.right +
-                    base.model.border.left + base.model.border.right;
-                noncontent_height = base.model.padding.top + base.model.padding.bottom +
-                    base.model.border.top + base.model.border.bottom;
-                base.position.size.height = height + noncontent_height;
+            // The associated box is the border box of this flow.
+            position.origin.y = model.margin.top;
 
-            }
-        }
+            noncontent_height = model.padding.top + model.padding.bottom + model.border.top +
+                model.border.bottom;
         
-        //TODO(eatkinson): compute heights properly using the 'height' property.
-        for &box in self.box.iter() {
-            let height_prop = 
-                MaybeAuto::from_style(box.style().Box.height,
-                                      Au(0)).specified_or_zero();
+            //TODO(eatkinson): compute heights properly using the 'height' property.
+            let height_prop = MaybeAuto::from_style(base.style().Box.height,
+                                                    Au::new(0)).specified_or_zero();
 
             height = geometry::max(height, height_prop) + noncontent_height;
             debug!("assign_height_float -- height: %?", height);
-            do box.with_mut_base |base| {
-                base.position.size.height = height;
-            }
-        }
-    }
-
-    pub fn build_display_list_float<E:ExtraDisplayListData>(&mut self,
-                                                            builder: &DisplayListBuilder,
-                                                            dirty: &Rect<Au>, 
-                                                            list: &Cell<DisplayList<E>>) 
-                                                            -> bool {
-
-        //TODO: implement iframe size messaging
-        if self.common.node.is_iframe_element() {
-            error!("float iframe size messaging not implemented yet");
-        }
-        let abs_rect = Rect(self.common.abs_position, self.common.position.size);
-        if !abs_rect.intersects(dirty) {
-            return true;
-        }
-
-
-        let offset = self.common.abs_position + self.rel_pos;
-        // add box that starts block context
-        for box in self.box.iter() {
-            box.build_display_list(builder, dirty, &offset, list)
-        }
-
-        // TODO: handle any out-of-flow elements
 
-        // go deeper into the flow tree
-        for child in self.common.child_iter() {
-            do child.with_mut_base |base| {
-                base.abs_position = offset + base.position.origin;
-            }
+            position.size.height = height;
         }
+    }
 
-        false
+    fn collapse_margins(&mut self,
+                        _: bool,
+                        _: &mut bool,
+                        _: &mut Au,
+                        _: &mut Au,
+                        collapsing: &mut Au,
+                        _: &mut Au) {
+        // Margins between a floated box and any other box do not collapse.
+        *collapsing = Au::new(0);
     }
 }
 
diff --git a/src/components/main/layout/float_context.rs b/src/components/main/layout/float_context.rs
index e9f39586081..5786cf369ae 100644
--- a/src/components/main/layout/float_context.rs
+++ b/src/components/main/layout/float_context.rs
@@ -11,7 +11,7 @@ use std::vec;
 use std::i32::max_value;
 
 #[deriving(Clone)]
-pub enum FloatType{
+pub enum FloatType {
     FloatLeft,
     FloatRight
 }
@@ -22,11 +22,12 @@ pub enum ClearType {
     ClearBoth
 }
 
-struct FloatContextBase{
-    float_data: ~[Option<FloatData>],
+struct FloatContextBase {
+    /// This is an option of a vector to avoid allocation in the fast path (no floats).
+    float_data: Option<~[Option<FloatData>]>,
     floats_used: uint,
-    max_y : Au,
-    offset: Point2D<Au>
+    max_y: Au,
+    offset: Point2D<Au>,
 }
 
 #[deriving(Clone)]
@@ -48,12 +49,12 @@ pub struct PlacementInfo{
 /// destroy the context on modification.
 pub enum FloatContext {
     Invalid,
-    Valid(~FloatContextBase)
+    Valid(FloatContextBase)
 }
 
 impl FloatContext {
     pub fn new(num_floats: uint) -> FloatContext {
-        Valid(~FloatContextBase::new(num_floats))
+        Valid(FloatContextBase::new(num_floats))
     }
 
     #[inline(always)]
@@ -68,7 +69,7 @@ impl FloatContext {
     fn with_mut_base<R>(&mut self, callback: &fn(&mut FloatContextBase) -> R) -> R {
         match *self {
             Invalid => fail!("Float context no longer available"),
-            Valid(ref mut base) => callback(&mut **base)
+            Valid(ref mut base) => callback(&mut *base)
         }
     }
 
@@ -76,7 +77,7 @@ impl FloatContext {
     pub fn with_base<R>(&self, callback: &fn(&FloatContextBase) -> R) -> R {
         match *self {
             Invalid => fail!("Float context no longer available"),
-            Valid(ref base) => callback(& **base)
+            Valid(ref base) => callback(&*base)
         }
     }
 
@@ -128,9 +129,12 @@ impl FloatContext {
 impl FloatContextBase{
     fn new(num_floats: uint) -> FloatContextBase {
         debug!("Creating float context of size %?", num_floats);
-        let new_data = vec::from_elem(num_floats, None);
         FloatContextBase {
-            float_data: new_data,
+            float_data: if num_floats == 0 {
+                None
+            } else {
+                Some(vec::from_elem(num_floats, None))
+            },
             floats_used: 0,
             max_y: Au(0),
             offset: Point2D(Au(0), Au(0))
@@ -144,7 +148,7 @@ impl FloatContextBase{
     fn last_float_pos(&self) -> Point2D<Au> {
         assert!(self.floats_used > 0, "Error: tried to access FloatContext with no floats in it");
 
-        match self.float_data[self.floats_used - 1] {
+        match self.float_data.get_ref()[self.floats_used - 1] {
             None => fail!("FloatContext error: floats should never be None here"),
             Some(float) => {
                 debug!("Returning float position: %?", float.bounds.origin + self.offset);
@@ -176,36 +180,38 @@ impl FloatContextBase{
         let mut r_bottom = None;
 
         // Find the float collisions for the given vertical range.
-        for float in self.float_data.iter() {
-            debug!("available_rect: Checking for collision against float");
-            match *float{
-                None => (),
-                Some(data) => {
-                    let float_pos = data.bounds.origin;
-                    let float_size = data.bounds.size;
-                    debug!("float_pos: %?, float_size: %?", float_pos, float_size);
-                    match data.f_type {
-                        FloatLeft => {
-                            if(float_pos.x + float_size.width > max_left && 
-                               float_pos.y + float_size.height > top && float_pos.y < top + height) {
-                                max_left = float_pos.x + float_size.width;
-                            
-                                l_top = Some(float_pos.y);
-                                l_bottom = Some(float_pos.y + float_size.height);
-
-                                debug!("available_rect: collision with left float: new max_left is %?",
-                                        max_left);
+        for floats in self.float_data.iter() {
+            for float in floats.iter() {
+                debug!("available_rect: Checking for collision against float");
+                match *float {
+                    None => (),
+                    Some(data) => {
+                        let float_pos = data.bounds.origin;
+                        let float_size = data.bounds.size;
+                        debug!("float_pos: %?, float_size: %?", float_pos, float_size);
+                        match data.f_type {
+                            FloatLeft => {
+                                if(float_pos.x + float_size.width > max_left && 
+                                   float_pos.y + float_size.height > top && float_pos.y < top + height) {
+                                    max_left = float_pos.x + float_size.width;
+                                
+                                    l_top = Some(float_pos.y);
+                                    l_bottom = Some(float_pos.y + float_size.height);
+
+                                    debug!("available_rect: collision with left float: new max_left is %?",
+                                            max_left);
+                                }
                             }
-                        }
-                        FloatRight => {
-                            if(float_pos.x < min_right && 
-                               float_pos.y + float_size.height > top && float_pos.y < top + height) {
-                                min_right = float_pos.x;
-
-                                r_top = Some(float_pos.y);
-                                r_bottom = Some(float_pos.y + float_size.height);
-                                debug!("available_rect: collision with right float: new min_right is %?",
-                                        min_right);
+                            FloatRight => {
+                                if(float_pos.x < min_right && 
+                                   float_pos.y + float_size.height > top && float_pos.y < top + height) {
+                                    min_right = float_pos.x;
+
+                                    r_top = Some(float_pos.y);
+                                    r_bottom = Some(float_pos.y + float_size.height);
+                                    debug!("available_rect: collision with right float: new min_right is %?",
+                                            min_right);
+                                }
                             }
                         }
                     }
@@ -242,9 +248,12 @@ impl FloatContextBase{
     }
 
     fn add_float(&mut self, info: &PlacementInfo) {
-        debug!("Floats_used: %?, Floats available: %?", self.floats_used, self.float_data.len());
-        assert!(self.floats_used < self.float_data.len() && 
-                self.float_data[self.floats_used].is_none());
+        assert!(self.float_data.is_some());
+        debug!("Floats_used: %?, Floats available: %?",
+               self.floats_used,
+               self.float_data.get_ref().len());
+        assert!(self.floats_used < self.float_data.get_ref().len() && 
+                self.float_data.get_ref()[self.floats_used].is_none());
 
         let new_info = PlacementInfo {
             width: info.width,
@@ -263,22 +272,24 @@ impl FloatContextBase{
             },
             f_type: info.f_type
         };
-        self.float_data[self.floats_used] = Some(new_float);
+        self.float_data.get_mut_ref()[self.floats_used] = Some(new_float);
         self.max_y = max(self.max_y, new_float.bounds.origin.y);
         self.floats_used += 1;
     }
 
     /// Returns true if the given rect overlaps with any floats.
     fn collides_with_float(&self, bounds: &Rect<Au>) -> bool {
-        for float in self.float_data.iter() {
-            match *float{
-                None => (),
-                Some(data) => {
-                    if data.bounds.translate(&self.offset).intersects(bounds) {
-                        return true;
+        for floats in self.float_data.iter() {
+            for float in floats.iter() {
+                match *float {
+                    None => (),
+                    Some(data) => {
+                        if data.bounds.translate(&self.offset).intersects(bounds) {
+                            return true;
+                        }
                     }
-                }
-            };
+                };
+            }
         }
 
         return false;
@@ -292,16 +303,18 @@ impl FloatContextBase{
         let left = left - self.offset.x;
         let mut max_height = None;
 
-        for float in self.float_data.iter() {
-            match *float {
-                None => (),
-                Some(f_data) => {
-                    if f_data.bounds.origin.y + f_data.bounds.size.height > top &&
-                       f_data.bounds.origin.x + f_data.bounds.size.width > left &&
-                       f_data.bounds.origin.x < left + width {
-                           let new_y = f_data.bounds.origin.y;
-                           max_height = Some(min(max_height.unwrap_or(new_y), new_y));
-                       }
+        for floats in self.float_data.iter() {
+            for float in floats.iter() {
+                match *float {
+                    None => (),
+                    Some(f_data) => {
+                        if f_data.bounds.origin.y + f_data.bounds.size.height > top &&
+                           f_data.bounds.origin.x + f_data.bounds.size.width > left &&
+                           f_data.bounds.origin.x < left + width {
+                               let new_y = f_data.bounds.origin.y;
+                               max_height = Some(min(max_height.unwrap_or(new_y), new_y));
+                           }
+                    }
                 }
             }
         }
@@ -361,19 +374,21 @@ impl FloatContextBase{
 
     fn clearance(&self, clear: ClearType) -> Au {
         let mut clearance = Au(0);
-        for float in self.float_data.iter() {
-            match *float {
-                None => (),
-                Some(f_data) => {
-                    match (clear, f_data.f_type) {
-                        (ClearLeft, FloatLeft) |
-                        (ClearRight, FloatRight) |
-                        (ClearBoth, _) => {
-                            clearance = max(
-                                clearance,
-                                self.offset.y + f_data.bounds.origin.y + f_data.bounds.size.height);
+        for floats in self.float_data.iter() {
+            for float in floats.iter() {
+                match *float {
+                    None => (),
+                    Some(f_data) => {
+                        match (clear, f_data.f_type) {
+                            (ClearLeft, FloatLeft) |
+                            (ClearRight, FloatRight) |
+                            (ClearBoth, _) => {
+                                clearance = max(
+                                    clearance,
+                                    self.offset.y + f_data.bounds.origin.y + f_data.bounds.size.height);
+                            }
+                            _ => ()
                         }
-                        _ => ()
                     }
                 }
             }
diff --git a/src/components/main/layout/flow.rs b/src/components/main/layout/flow.rs
index 273272fbe04..472950b4a96 100644
--- a/src/components/main/layout/flow.rs
+++ b/src/components/main/layout/flow.rs
@@ -25,207 +25,275 @@
 ///   line breaks and mapping to CSS boxes, for the purpose of handling `getClientRects()` and
 ///   similar methods.
 
-use layout::block::BlockFlowData;
-use layout::float::FloatFlowData;
+use css::node_style::StyledNode;
+use layout::block::BlockFlow;
 use layout::box::RenderBox;
 use layout::context::LayoutContext;
+use layout::float::FloatFlow;
 use layout::display_list_builder::{DisplayListBuilder, ExtraDisplayListData};
-use layout::inline::{InlineFlowData};
-use layout::float_context::{FloatContext, Invalid, FloatType};
+use layout::float_context::{FloatContext, Invalid};
 use layout::incremental::RestyleDamage;
-use css::node_style::StyledNode;
+use layout::inline::InlineFlow;
+
 use extra::dlist::{DList,MutDListIterator};
 use extra::container::Deque;
-
-use std::cell::Cell;
-use std::io::stderr;
 use geom::point::Point2D;
 use geom::rect::Rect;
 use gfx::display_list::DisplayList;
 use servo_util::geometry::Au;
 use script::dom::node::{AbstractNode, LayoutView};
+use std::cast;
+use std::cell::Cell;
 
-/// The type of the formatting context and data specific to each context, such as line box
-/// structures or float lists.
-pub enum FlowContext {
-    AbsoluteFlow(~FlowData), 
-    BlockFlow(~BlockFlowData),
-    FloatFlow(~FloatFlowData),
-    InlineBlockFlow(~FlowData),
-    InlineFlow(~InlineFlowData),
-    TableFlow(~FlowData),
-}
+/// Virtual methods that make up a float context.
+///
+/// Note that virtual methods have a cost; we should not overuse them in Servo. Consider adding
+/// methods to `ImmutableFlowUtils` or `MutableFlowUtils` before adding more methods here.
+pub trait FlowContext {
+    // RTTI
+    //
+    // TODO(pcwalton): Use Rust's RTTI, once that works.
 
-pub enum FlowContextType {
-    Flow_Absolute, 
-    Flow_Block,
-    Flow_Float(FloatType),
-    Flow_InlineBlock,
-    Flow_Inline,
-    Flow_Root,
-    Flow_Table
-}
+    /// Returns the class of flow that this is.
+    fn class(&self) -> FlowClass;
 
-impl FlowContext {
-    pub fn each_bu_sub_inorder (&mut self, callback: &fn(&mut FlowContext) -> bool) -> bool {
-        for kid in self.child_iter() {
-            // FIXME: Work around rust#2202. We should be able to pass the callback directly.
-            if !kid.each_bu_sub_inorder(|a| callback(a)) {
-                return false;
-            }
-        }
+    /// If this is a block flow, returns the underlying object. Fails otherwise.
+    fn as_block<'a>(&'a mut self) -> &'a mut BlockFlow {
+        fail!("called as_block() on a non-block flow")
+    }
 
-        if !self.is_inorder() {
-            callback(self)
-        } else {
-            true
-        }
+    /// If this is an inline flow, returns the underlying object, borrowed immutably. Fails
+    /// otherwise.
+    fn as_immutable_inline<'a>(&'a self) -> &'a InlineFlow {
+        fail!("called as_immutable_inline() on a non-inline flow")
     }
 
-    pub fn each_preorder_prune(&mut self, prune: &fn(&mut FlowContext) -> bool, 
-                               callback: &fn(&mut FlowContext) -> bool) 
-                               -> bool {
-        if prune(self) {
-            return true;
-        }
+    /// If this is an inline flow, returns the underlying object. Fails otherwise.
+    fn as_inline<'a>(&'a mut self) -> &'a mut InlineFlow {
+        fail!("called as_inline() on a non-inline flow")
+    }
 
-        if !callback(self) {
-            return false;
-        }
+    /// If this is a float flow, returns the underlying object. Fails otherwise.
+    fn as_float<'a>(&'a mut self) -> &'a mut FloatFlow {
+        fail!("called as_float() on a non-float flow")
+    }
 
-        for kid in self.child_iter() {
-            // FIXME: Work around rust#2202. We should be able to pass the callback directly.
-            if !kid.each_preorder_prune(|a| prune(a), |a| callback(a)) {
-                return false;
-            }
-        }
+    // Main methods
 
-        true
+    /// Pass 1 of reflow: computes minimum and preferred widths.
+    fn bubble_widths(&mut self, _ctx: &mut LayoutContext) {
+        fail!("bubble_widths not yet implemented")
     }
 
-    pub fn each_postorder_prune(&mut self, prune: &fn(&mut FlowContext) -> bool, 
-                                callback: &fn(&mut FlowContext) -> bool) 
-                                -> bool {
-        if prune(self) {
-            return true;
-        }
+    /// Pass 2 of reflow: computes width.
+    fn assign_widths(&mut self, _ctx: &mut LayoutContext) {
+        fail!("assign_widths not yet implemented")
+    }
 
-        for kid in self.child_iter() {
-            // FIXME: Work around rust#2202. We should be able to pass the callback directly.
-            if !kid.each_postorder_prune(|a| prune(a), |a| callback(a)) {
-                return false;
-            }
-        }
+    /// Pass 3 of reflow: computes height.
+    fn assign_height(&mut self, _ctx: &mut LayoutContext) {
+        fail!("assign_height not yet implemented")
+    }
 
-        callback(self)
+    /// In-order version of pass 3 of reflow: computes heights with floats present.
+    fn assign_height_inorder(&mut self, _ctx: &mut LayoutContext) {
+        fail!("assign_height_inorder not yet implemented")
     }
 
-    pub fn each_preorder(&mut self, callback: &fn(&mut FlowContext) -> bool) -> bool {
-        self.each_preorder_prune(|_| false, callback)
+    /// Collapses margins with the parent flow. This runs as part of assign-heights.
+    fn collapse_margins(&mut self,
+                        _top_margin_collapsible: bool,
+                        _first_in_flow: &mut bool,
+                        _margin_top: &mut Au,
+                        _top_offset: &mut Au,
+                        _collapsing: &mut Au,
+                        _collapsible: &mut Au) {
+        fail!("collapse_margins not yet implemented")
     }
 
-    pub fn each_postorder(&mut self, callback: &fn(&mut FlowContext) -> bool) -> bool {
-        self.each_postorder_prune(|_| false, callback)
+    /// Returns a debugging string describing this flow.
+    fn debug_str(&self) -> ~str {
+        ~"???"
     }
 }
 
-impl<'self> FlowContext {
-    pub fn is_block_like(&self) -> bool {
-        match *self {
-            BlockFlow(*) | FloatFlow(*) => true,
-            _ => false,
-        }
+// Base access
+
+#[inline(always)]
+pub fn base<'a>(this: &'a FlowContext) -> &'a FlowData {
+    unsafe {
+        let (_, ptr): (uint, &FlowData) = cast::transmute(this);
+        ptr
     }
+}
 
-    pub fn is_leaf(&self) -> bool {
-        do self.with_base |base| {
-            base.children.len() == 0
-        }
+#[inline(always)]
+pub fn mut_base<'a>(this: &'a mut FlowContext) -> &'a mut FlowData {
+    unsafe {
+        let (_, ptr): (uint, &mut FlowData) = cast::transmute(this);
+        ptr
     }
+}
 
-    pub fn add_new_child(&mut self, new_child: FlowContext) {
-        let cell = Cell::new(new_child);
-        do self.with_mut_base |base| {
-            base.children.push_back(cell.take());
+/// Returns the last child of this flow.
+pub fn last_child<'a>(flow: &'a mut FlowContext) -> Option<&'a mut ~FlowContext:> {
+    mut_base(flow).children.back_mut()
+}
+
+/// Iterates over the children of this flow.
+pub fn child_iter<'a>(flow: &'a mut FlowContext) -> MutDListIterator<'a,~FlowContext:> {
+    mut_base(flow).children.mut_iter()
+}
+
+pub trait ImmutableFlowUtils {
+    // Convenience functions
+
+    /// Returns true if this flow is a block or a float flow.
+    fn is_block_like(self) -> bool;
+
+    /// Returns true if this flow has no children.
+    fn is_leaf(self) -> bool;
+
+    /// Returns true if this flow is a block flow, an inline flow, or a float flow.
+    fn starts_block_flow(self) -> bool;
+
+    /// Returns true if this flow is an inline flow.
+    fn starts_inline_flow(self) -> bool;
+
+    /// Dumps the flow tree for debugging.
+    fn dump(self);
+}
+
+pub trait MutableFlowUtils {
+    // Traversals
+
+    /// Traverses the tree in preorder.
+    fn traverse_preorder<T:PreorderFlowTraversal>(self, traversal: &mut T) -> bool;
+
+    /// Traverses the tree in postorder.
+    fn traverse_postorder<T:PostorderFlowTraversal>(self, traversal: &mut T) -> bool;
+
+    // Mutators
+
+    /// Adds a new flow as a child of this flow.
+    fn add_new_child(self, new_child: ~FlowContext:);
+
+    /// Invokes a closure with the first child of this flow.
+    fn with_first_child<R>(self, f: &fn(Option<&mut ~FlowContext:>) -> R) -> R;
+
+    /// Invokes a closure with the last child of this flow.
+    fn with_last_child<R>(self, f: &fn(Option<&mut ~FlowContext:>) -> R) -> R;
+
+    /// Removes the first child of this flow and destroys it.
+    fn remove_first(self);
+
+    /// Removes the last child of this flow and destroys it.
+    fn remove_last(self);
+
+    /// Builds a display list for this flow and its children.
+    fn build_display_list<E:ExtraDisplayListData>(
+                          self,
+                          builder: &DisplayListBuilder,
+                          dirty: &Rect<Au>,
+                          list: &Cell<DisplayList<E>>)
+                          -> bool;
+}
+
+pub enum FlowClass {
+    AbsoluteFlowClass,
+    BlockFlowClass,
+    FloatFlowClass,
+    InlineBlockFlowClass,
+    InlineFlowClass,
+    TableFlowClass,
+}
+
+// Miscellaneous flows that are not yet implemented.
+
+pub struct AbsoluteFlow {
+    base: FlowData,
+}
+
+impl AbsoluteFlow {
+    pub fn new(base: FlowData) -> AbsoluteFlow {
+        AbsoluteFlow {
+            base: base,
         }
     }
+}
 
-    pub fn with_first_child<R>(&mut self, cb: &fn(Option<&mut FlowContext>) -> R) -> R {
-        do self.with_mut_base |base| {
-            cb(base.children.front_mut())
-        }
+impl FlowContext for AbsoluteFlow {
+    fn class(&self) -> FlowClass {
+        AbsoluteFlowClass
     }
+}
+
+pub struct InlineBlockFlow {
+    base: FlowData,
+}
 
-    pub fn with_last_child<R>(&mut self, cb: &fn(Option<&mut FlowContext>) -> R) -> R {
-        do self.with_mut_base |base| {
-            cb(base.children.back_mut())
+impl InlineBlockFlow {
+    pub fn new(base: FlowData) -> InlineBlockFlow {
+        InlineBlockFlow {
+            base: base,
         }
     }
+}
 
-    pub fn last_child(&'self mut self) -> Option<&'self mut FlowContext> {
-        self.mut_base().children.back_mut()
+impl FlowContext for InlineBlockFlow {
+    fn class(&self) -> FlowClass {
+        InlineBlockFlowClass
     }
+}
 
-    pub fn remove_first(&mut self) {
-        do self.with_mut_base |base| {
-            base.children.pop_front();
-        }
-    }
+pub struct TableFlow {
+    base: FlowData,
+}
 
-    pub fn remove_last(&mut self) {
-        do self.with_mut_base |base| {
-            base.children.pop_back();
+impl TableFlow {
+    pub fn new(base: FlowData) -> TableFlow {
+        TableFlow {
+            base: base,
         }
     }
+}
 
-    pub fn child_iter<'a>(&'a mut self) -> MutDListIterator<'a, FlowContext> {
-        self.mut_base().children.mut_iter()
+impl FlowContext for TableFlow {
+    fn class(&self) -> FlowClass {
+        TableFlowClass
     }
-
 }
 
-impl<'self> FlowContext {
-    pub fn with_base<R>(&self, callback: &fn(&FlowData) -> R) -> R {
-        match *self {
-            AbsoluteFlow(ref info) => callback(&**info),
-            BlockFlow(ref info) => {
-                callback(&info.common)
-            }
-            FloatFlow(ref info) => callback(&info.common),
-            InlineBlockFlow(ref info) => callback(&**info),
-            InlineFlow(ref info) => {
-                callback(&info.common)
-            }
-            TableFlow(ref info) => callback(&**info)
-        }
+/// A top-down traversal.
+pub trait PreorderFlowTraversal {
+    /// The operation to perform. Return true to continue or false to stop.
+    fn process(&mut self, flow: &mut FlowContext) -> bool;
+
+    /// Returns true if this node should be pruned. If this returns true, we skip the operation
+    /// entirely and do not process any descendant nodes. This is called *before* child nodes are
+    /// visited. The default implementation never prunes any nodes.
+    fn should_prune(&mut self, _flow: &mut FlowContext) -> bool {
+        false
     }
-    pub fn with_mut_base<R>(&mut self, callback: &fn(&mut FlowData) -> R) -> R {
-        match *self {
-            AbsoluteFlow(ref mut info) => callback(&mut **info),
-            BlockFlow(ref mut info) => {
-                callback(&mut info.common)
-            }
-            FloatFlow(ref mut info) => callback(&mut info.common),
-            InlineBlockFlow(ref mut info) => callback(&mut **info),
-            InlineFlow(ref mut info) => {
-                callback(&mut info.common)
-            }
-            TableFlow(ref mut info) => callback(&mut **info),
-        }
+}
+
+/// A bottom-up traversal, with a optional in-order pass.
+pub trait PostorderFlowTraversal {
+    /// The operation to perform. Return true to continue or false to stop.
+    fn process(&mut self, flow: &mut FlowContext) -> bool;
+
+    /// Returns false if this node must be processed in-order. If this returns false, we skip the
+    /// operation for this node, but continue processing the descendants. This is called *after*
+    /// child nodes are visited.
+    fn should_process(&mut self, _flow: &mut FlowContext) -> bool {
+        true
     }
-    pub fn mut_base(&'self mut self) -> &'self mut FlowData {
-        match *self {
-            AbsoluteFlow(ref mut info) => &mut(**info),
-            BlockFlow(ref mut info) => {
-                &mut info.common
-            }
-            FloatFlow(ref mut info) => &mut info.common,
-            InlineBlockFlow(ref mut info) => &mut(**info),
-            InlineFlow(ref mut info) => {
-                &mut info.common
-            }
-            TableFlow(ref mut info) => &mut(**info),
-        }
+
+    /// Returns true if this node should be pruned. If this returns true, we skip the operation
+    /// entirely and do not process any descendant nodes. This is called *before* child nodes are
+    /// visited. The default implementation never prunes any nodes.
+    fn should_prune(&mut self, _flow: &mut FlowContext) -> bool {
+        false
     }
 }
 
@@ -237,7 +305,7 @@ pub struct FlowData {
     node: AbstractNode<LayoutView>,
     restyle_damage: RestyleDamage,
 
-    children: DList<FlowContext>,
+    children: DList<~FlowContext:>,
 
     /* TODO (Issue #87): debug only */
     id: int,
@@ -256,11 +324,12 @@ pub struct FlowData {
 }
 
 pub struct BoxIterator {
-    priv boxes: ~[RenderBox],
+    priv boxes: ~[@RenderBox],
     priv index: uint,
 }
-impl Iterator<RenderBox> for BoxIterator {
-    fn next(&mut self) -> Option<RenderBox> {
+
+impl Iterator<@RenderBox> for BoxIterator {
+    fn next(&mut self) -> Option<@RenderBox> {
         if self.index >= self.boxes.len() {
             None
         } else {
@@ -270,6 +339,7 @@ impl Iterator<RenderBox> for BoxIterator {
         }
     }
 }
+
 impl FlowData {
     pub fn new(id: int, node: AbstractNode<LayoutView>) -> FlowData {
         FlowData {
@@ -280,237 +350,135 @@ impl FlowData {
 
             id: id,
 
-            min_width: Au(0),
-            pref_width: Au(0),
+            min_width: Au::new(0),
+            pref_width: Au::new(0),
             position: Au::zero_rect(),
             floats_in: Invalid,
             floats_out: Invalid,
             num_floats: 0,
-            abs_position: Point2D(Au(0), Au(0)),
+            abs_position: Point2D(Au::new(0), Au::new(0)),
             is_inorder: false
         }
     }
 
-    pub fn child_iter<'a>(&'a mut self) -> MutDListIterator<'a, FlowContext> {
+    pub fn child_iter<'a>(&'a mut self) -> MutDListIterator<'a,~FlowContext:> {
         self.children.mut_iter()
     }
-
 }
 
-impl<'self> FlowContext {
-    /// A convenience method to return the position of this flow. Fails if the flow is currently
-    /// being borrowed mutably.
-    #[inline(always)]
-    pub fn position(&self) -> Rect<Au> {
-        do self.with_base |common_info| {
-            common_info.position
-        }
-    }
-
-    #[inline(always)]
-    pub fn is_inorder(&self) -> bool {
-        do self.with_base |common_info| {
-            common_info.is_inorder
+impl<'self> ImmutableFlowUtils for &'self FlowContext {
+    /// Returns true if this flow is a block or a float flow.
+    fn is_block_like(self) -> bool {
+        match self.class() {
+            BlockFlowClass | FloatFlowClass => true,
+            AbsoluteFlowClass | InlineBlockFlowClass | InlineFlowClass | TableFlowClass => false,
         }
     }
 
-    /// A convenience method to return the ID of this flow. Fails if the flow is currently being
-    /// borrowed mutably.
-    #[inline(always)]
-    pub fn id(&self) -> int {
-        do self.with_base |info| {
-            info.id
-        }
+    /// Returns true if this flow has no children.
+    fn is_leaf(self) -> bool {
+        base(self).children.len() == 0
     }
 
-    pub fn inline(&'self mut self) -> &'self mut InlineFlowData {
-        match *self {
-            InlineFlow(ref mut info) => &mut (**info),
-            _ => fail!(fmt!("Tried to access inline data of non-inline: f%d", self.id()))
+    /// Returns true if this flow is a block flow, an inline-block flow, or a float flow.
+    fn starts_block_flow(self) -> bool {
+        match self.class() {
+            BlockFlowClass | InlineBlockFlowClass | FloatFlowClass => true,
+            AbsoluteFlowClass | InlineFlowClass | TableFlowClass => false,
         }
     }
 
-    pub fn imm_inline(&'self self) -> &'self InlineFlowData {
-        match *self {
-            InlineFlow(ref info) => &**info,
-            _ => fail!(fmt!("Tried to access inline data of non-inline: f%d", self.id()))
+    /// Returns true if this flow is a block flow, an inline flow, or a float flow.
+    fn starts_inline_flow(self) -> bool {
+        match self.class() {
+            InlineFlowClass => true,
+            AbsoluteFlowClass | BlockFlowClass | FloatFlowClass | InlineBlockFlowClass |
+            TableFlowClass => false,
         }
     }
 
-    pub fn block(&'self mut self) -> &'self mut BlockFlowData {
-        match *self {
-            BlockFlow(ref mut info) => &mut (**info),
-            _ => fail!(fmt!("Tried to access block data of non-block: f%d", self.id()))
-        }
+    /// Dumps the flow tree for debugging.
+    fn dump(self) {
+        // TODO(pcwalton): Fill this in.
     }
+}
 
-    pub fn root(&'self mut self) -> &'self mut BlockFlowData {
-        match *self {
-            BlockFlow(ref mut info) if info.is_root => &mut (**info),
-            _ => fail!(fmt!("Tried to access root block data of non-root: f%d", self.id()))
+impl<'self> MutableFlowUtils for &'self mut FlowContext {
+    /// Traverses the tree in preorder.
+    fn traverse_preorder<T:PreorderFlowTraversal>(self, traversal: &mut T) -> bool {
+        if traversal.should_prune(self) {
+            return true
         }
-    }
 
-    pub fn bubble_widths(&mut self, ctx: &mut LayoutContext) {
-
-        debug!("FlowContext: bubbling widths for f%?", self.id());
-        match *self {
-            BlockFlow(ref mut info)  => info.bubble_widths_block(ctx),
-            InlineFlow(ref mut info) => info.bubble_widths_inline(ctx),
-            FloatFlow(ref mut info)  => info.bubble_widths_float(ctx),
-            _ => fail!(fmt!("Tried to bubble_widths of flow: f%d", self.id()))
+        if !traversal.process(self) {
+            return false
         }
-    }
 
-    pub fn assign_widths(&mut self, ctx: &mut LayoutContext) {
-
-        debug!("FlowContext: assigning widths for f%?", self.id());
-        match *self {
-            BlockFlow(ref mut info)  => info.assign_widths_block(ctx),
-            InlineFlow(ref mut info) => info.assign_widths_inline(ctx),
-            FloatFlow(ref mut info)  => info.assign_widths_float(),
-            _ => fail!(fmt!("Tried to assign_widths of flow: f%d", self.id()))
+        for kid in child_iter(self) {
+            if !kid.traverse_preorder(traversal) {
+                return false
+            }
         }
-    }
-
-    pub fn assign_height(&mut self, ctx: &mut LayoutContext) {
 
-        debug!("FlowContext: assigning height for f%?", self.id());
-        match *self {
-            BlockFlow(ref mut info)  => info.assign_height_block(ctx),
-            InlineFlow(ref mut info) => info.assign_height_inline(ctx),
-            FloatFlow(ref mut info)  => info.assign_height_float(ctx),
-            _ => fail!(fmt!("Tried to assign_height of flow: f%d", self.id()))
-        }
+        true
     }
 
-    pub fn assign_height_inorder(&mut self, ctx: &mut LayoutContext) {
-        match *self {
-            BlockFlow(ref mut info)  => info.assign_height_inorder_block(ctx),
-            InlineFlow(ref mut info) => info.assign_height_inorder_inline(ctx),
-            FloatFlow(ref mut info)  => info.assign_height_inorder_float(),
-            _ => fail!(fmt!("Tried to assign_height of flow: f%d", self.id()))
+    /// Traverses the tree in postorder.
+    fn traverse_postorder<T:PostorderFlowTraversal>(self, traversal: &mut T) -> bool {
+        if traversal.should_prune(self) {
+            return true
         }
-    }
 
-    pub fn build_display_list<E:ExtraDisplayListData>(&mut self,
-                                                     builder: &DisplayListBuilder,
-                                                     dirty: &Rect<Au>,
-                                                     list: &Cell<DisplayList<E>>)
-                                                     -> bool {
-
-        
-        debug!("FlowContext: building display list for f%?", self.id());
-        match *self {
-            BlockFlow(ref mut info)  => info.build_display_list_block(builder, dirty, list),
-            InlineFlow(ref mut info) => info.build_display_list_inline(builder, dirty, list),
-            FloatFlow(ref mut info)  => info.build_display_list_float(builder, dirty, list),
-            _ => {
-                fail!("Tried to build_display_list_recurse of flow: %?", self)
+        for kid in child_iter(self) {
+            if !kid.traverse_postorder(traversal) {
+                return false
             }
         }
 
-    }
-    /// A convenience method to return the restyle damage of this flow. Fails if the flow is
-    /// currently being borrowed mutably.
-    #[inline(always)]
-    pub fn restyle_damage(&self) -> RestyleDamage {
-        do self.with_base |info| {
-            info.restyle_damage
+        if !traversal.should_process(self) {
+            return true
         }
-    }
 
-
-    // Actual methods that do not require much flow-specific logic
-    pub fn foldl_all_boxes<B:Clone>(&mut self, seed: B, cb: &fn(a: B, b: RenderBox) -> B) -> B {
-        match *self {
-            BlockFlow(ref mut block) => {
-                do block.box.as_ref().map_default(seed.clone()) |box| {
-                    cb(seed.clone(), *box)
-                }
-            }
-            InlineFlow(ref mut inline) => {
-                do inline.boxes.iter().fold(seed) |acc, box| {
-                    cb(acc.clone(), *box)
-                }
-            }
-            _ => fail!(fmt!("Don't know how to iterate node's RenderBoxes for %?", self)),
-        }
+        traversal.process(self)
     }
 
-    pub fn foldl_boxes_for_node<B:Clone>(&mut self,
-                                        node: AbstractNode<LayoutView>,
-                                        seed: B,
-                                        callback: &fn(a: B, RenderBox) -> B)
-                                        -> B {
-        do self.foldl_all_boxes(seed) |acc, box| {
-            if box.node() == node {
-                callback(acc, box)
-            } else {
-                acc
-            }
-        }
+    /// Adds a new flow as a child of this flow.
+    fn add_new_child(self, new_child: ~FlowContext:) {
+        mut_base(self).children.push_back(new_child)
     }
 
-    pub fn iter_all_boxes(&mut self) -> BoxIterator {
-        BoxIterator {
-            boxes: match *self {
-                BlockFlow(ref mut block) => block.box.as_ref().map_default(~[], |&x| ~[x]),
-                InlineFlow(ref mut inline) => inline.boxes.clone(),
-                _ => fail!(fmt!("Don't know how to iterate node's RenderBoxes for %?", self))
-            },
-            index: 0,
-        }
+    /// Invokes a closure with the first child of this flow.
+    fn with_first_child<R>(self, f: &fn(Option<&mut ~FlowContext:>) -> R) -> R {
+        f(mut_base(self).children.front_mut())
     }
 
-    /// Dumps the flow tree for debugging.
-    pub fn dump(&mut self) {
-        self.dump_indent(0);
+    /// Invokes a closure with the last child of this flow.
+    fn with_last_child<R>(self, f: &fn(Option<&mut ~FlowContext:>) -> R) -> R {
+        f(mut_base(self).children.back_mut())
     }
 
-    /// Dumps the flow tree, for debugging, with indentation.
-    pub fn dump_indent(&mut self, indent: uint) {
-        let mut s = ~"|";
-        for _ in range(0, indent) {
-            s.push_str("---- ");
-        }
-
-        s.push_str(self.debug_str());
-        stderr().write_line(s);
+    /// Removes the first child of this flow and destroys it.
+    fn remove_first(self) {
+        let _ = mut_base(self).children.pop_front();
+    }
 
-        // FIXME: this should have a pure/const version?
-        for child in self.child_iter() {
-            child.dump_indent(indent + 1)
-        }
+    /// Removes the last child of this flow and destroys it.
+    fn remove_last(self) {
+        let _ = mut_base(self).children.pop_back();
     }
-    
-    pub fn debug_str(&self) -> ~str {
-        let repr = match *self {
-            InlineFlow(ref inline) => {
-                let mut s = inline.boxes.iter().fold(~"InlineFlow(children=", |s, box| {
-                    fmt!("%s b%d", s, box.id())
-                });
-                s.push_str(")");
-                s
-            },
-            BlockFlow(ref block) => {
-                match block.box {
-                    Some(box) => fmt!("BlockFlow(box=b%d)", box.id()),
-                    None => ~"BlockFlow",
-                }
-            },
-            FloatFlow(ref float) => {
-                match float.box {
-                    Some(box) => fmt!("FloatFlow(box=b%d)", box.id()),
-                    None => ~"FloatFlow",
-                }
-            },
-            _ => ~"(Unknown flow)"
-        };
-
-        do self.with_base |base| {
-            fmt!("f%? %? floats %? size %? damage %?", base.id, repr, base.num_floats,
-                 base.position, base.restyle_damage)
+
+    fn build_display_list<E:ExtraDisplayListData>(
+                          self,
+                          builder: &DisplayListBuilder,
+                          dirty: &Rect<Au>,
+                          list: &Cell<DisplayList<E>>)
+                          -> bool {
+        debug!("FlowContext: building display list for f%?", base(self).id);
+        match self.class() {
+            BlockFlowClass => self.as_block().build_display_list_block(builder, dirty, list),
+            InlineFlowClass => self.as_inline().build_display_list_inline(builder, dirty, list),
+            FloatFlowClass => self.as_float().build_display_list_float(builder, dirty, list),
+            _ => fail!("Tried to build_display_list_recurse of flow: %?", self),
         }
     }
 }
diff --git a/src/components/main/layout/inline.rs b/src/components/main/layout/inline.rs
index cad2ec09a2f..037b782edf0 100644
--- a/src/components/main/layout/inline.rs
+++ b/src/components/main/layout/inline.rs
@@ -3,28 +3,27 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use css::node_style::StyledNode;
-use std::cell::Cell;
 use layout::box::{CannotSplit, GenericRenderBoxClass, ImageRenderBoxClass, RenderBox};
-use layout::box::{SplitDidFit, SplitDidNotFit, TextRenderBoxClass};
+use layout::box::{RenderBoxUtils, SplitDidFit, SplitDidNotFit, TextRenderBoxClass};
 use layout::context::LayoutContext;
 use layout::display_list_builder::{DisplayListBuilder, ExtraDisplayListData};
-use layout::flow::{FlowContext, FlowData, InlineFlow};
+use layout::flow::{FlowClass, FlowContext, FlowData, InlineFlowClass};
+use layout::flow;
 use layout::float_context::FloatContext;
 use layout::util::{ElementMapping};
 use layout::float_context::{PlacementInfo, FloatLeft};
 
-use std::u16;
-use std::util;
+use extra::container::Deque;
+use extra::ringbuf::RingBuf;
 use geom::{Point2D, Rect, Size2D};
 use gfx::display_list::DisplayList;
-use servo_util::geometry::Au;
-use style::computed_values::line_height;
 use style::computed_values::text_align;
 use style::computed_values::vertical_align;
+use servo_util::geometry::Au;
 use servo_util::range::Range;
-use servo_util::tree::TreeNodeRef;
-use extra::container::Deque;
-use extra::ringbuf::RingBuf;
+use std::cell::Cell;
+use std::u16;
+use std::util;
 
 /*
 Lineboxes are represented as offsets into the child list, rather than
@@ -62,13 +61,15 @@ struct LineBox {
 
 struct LineboxScanner {
     floats: FloatContext,
-    new_boxes: ~[RenderBox],
-    work_list: @mut RingBuf<RenderBox>,
+    new_boxes: ~[@RenderBox],
+    work_list: @mut RingBuf<@RenderBox>,
     pending_line: LineBox,
     lines: ~[LineBox],
     cur_y: Au,
 }
 
+local_data_key!(local_linebox_scanner: LineboxScanner)
+
 impl LineboxScanner {
     pub fn new(float_ctx: FloatContext) -> LineboxScanner {
         LineboxScanner {
@@ -77,33 +78,45 @@ impl LineboxScanner {
             work_list: @mut RingBuf::new(),
             pending_line: LineBox {
                 range: Range::empty(), 
-                bounds: Rect(Point2D(Au(0), Au(0)), Size2D(Au(0), Au(0))), 
-                green_zone: Size2D(Au(0), Au(0))
+                bounds: Rect(Point2D(Au::new(0), Au::new(0)), Size2D(Au::new(0), Au::new(0))), 
+                green_zone: Size2D(Au::new(0), Au::new(0))
             },
             lines: ~[],
-            cur_y: Au(0)
+            cur_y: Au::new(0)
         }
     }
+
+    fn reinitialize(&mut self, float_ctx: FloatContext) {
+        self.floats = float_ctx;
+        self.new_boxes.truncate(0);
+        self.work_list.clear();
+        self.pending_line.range = Range::empty();
+        self.pending_line.bounds = Rect(Point2D(Au::new(0), Au::new(0)),
+                                        Size2D(Au::new(0), Au::new(0)));
+        self.pending_line.green_zone = Size2D(Au::new(0), Au::new(0));
+        self.lines.truncate(0);
+        self.cur_y = Au::new(0);
+    }
     
     pub fn floats_out(&mut self) -> FloatContext {
         self.floats.clone()
     }
 
-    fn reset_scanner(&mut self, flow: &mut InlineFlowData) {
-        debug!("Resetting line box scanner's state for flow f%d.", flow.common.id);
+    fn reset_scanner(&mut self, flow: &mut InlineFlow) {
+        debug!("Resetting line box scanner's state for flow f%d.", flow.base.id);
         self.lines = ~[];
         self.new_boxes = ~[];
-        self.cur_y = Au(0);
+        self.cur_y = Au::new(0);
         self.reset_linebox();
     }
 
     fn reset_linebox(&mut self) {
         self.pending_line.range.reset(0,0);
-        self.pending_line.bounds = Rect(Point2D(Au(0), self.cur_y), Size2D(Au(0), Au(0)));
-        self.pending_line.green_zone = Size2D(Au(0), Au(0))     
+        self.pending_line.bounds = Rect(Point2D(Au::new(0), self.cur_y), Size2D(Au::new(0), Au::new(0)));
+        self.pending_line.green_zone = Size2D(Au::new(0), Au::new(0))
     }
 
-    pub fn scan_for_lines(&mut self, flow: &mut InlineFlowData) {
+    pub fn scan_for_lines(&mut self, flow: &mut InlineFlow) {
         self.reset_scanner(flow);
 
         let mut i = 0u;
@@ -115,11 +128,11 @@ impl LineboxScanner {
                     break
                 }
                 let box = flow.boxes[i]; i += 1;
-                debug!("LineboxScanner: Working with box from box list: b%d", box.id());
+                debug!("LineboxScanner: Working with box from box list: b%d", box.base().id());
                 box
             } else {
-                    let box = self.work_list.pop_front().unwrap();
-                debug!("LineboxScanner: Working with box from work list: b%d", box.id());
+                let box = self.work_list.pop_front().unwrap();
+                debug!("LineboxScanner: Working with box from work list: b%d", box.base().id());
                 box
             };
 
@@ -139,16 +152,15 @@ impl LineboxScanner {
             self.flush_current_line();
         }
 
-
         flow.elems.repair_for_box_changes(flow.boxes, self.new_boxes);
 
         self.swap_out_results(flow);
     }
 
-    fn swap_out_results(&mut self, flow: &mut InlineFlowData) {
+    fn swap_out_results(&mut self, flow: &mut InlineFlow) {
         debug!("LineboxScanner: Propagating scanned lines[n=%u] to inline flow f%d",
                self.lines.len(),
-               flow.common.id);
+               flow.base.id);
 
         util::swap(&mut flow.boxes, &mut self.new_boxes);
         util::swap(&mut flow.lines, &mut self.lines);
@@ -165,47 +177,10 @@ impl LineboxScanner {
         self.reset_linebox();
     }
 
-    fn calculate_line_height(&self, box: RenderBox, font_size: Au) -> Au { 
-        match box.line_height() {
-            line_height::Normal => font_size.scale_by(1.14),
-            line_height::Number(l) => font_size.scale_by(l),
-            line_height::Length(l) => l
-        }
-    }
-
-    fn box_height(&self, box: RenderBox) -> Au {
-        match box {
-            ImageRenderBoxClass(image_box) => {
-                let size = image_box.image.get_size();
-                let height = Au::from_px(size.unwrap_or(Size2D(0, 0)).height);
-                image_box.base.position.size.height = height;
-                debug!("box_height: found image height: %?", height);
-                height
-            }
-            TextRenderBoxClass(text_box) => {
-                let range = &text_box.range;
-                let run = &text_box.run;
-
-                // Compute the height based on the line-height and font size
-                let text_bounds = run.metrics_for_range(range).bounding_box;
-                let em_size = text_bounds.size.height;
-                let line_height = self.calculate_line_height(box, em_size);
-
-                line_height
-            }
-            GenericRenderBoxClass(_) => {
-                Au(0)
-            }
-            _ => {
-                fail!(fmt!("Tried to get height of unknown Box variant: %s", box.debug_str()))
-            }
-        }
-    }
-
     // FIXME(eatkinson): this assumes that the tallest box in the line determines the line height
     // This might not be the case with some weird text fonts.
-    fn new_height_for_line(&self, new_box: RenderBox) -> Au {
-        let box_height = self.box_height(new_box);
+    fn new_height_for_line(&self, new_box: &RenderBox) -> Au {
+        let box_height = new_box.box_height();
         if box_height > self.pending_line.bounds.size.height {
             box_height
         } else {
@@ -216,9 +191,12 @@ impl LineboxScanner {
     /// Computes the position of a line that has only the provided RenderBox.
     /// Returns: the bounding rect of the line's green zone (whose origin coincides
     /// with the line's origin) and the actual width of the first box after splitting.
-    fn initial_line_placement (&self, first_box: RenderBox, ceiling: Au, flow: &mut InlineFlowData) -> (Rect<Au>, Au) {
+    fn initial_line_placement(&self, first_box: @RenderBox, ceiling: Au, flow: &mut InlineFlow)
+                              -> (Rect<Au>, Au) {
         debug!("LineboxScanner: Trying to place first box of line %?", self.lines.len());
-        debug!("LineboxScanner: box size: %?", first_box.position().size);
+
+        let first_box_size = first_box.base().position.get().size;
+        debug!("LineboxScanner: box size: %?", first_box_size);
         let splitable = first_box.can_split();
         let line_is_empty: bool = self.pending_line.range.length() == 0;
 
@@ -226,34 +204,36 @@ impl LineboxScanner {
         // We will move it later if it has nonzero width
         // and that causes problems.
         let placement_width = if splitable {
-            Au(0)
+            Au::new(0)
         } else {
-            first_box.position().size.width
+            first_box_size.width
         };
 
         let mut info = PlacementInfo {
             width: placement_width,
-            height: first_box.position().size.height,
+            height: first_box_size.height,
             ceiling: ceiling,
-            max_width: flow.common.position.size.width,
+            max_width: flow.base.position.size.width,
             f_type: FloatLeft
         };
 
         let line_bounds = self.floats.place_between_floats(&info);
 
-        debug!("LineboxScanner: found position for line: %? using placement_info: %?", line_bounds, info);
+        debug!("LineboxScanner: found position for line: %? using placement_info: %?",
+               line_bounds,
+               info);
         
         // Simple case: if the box fits, then we can stop here
-        if line_bounds.size.width > first_box.position().size.width {
+        if line_bounds.size.width > first_box_size.width {
             debug!("LineboxScanner: case=box fits");
-            return (line_bounds, first_box.position().size.width);
+            return (line_bounds, first_box_size.width);
         }
 
         // If not, but we can't split the box, then we'll place
         // the line here and it will overflow.
         if !splitable {
             debug!("LineboxScanner: case=line doesn't fit, but is unsplittable");
-            return (line_bounds, first_box.position().size.width);
+            return (line_bounds, first_box_size.width);
         }
 
         // Otherwise, try and split the box
@@ -264,15 +244,15 @@ impl LineboxScanner {
             CannotSplit(_) => {
                 error!("LineboxScanner: Tried to split unsplittable render box! %s",
                         first_box.debug_str());
-                return (line_bounds, first_box.position().size.width);
+                return (line_bounds, first_box_size.width);
             }
             SplitDidFit(left, right) => {
 
                 debug!("LineboxScanner: case=box split and fit");
                 let actual_box_width = match (left, right) {
-                    (Some(l_box), Some(_))  => l_box.position().size.width,
-                    (Some(l_box), None)     => l_box.position().size.width,
-                    (None, Some(r_box))     => r_box.position().size.width,
+                    (Some(l_box), Some(_))  => l_box.base().position.get().size.width,
+                    (Some(l_box), None)     => l_box.base().position.get().size.width,
+                    (None, Some(r_box))     => r_box.base().position.get().size.width,
                     (None, None)            => fail!("This case makes no sense.")
                 };
                 return (line_bounds, actual_box_width);
@@ -284,9 +264,9 @@ impl LineboxScanner {
 
                 debug!("LineboxScanner: case=box split and fit didn't fit; trying to push it down");
                 let actual_box_width = match (left, right) {
-                    (Some(l_box), Some(_))  => l_box.position().size.width,
-                    (Some(l_box), None)     => l_box.position().size.width,
-                    (None, Some(r_box))     => r_box.position().size.width,
+                    (Some(l_box), Some(_))  => l_box.base().position.get().size.width,
+                    (Some(l_box), None)     => l_box.base().position.get().size.width,
+                    (None, Some(r_box))     => r_box.base().position.get().size.width,
                     (None, None)            => fail!("This case makes no sense.")
                 };
 
@@ -301,7 +281,7 @@ impl LineboxScanner {
     }
 
     /// Returns false only if we should break the line.
-    fn try_append_to_line(&mut self, in_box: RenderBox, flow: &mut InlineFlowData) -> bool {
+    fn try_append_to_line(&mut self, in_box: @RenderBox, flow: &mut InlineFlow) -> bool {
         let line_is_empty: bool = self.pending_line.range.length() == 0;
 
         if line_is_empty {
@@ -313,7 +293,7 @@ impl LineboxScanner {
         debug!("LineboxScanner: Trying to append box to line %u (box size: %?, green zone: \
                 %?): %s",
                self.lines.len(),
-               in_box.position().size,
+               in_box.base().position.get().size,
                self.pending_line.green_zone,
                in_box.debug_str());
 
@@ -326,6 +306,8 @@ impl LineboxScanner {
 
         let new_height = self.new_height_for_line(in_box);
         if new_height > green_zone.height {
+            debug!("LineboxScanner: entering float collision avoider!");
+
             // Uh-oh. Adding this box is going to increase the height,
             // and because of that we will collide with some floats.
 
@@ -367,9 +349,10 @@ impl LineboxScanner {
         // horizontally. We'll try to place the whole box on this line and break somewhere
         // if it doesn't fit.
 
-        let new_width = self.pending_line.bounds.size.width + in_box.position().size.width;
+        let new_width = self.pending_line.bounds.size.width +
+            in_box.base().position.get().size.width;
 
-        if(new_width <= green_zone.width){
+        if new_width <= green_zone.width {
             debug!("LineboxScanner: case=box fits without splitting");
             self.push_box_to_line(in_box);
             return true;
@@ -441,28 +424,29 @@ impl LineboxScanner {
     }
 
     // unconditional push
-    fn push_box_to_line(&mut self, box: RenderBox) {
-        debug!("LineboxScanner: Pushing box b%d to line %u", box.id(), self.lines.len());
+    fn push_box_to_line(&mut self, box: @RenderBox) {
+        debug!("LineboxScanner: Pushing box b%d to line %u", box.base().id(), self.lines.len());
 
         if self.pending_line.range.length() == 0 {
             assert!(self.new_boxes.len() <= (u16::max_value as uint));
             self.pending_line.range.reset(self.new_boxes.len(), 0);
         }
         self.pending_line.range.extend_by(1);
-        self.pending_line.bounds.size.width = self.pending_line.bounds.size.width + box.position().size.width;
+        self.pending_line.bounds.size.width = self.pending_line.bounds.size.width +
+            box.base().position.get().size.width;
         self.pending_line.bounds.size.height = Au::max(self.pending_line.bounds.size.height, 
-                                                             box.position().size.height);
+                                                       box.base().position.get().size.height);
         self.new_boxes.push(box);
     }
 }
 
-pub struct InlineFlowData {
+pub struct InlineFlow {
     /// Data common to all flows.
-    common: FlowData,
+    base: FlowData,
 
     // A vec of all inline render boxes. Several boxes may
     // correspond to one Node/Element.
-    boxes: ~[RenderBox],
+    boxes: ~[@RenderBox],
     // vec of ranges into boxes that represents line positions.
     // these ranges are disjoint, and are the result of inline layout.
     // also some metadata used for positioning lines
@@ -473,10 +457,10 @@ pub struct InlineFlowData {
     elems: ElementMapping
 }
 
-impl InlineFlowData {
-    pub fn new(common: FlowData) -> InlineFlowData {
-        InlineFlowData {
-            common: common,
+impl InlineFlow {
+    pub fn new(base: FlowData) -> InlineFlow {
+        InlineFlow {
+            base: base,
             boxes: ~[],
             lines: ~[],
             elems: ElementMapping::new(),
@@ -489,87 +473,103 @@ impl InlineFlowData {
         }
         self.boxes = ~[];
     }
-}
 
-pub trait InlineLayout {
-    fn starts_inline_flow(&self) -> bool;
-}
+    pub fn build_display_list_inline<E:ExtraDisplayListData>(&self,
+                                                             builder: &DisplayListBuilder,
+                                                             dirty: &Rect<Au>,
+                                                             list: &Cell<DisplayList<E>>)
+                                                             -> bool {
 
-impl InlineLayout for FlowContext {
-    fn starts_inline_flow(&self) -> bool {
-        match *self {
-            InlineFlow(*) => true,
-            _ => false
+        //TODO: implement inline iframe size messaging
+        if self.base.node.is_iframe_element() {
+            error!("inline iframe size messaging not implemented yet");
         }
+
+        let abs_rect = Rect(self.base.abs_position, self.base.position.size);
+        if !abs_rect.intersects(dirty) {
+            return true;
+        }
+
+        // TODO(#228): Once we form line boxes and have their cached bounds, we can be smarter and
+        // not recurse on a line if nothing in it can intersect the dirty region.
+        debug!("FlowContext[%d]: building display list for %u inline boxes",
+               self.base.id,
+               self.boxes.len());
+
+        for box in self.boxes.iter() {
+            box.build_display_list(builder, dirty, &self.base.abs_position, list)
+        }
+
+        // TODO(#225): Should `inline-block` elements have flows as children of the inline flow or
+        // should the flow be nested inside the box somehow?
+        
+        // For now, don't traverse the subtree rooted here
+        true
     }
 }
 
-impl InlineFlowData {
-    pub fn bubble_widths_inline(&mut self, ctx: &mut LayoutContext) {
+impl FlowContext for InlineFlow {
+    fn class(&self) -> FlowClass {
+        InlineFlowClass
+    }
+
+    fn as_immutable_inline<'a>(&'a self) -> &'a InlineFlow {
+        self
+    }
+
+    fn as_inline<'a>(&'a mut self) -> &'a mut InlineFlow {
+        self
+    }
+
+    fn bubble_widths(&mut self, _: &mut LayoutContext) {
         let mut num_floats = 0;
 
-        for kid in self.common.child_iter() {
-            do kid.with_mut_base |base| {
-                num_floats += base.num_floats;
-                base.floats_in = FloatContext::new(base.num_floats);
-            }
+        for kid in self.base.child_iter() {
+            let child_base = flow::mut_base(*kid);
+            num_floats += child_base.num_floats;
+            child_base.floats_in = FloatContext::new(child_base.num_floats);
         }
 
         {
             let this = &mut *self;
 
-            let mut min_width = Au(0);
-            let mut pref_width = Au(0);
+            let mut min_width = Au::new(0);
+            let mut pref_width = Au::new(0);
 
             for box in this.boxes.iter() {
-                debug!("FlowContext[%d]: measuring %s", self.common.id, box.debug_str());
-                min_width = Au::max(min_width, box.get_min_width(ctx));
-                pref_width = Au::max(pref_width, box.get_pref_width(ctx));
+                debug!("FlowContext[%d]: measuring %s", self.base.id, box.debug_str());
+                let (this_minimum_width, this_preferred_width) =
+                    box.minimum_and_preferred_widths();
+                min_width = Au::max(min_width, this_minimum_width);
+                pref_width = Au::max(pref_width, this_preferred_width);
             }
 
-            this.common.min_width = min_width;
-            this.common.pref_width = pref_width;
-            this.common.num_floats = num_floats;
+            this.base.min_width = min_width;
+            this.base.pref_width = pref_width;
+            this.base.num_floats = num_floats;
         }
     }
 
     /// Recursively (top-down) determines the actual width of child contexts and boxes. When called
     /// on this context, the context has had its width set by the parent context.
-    pub fn assign_widths_inline(&mut self, _: &LayoutContext) {
+    fn assign_widths(&mut self, _: &mut LayoutContext) {
         // Initialize content box widths if they haven't been initialized already.
         //
         // TODO: Combine this with `LineboxScanner`'s walk in the box list, or put this into
         // `RenderBox`.
 
-        debug!("assign_widths_inline: floats_in: %?", self.common.floats_in);
+        debug!("assign_widths_inline: floats_in: %?", self.base.floats_in);
         {
             let this = &mut *self;
             for &box in this.boxes.iter() {
-                match box {
-                    ImageRenderBoxClass(image_box) => {
-                        let width = box.image_width(image_box);
-                        image_box.base.position.size.width = width;
-                    }
-                    TextRenderBoxClass(_) => {
-                        // Text boxes are preinitialized.
-                    }
-                    GenericRenderBoxClass(generic_box) => {
-                        // TODO(#225): There will be different cases here for `inline-block` and
-                        // other replaced content.
-                        // FIXME(pcwalton): This seems clownshoes; can we remove?
-                        generic_box.position.size.width = Au::from_px(45);
-                    }
-                    // FIXME(pcwalton): This isn't very type safe!
-                    _ => fail!(fmt!("Tried to assign width to unknown Box variant: %?", box)),
-                }
-            } // End of for loop.
+                box.assign_width();
+            }
         }
 
-        for kid in self.common.child_iter() {
-            do kid.with_mut_base |base| {
-                base.position.size.width = self.common.position.size.width;
-                base.is_inorder = self.common.is_inorder;
-            }
+        for kid in self.base.child_iter() {
+            let child_base = flow::mut_base(*kid);
+            child_base.position.size.width = self.base.position.size.width;
+            child_base.is_inorder = self.base.is_inorder;
         }
         // There are no child contexts, so stop here.
 
@@ -580,16 +580,15 @@ impl InlineFlowData {
         // 'inline-block' box that created this flow before recursing.
     }
 
-    pub fn assign_height_inorder_inline(&mut self, ctx: &mut LayoutContext) {
-        for kid in self.common.child_iter() {
+    fn assign_height_inorder(&mut self, ctx: &mut LayoutContext) {
+        for kid in self.base.child_iter() {
             kid.assign_height_inorder(ctx);
         }
-        self.assign_height_inline(ctx);
+        self.assign_height(ctx);
     }
 
-    pub fn assign_height_inline(&mut self, _: &LayoutContext) {
-
-        debug!("assign_height_inline: assigning height for flow %?", self.common.id);
+    fn assign_height(&mut self, _: &mut LayoutContext) {
+        debug!("assign_height_inline: assigning height for flow %?", self.base.id);
 
         // Divide the boxes into lines
         // TODO(#226): Get the CSS `line-height` property from the containing block's style to
@@ -597,33 +596,37 @@ impl InlineFlowData {
         //
         // TODO(#226): Get the CSS `line-height` property from each non-replaced inline element to
         // determine its height for computing linebox height.
-        debug!("assign_height_inline: floats_in: %?", self.common.floats_in);
-        let scanner_floats = self.common.floats_in.clone();
+        //
+        // TODO(pcwalton): Cache the linebox scanner?
+        debug!("assign_height_inline: floats_in: %?", self.base.floats_in);
+
+        let scanner_floats = self.base.floats_in.clone();
         let mut scanner = LineboxScanner::new(scanner_floats);
+
+        // Access the linebox scanner.
         scanner.scan_for_lines(self);
 
-        let mut line_height_offset = Au(0);
+        let mut line_height_offset = Au::new(0);
 
         // Now, go through each line and lay out the boxes inside
         for line in self.lines.mut_iter() {
             // We need to distribute extra width based on text-align.
             let mut slack_width = line.green_zone.width - line.bounds.size.width;
-            if slack_width < Au(0) {
-                slack_width = Au(0);
+            if slack_width < Au::new(0) {
+                slack_width = Au::new(0);
             }
-            //assert!(slack_width >= Au(0), "Too many boxes on line");
+            //assert!(slack_width >= Au::new(0), "Too many boxes on line");
 
             // Get the text alignment.
             // TODO(Issue #222): use 'text-align' property from InlineFlow's
             // block container, not from the style of the first box child.
-            let linebox_align;
-            if line.range.begin() < self.boxes.len() {
+            let linebox_align = if line.range.begin() < self.boxes.len() {
                 let first_box = self.boxes[line.range.begin()];
-                linebox_align = first_box.text_align();
+                first_box.base().nearest_ancestor_element().style().Text.text_align
             } else {
                 // Nothing to lay out, so assume left alignment.
-                linebox_align = text_align::left;
-            }
+                text_align::left
+            };
 
             // Set the box x positions
             let mut offset_x = line.bounds.origin.x;
@@ -632,28 +635,25 @@ impl InlineFlowData {
                 // TODO(Issue #213): implement `text-align: justify`
                 text_align::left | text_align::justify => {
                     for i in line.range.eachi() {
-                        do self.boxes[i].with_mut_base |base| {
-                            base.position.origin.x = offset_x;
-                            offset_x = offset_x + base.position.size.width;
-                        }
+                        let box = self.boxes[i].base();
+                        box.position.mutate().ptr.origin.x = offset_x;
+                        offset_x = offset_x + box.position.get().size.width;
                     }
                 }
                 text_align::center => {
                     offset_x = offset_x + slack_width.scale_by(0.5);
                     for i in line.range.eachi() {
-                        do self.boxes[i].with_mut_base |base| {
-                            base.position.origin.x = offset_x;
-                            offset_x = offset_x + base.position.size.width;
-                        }
+                        let box = self.boxes[i].base();
+                        box.position.mutate().ptr.origin.x = offset_x;
+                        offset_x = offset_x + box.position.get().size.width;
                     }
                 }
                 text_align::right => {
                     offset_x = offset_x + slack_width;
                     for i in line.range.eachi() {
-                        do self.boxes[i].with_mut_base |base| {
-                            base.position.origin.x = offset_x;
-                            offset_x = offset_x + base.position.size.width;
-                        }
+                        let box = self.boxes[i].base();
+                        box.position.mutate().ptr.origin.x = offset_x;
+                        offset_x = offset_x + box.position.get().size.width;
                     }
                 }
             };
@@ -663,46 +663,53 @@ impl InlineFlowData {
             line.bounds.origin.y = line.bounds.origin.y + line_height_offset;
 
             // Calculate the distance from baseline to the top of the linebox.
-            let mut topmost = Au(0);
+            let mut topmost = Au::new(0);
             // Calculate the distance from baseline to the bottom of the linebox.
-            let mut bottommost = Au(0);
+            let mut bottommost = Au::new(0);
 
             // Calculate the biggest height among boxes with 'top' value.
-            let mut biggest_top = Au(0);
+            let mut biggest_top = Au::new(0);
             // Calculate the biggest height among boxes with 'bottom' value.
-            let mut biggest_bottom = Au(0);
+            let mut biggest_bottom = Au::new(0);
 
             for box_i in line.range.eachi() {
                 let cur_box = self.boxes[box_i];
 
-                let (top_from_base, bottom_from_base, ascent) = match cur_box {
-                    ImageRenderBoxClass(image_box) => {
-                        let mut height = cur_box.image_height(image_box);
+                let (top_from_base, bottom_from_base, ascent) = match cur_box.class() {
+                    ImageRenderBoxClass => {
+                        let image_box = cur_box.as_image_render_box();
+                        let mut height = image_box.image_height();
 
                         // TODO: margin, border, padding's top and bottom should be calculated in advance,
                         // since baseline of image is bottom margin edge.
-                        let mut top = Au(0);
-                        let mut bottom = Au(0);
-                        do cur_box.with_model |model| {
+                        let mut top;
+                        let mut bottom;
+                        {
+                            let model = image_box.base.model.get();
                             top = model.border.top + model.padding.top + model.margin.top;
-                            bottom = model.border.bottom + model.padding.bottom + model.margin.bottom;
+                            bottom = model.border.bottom + model.padding.bottom +
+                                model.margin.bottom;
                         }
+
                         let noncontent_height = top + bottom;
                         height = height + noncontent_height;
-                        image_box.base.position.size.height = height;
-                        image_box.base.position.translate(&Point2D(Au(0), -height));
+
+                        let position_ref = image_box.base.position.mutate();
+                        position_ref.ptr.size.height = height;
+                        position_ref.ptr.translate(&Point2D(Au::new(0), -height));
 
                         let ascent = height + bottom;
-                        (height, Au(0), ascent)
+                        (height, Au::new(0), ascent)
                     },
-                    TextRenderBoxClass(text_box) => {
+                    TextRenderBoxClass => {
+                        let text_box = cur_box.as_text_render_box();
                         let range = &text_box.range;
                         let run = &text_box.run;
                         
                         // Compute the height based on the line-height and font size
                         let text_bounds = run.metrics_for_range(range).bounding_box;
                         let em_size = text_bounds.size.height;
-                        let line_height = scanner.calculate_line_height(cur_box, em_size);
+                        let line_height = text_box.base.calculate_line_height(em_size);
 
                         // Find the top and bottom of the content area.
                         // Those are used in text-top and text-bottom value of 'vertical-align'
@@ -710,12 +717,15 @@ impl InlineFlowData {
                        
                         // Offset from the top of the box is 1/2 of the leading + ascent
                         let text_offset = text_ascent + (line_height - em_size).scale_by(0.5);
-                        text_bounds.translate(&Point2D(text_box.base.position.origin.x, Au(0)));
+                        text_bounds.translate(&Point2D(text_box.base.position.get().origin.x,
+                                                       Au::new(0)));
 
                         (text_offset, line_height - text_offset, text_ascent)
                     },
-                    GenericRenderBoxClass(generic_box) => {
-                        (generic_box.position.size.height, Au(0), generic_box.position.size.height)
+                    GenericRenderBoxClass => {
+                        let base = cur_box.base();
+                        let height = base.position.get().size.height;
+                        (height, Au::new(0), height)
                     },
                     // FIXME(pcwalton): This isn't very type safe!
                     _ => {
@@ -734,44 +744,40 @@ impl InlineFlowData {
 
                 // It should calculate the distance from baseline to the top of parent's content area.
                 // But, it is assumed now as font size of parent.
-                let mut parent_text_top = Au(0);
+                let mut parent_text_top;
                 // It should calculate the distance from baseline to the bottom of parent's content area.
                 // But, it is assumed now as 0.
-                let parent_text_bottom  = Au(0);
-                do cur_box.with_mut_base |base| {
-                    // Get parent node
-                    let parent = match base.node.parent_node() {
-                        None => base.node,
-                        Some(parent) => parent,
-                    };
-
-                    let font_size = parent.style().Font.font_size;
-                    parent_text_top = font_size;
-                }
+                let parent_text_bottom  = Au::new(0);
+                let cur_box_base = cur_box.base();
+                // Get parent node
+                let parent = cur_box_base.node.parent_node();
+
+                let font_size = parent.unwrap().style().Font.font_size;
+                parent_text_top = font_size;
 
                 // This flag decides whether topmost and bottommost are updated or not.
                 // That is, if the box has top or bottom value, no_update_flag becomes true.
                 let mut no_update_flag = false;
                 // Calculate a relative offset from baseline.
-                let offset = match cur_box.vertical_align() {
+                let offset = match cur_box_base.vertical_align() {
                     vertical_align::baseline => {
                         -ascent
                     },
                     vertical_align::middle => {
                         // TODO: x-height value should be used from font info.
-                        let xheight = Au(0);
-                        -(xheight + scanner.box_height(cur_box)).scale_by(0.5)
+                        let xheight = Au::new(0);
+                        -(xheight + cur_box.box_height()).scale_by(0.5)
                     },
                     vertical_align::sub => {
                         // TODO: The proper position for subscripts should be used.
                         // Lower the baseline to the proper position for subscripts
-                        let sub_offset = Au(0);
+                        let sub_offset = Au::new(0);
                         (sub_offset - ascent)
                     },
                     vertical_align::super_ => {
                         // TODO: The proper position for superscripts should be used.
                         // Raise the baseline to the proper position for superscripts
-                        let super_offset = Au(0);
+                        let super_offset = Au::new(0);
                         (-super_offset - ascent)
                     },
                     vertical_align::text_top => {
@@ -808,8 +814,9 @@ impl InlineFlowData {
                         -(length + ascent)
                     },
                     vertical_align::Percentage(p) => {
-                        let pt_size = cur_box.font_style().pt_size; 
-                        let line_height = scanner.calculate_line_height(cur_box, Au::from_pt(pt_size));
+                        let pt_size = cur_box.base().font_style().pt_size; 
+                        let line_height = cur_box.base()
+                                                 .calculate_line_height(Au::from_pt(pt_size));
                         let percent_offset = line_height.scale_by(p);
                         -(percent_offset + ascent)
                     }
@@ -824,9 +831,7 @@ impl InlineFlowData {
                     bottommost = bottom_from_base;
                 }
 
-                do cur_box.with_mut_base |base| {
-                    base.position.origin.y = line.bounds.origin.y + offset;
-                }
+                cur_box.base().position.mutate().ptr.origin.y = line.bounds.origin.y + offset;
             }
 
             // Calculate the distance from baseline to the top of the biggest box with 'bottom' value.
@@ -849,21 +854,15 @@ impl InlineFlowData {
             // All boxes' y position is updated following the new baseline offset.
             for box_i in line.range.eachi() {
                 let cur_box = self.boxes[box_i];
-                let adjust_offset = match cur_box.vertical_align() {
-                    vertical_align::top => {
-                        Au(0)
-                    },
-                    vertical_align::bottom => {
-                        baseline_offset + bottommost
-                    },
-                    _ => {
-                        baseline_offset
-                    }
+                let cur_box_base = cur_box.base();
+                let adjust_offset = match cur_box_base.vertical_align() {
+                    vertical_align::top => Au::new(0),
+                    vertical_align::bottom => baseline_offset + bottommost,
+                    _ => baseline_offset,
                 };
 
-                do cur_box.with_mut_base |base| {
-                    base.position.origin.y = base.position.origin.y + adjust_offset;
-                }
+                cur_box_base.position.mutate().ptr.origin.y =
+                    cur_box_base.position.get().origin.y + adjust_offset;
             }
 
             // This is used to set the top y position of the next linebox in the next loop.
@@ -871,48 +870,30 @@ impl InlineFlowData {
             line.bounds.size.height = topmost + bottommost;
         } // End of `lines.each` loop.
 
-        self.common.position.size.height = 
+        self.base.position.size.height = 
             if self.lines.len() > 0 {
                 self.lines.last().bounds.origin.y + self.lines.last().bounds.size.height
             } else {
-                Au(0)
+                Au::new(0)
             };
 
-        self.common.floats_out = scanner.floats_out().translate(Point2D(Au(0), 
-                                                                -self.common.position.size.height));
+        self.base.floats_out = scanner.floats_out()
+                                      .translate(Point2D(Au::new(0),
+                                                         -self.base.position.size.height));
     }
 
-    pub fn build_display_list_inline<E:ExtraDisplayListData>(&self,
-                                                             builder: &DisplayListBuilder,
-                                                             dirty: &Rect<Au>,
-                                                             list: &Cell<DisplayList<E>>)
-                                                             -> bool {
-
-        //TODO: implement inline iframe size messaging
-        if self.common.node.is_iframe_element() {
-            error!("inline iframe size messaging not implemented yet");
-        }
-
-        let abs_rect = Rect(self.common.abs_position, self.common.position.size);
-        if !abs_rect.intersects(dirty) {
-            return true;
+    fn collapse_margins(&mut self,
+                        _: bool,
+                        _: &mut bool,
+                        _: &mut Au,
+                        _: &mut Au,
+                        collapsing: &mut Au,
+                        collapsible: &mut Au) {
+        *collapsing = Au::new(0);
+        // Non-empty inline flows prevent collapsing between the previous margion and the next.
+        if self.base.position.size.height > Au::new(0) {
+            *collapsible = Au::new(0);
         }
-
-        // TODO(#228): Once we form line boxes and have their cached bounds, we can be smarter and
-        // not recurse on a line if nothing in it can intersect the dirty region.
-        debug!("FlowContext[%d]: building display list for %u inline boxes",
-               self.common.id,
-               self.boxes.len());
-
-        for box in self.boxes.iter() {
-            box.build_display_list(builder, dirty, &self.common.abs_position, list)
-        }
-
-        // TODO(#225): Should `inline-block` elements have flows as children of the inline flow or
-        // should the flow be nested inside the box somehow?
-        
-        // For now, don't traverse the subtree rooted here
-        true
     }
 }
 
diff --git a/src/components/main/layout/layout_task.rs b/src/components/main/layout/layout_task.rs
index 07a638a2604..cb029942710 100644
--- a/src/components/main/layout/layout_task.rs
+++ b/src/components/main/layout/layout_task.rs
@@ -11,7 +11,9 @@ use layout::aux::LayoutAuxMethods;
 use layout::box_builder::LayoutTreeBuilder;
 use layout::context::LayoutContext;
 use layout::display_list_builder::{DisplayListBuilder};
-use layout::flow::FlowContext;
+use layout::flow::{FlowContext, ImmutableFlowUtils, MutableFlowUtils, PreorderFlowTraversal};
+use layout::flow::{PostorderFlowTraversal};
+use layout::flow;
 use layout::incremental::{RestyleDamage, BubbleWidths};
 
 use std::cast::transmute;
@@ -42,7 +44,7 @@ use script::layout_interface::{ReflowForDisplay, ReflowMsg};
 use script::script_task::{ReflowCompleteMsg, ScriptChan, SendEventMsg};
 use servo_msg::constellation_msg::{ConstellationChan, PipelineId};
 use servo_net::image_cache_task::{ImageCacheTask, ImageResponseMsg};
-use servo_net::local_image_cache::LocalImageCache;
+use servo_net::local_image_cache::{ImageResponder, LocalImageCache};
 use servo_util::tree::TreeNodeRef;
 use servo_util::time::{ProfilerChan, profile};
 use servo_util::time;
@@ -67,6 +69,129 @@ struct LayoutTask {
     profiler_chan: ProfilerChan,
 }
 
+/// The damage computation traversal.
+#[deriving(Clone)]
+struct ComputeDamageTraversal;
+
+impl PostorderFlowTraversal for ComputeDamageTraversal {
+    #[inline]
+    fn process(&mut self, flow: &mut FlowContext) -> bool {
+        let mut damage = flow::base(flow).restyle_damage;
+        for child in flow::child_iter(flow) {
+            damage.union_in_place(flow::base(*child).restyle_damage)
+        }
+        flow::mut_base(flow).restyle_damage = damage;
+        true
+    }
+}
+
+/// Propagates restyle damage up and down the tree as appropriate.
+///
+/// FIXME(pcwalton): Merge this with flow tree building and/or other traversals.
+struct PropagateDamageTraversal {
+    resized: bool,
+}
+
+impl PreorderFlowTraversal for PropagateDamageTraversal {
+    #[inline]
+    fn process(&mut self, flow: &mut FlowContext) -> bool {
+        // Also set any damage implied by resize.
+        if self.resized {
+            flow::mut_base(flow).restyle_damage.union_in_place(RestyleDamage::for_resize())
+        }
+
+        let prop = flow::base(flow).restyle_damage.propagate_down();
+        if prop.is_nonempty() {
+            for kid_ctx in flow::child_iter(flow) {
+                flow::mut_base(*kid_ctx).restyle_damage.union_in_place(prop)
+            }
+        }
+        true
+    }
+}
+
+/// The bubble-widths traversal, the first part of layout computation. This computes preferred
+/// and intrinsic widths and bubbles them up the tree.
+struct BubbleWidthsTraversal<'self>(&'self mut LayoutContext);
+
+impl<'self> PostorderFlowTraversal for BubbleWidthsTraversal<'self> {
+    #[inline]
+    fn process(&mut self, flow: &mut FlowContext) -> bool {
+        flow.bubble_widths(**self);
+        true
+    }
+
+    #[inline]
+    fn should_prune(&mut self, flow: &mut FlowContext) -> bool {
+        flow::mut_base(flow).restyle_damage.lacks(BubbleWidths)
+    }
+}
+
+/// The assign-widths traversal. In Gecko this corresponds to `Reflow`.
+struct AssignWidthsTraversal<'self>(&'self mut LayoutContext);
+
+impl<'self> PreorderFlowTraversal for AssignWidthsTraversal<'self> {
+    #[inline]
+    fn process(&mut self, flow: &mut FlowContext) -> bool {
+        flow.assign_widths(**self);
+        true
+    }
+}
+
+/// The assign-heights traversal, the last (and most expensive) part of layout computation.
+/// Determines the final heights for all layout objects. In Gecko this corresponds to
+/// `FinishAndStoreOverflow`.
+struct AssignHeightsTraversal<'self>(&'self mut LayoutContext);
+
+impl<'self> PostorderFlowTraversal for AssignHeightsTraversal<'self> {
+    #[inline]
+    fn process(&mut self, flow: &mut FlowContext) -> bool {
+        flow.assign_height(**self);
+        true
+    }
+
+    #[inline]
+    fn should_process(&mut self, flow: &mut FlowContext) -> bool {
+        !flow::base(flow).is_inorder
+    }
+}
+
+/// The display list building traversal. In WebKit this corresponds to `paint`. In Gecko this
+/// corresponds to `BuildDisplayListForChild`.
+struct DisplayListBuildingTraversal<'self> {
+    builder: DisplayListBuilder<'self>,
+    root_pos: Rect<Au>,
+    display_list: ~Cell<DisplayList<AbstractNode<()>>>,
+}
+
+impl<'self> PreorderFlowTraversal for DisplayListBuildingTraversal<'self> {
+    #[inline]
+    fn process(&mut self, _: &mut FlowContext) -> bool {
+        true
+    }
+
+    #[inline]
+    fn should_prune(&mut self, flow: &mut FlowContext) -> bool {
+        flow.build_display_list(&self.builder, &self.root_pos, self.display_list)
+    }
+}
+
+struct LayoutImageResponder {
+    id: PipelineId,
+    script_chan: ScriptChan,
+}
+
+impl ImageResponder for LayoutImageResponder {
+    fn respond(&self) -> ~fn(ImageResponseMsg) {
+        let id = self.id.clone();
+        let script_chan = self.script_chan.clone();
+        let f: ~fn(ImageResponseMsg) = |_| {
+            script_chan.send(SendEventMsg(id.clone(), ReflowEvent))
+        };
+        f
+    }
+}
+
 impl LayoutTask {
     pub fn create(id: PipelineId,
                   port: Port<Msg>,
@@ -180,14 +305,13 @@ impl LayoutTask {
 
         // FIXME: Bad copy!
         let doc_url = data.url.clone();
-        let script_chan = data.script_chan.clone();
 
         debug!("layout: received layout request for: %s", doc_url.to_str());
         debug!("layout: damage is %?", data.damage);
         debug!("layout: parsed Node tree");
         debug!("%?", node.dump());
         // Reset the image cache.
-        self.local_image_cache.next_round(self.make_on_image_available_cb(script_chan));
+        self.local_image_cache.next_round(self.make_on_image_available_cb());
 
         self.doc_url = Some(doc_url);
         let screen_size = Size2D(Au::from_px(data.window_size.width as int),
@@ -218,10 +342,10 @@ impl LayoutTask {
         }
 
         // Construct the flow tree.
-        let mut layout_root: FlowContext = do profile(time::LayoutTreeBuilderCategory,
-                                                  self.profiler_chan.clone()) {
+        let mut layout_root: ~FlowContext: = do profile(time::LayoutTreeBuilderCategory,
+                                                   self.profiler_chan.clone()) {
             let mut builder = LayoutTreeBuilder::new();
-            let layout_root: FlowContext = match builder.construct_trees(&layout_ctx, *node) {
+            let layout_root: ~FlowContext: = match builder.construct_trees(&layout_ctx, *node) {
                 Ok(root) => root,
                 Err(*) => fail!(~"Root flow should always exist")
             };
@@ -229,41 +353,11 @@ impl LayoutTask {
             layout_root
         };
 
-        // Propagate restyle damage up and down the tree, as appropriate.
-        // FIXME: Merge this with flow tree building and/or the other traversals.
-        do layout_root.each_preorder |flow| {
-            // Also set any damage implied by resize.
-            if resized {
-                do flow.with_mut_base |base| {
-                    base.restyle_damage.union_in_place(RestyleDamage::for_resize());
-                }
-            }
-
-            let prop = flow.with_base(|base| base.restyle_damage.propagate_down());
-            if prop.is_nonempty() {
-                for kid_ctx in flow.child_iter() {
-                    do kid_ctx.with_mut_base |kid| {
-                        kid.restyle_damage.union_in_place(prop);
-                    }
-                }
-            }
-            true
-        };
-
-        do layout_root.each_postorder |flow| {
-            let mut damage = do flow.with_base |base| {
-                base.restyle_damage
-            };
-            for child in flow.child_iter() {
-                do child.with_base |child_base| {
-                    damage.union_in_place(child_base.restyle_damage);
-                }
-            }
-            do flow.with_mut_base |base| {
-                base.restyle_damage = damage;
-            }
-            true
-        };
+        // Propagate damage.
+        layout_root.traverse_preorder(&mut PropagateDamageTraversal {
+            resized: resized,
+        });
+        layout_root.traverse_postorder(&mut ComputeDamageTraversal.clone());
 
         debug!("layout: constructed Flow tree");
         debug!("%?", layout_root.dump());
@@ -271,51 +365,37 @@ impl LayoutTask {
         // Perform the primary layout passes over the flow tree to compute the locations of all
         // the boxes.
         do profile(time::LayoutMainCategory, self.profiler_chan.clone()) {
-            do layout_root.each_postorder_prune(|f| f.restyle_damage().lacks(BubbleWidths)) |flow| {
-                flow.bubble_widths(&mut layout_ctx);
-                true
-            };
+            let _ = layout_root.traverse_postorder(&mut BubbleWidthsTraversal(&mut layout_ctx));
 
-            // FIXME: We want to do
+            // FIXME(kmc): We want to do
             //     for flow in layout_root.traverse_preorder_prune(|f| f.restyle_damage().lacks(Reflow)) 
             // but FloatContext values can't be reused, so we need to recompute them every time.
             // NOTE: this currently computes borders, so any pruning should separate that operation out.
-            debug!("assigning widths");
-            do layout_root.each_preorder |flow| {
-                flow.assign_widths(&mut layout_ctx);
-                true
-            };
+            let _ = layout_root.traverse_preorder(&mut AssignWidthsTraversal(&mut layout_ctx));
 
             // For now, this is an inorder traversal
             // FIXME: prune this traversal as well
-            debug!("assigning height");
-            do layout_root.each_bu_sub_inorder |flow| {
-                flow.assign_height(&mut layout_ctx);
-                true
-            };
+            let _ = layout_root.traverse_postorder(&mut AssignHeightsTraversal(&mut layout_ctx));
         }
 
         // Build the display list if necessary, and send it to the renderer.
         if data.goal == ReflowForDisplay {
             do profile(time::LayoutDispListBuildCategory, self.profiler_chan.clone()) {
-                let builder = DisplayListBuilder {
-                    ctx: &layout_ctx,
-                };
-
-                let display_list = ~Cell::new(DisplayList::<AbstractNode<()>>::new());
-
                 // TODO: Set options on the builder before building.
                 // TODO: Be smarter about what needs painting.
-                let root_pos = &layout_root.position().clone();
-                layout_root.each_preorder_prune(|flow| {  
-                    flow.build_display_list(&builder, root_pos, display_list) 
-                }, |_| { true } );
-
-                let root_size = do layout_root.with_base |base| {
-                    base.position.size
+                let mut traversal = DisplayListBuildingTraversal {
+                    builder: DisplayListBuilder {
+                        ctx: &layout_ctx,
+                    },
+                    root_pos: flow::base(layout_root).position.clone(),
+                    display_list: ~Cell::new(DisplayList::<AbstractNode<()>>::new()),
                 };
 
-                let display_list = Arc::new(display_list.take());
+                let _ = layout_root.traverse_preorder(&mut traversal);
+
+                let root_size = flow::base(layout_root).position.size;
+
+                let display_list = Arc::new(traversal.display_list.take());
 
                 for i in range(0,display_list.get().list.len()) {
                     let node: AbstractNode<LayoutView> = unsafe {
@@ -445,7 +525,7 @@ impl LayoutTask {
                         Some(ref list) => {
                             let display_list = list.get();
                             let (x, y) = (Au::from_frac_px(point.x as f64),
-                                    Au::from_frac_px(point.y as f64));
+                                          Au::from_frac_px(point.y as f64));
                             let mut resp = Err(());
                             // iterate in reverse to ensure we have the most recently painted render box
                             for display_item in display_list.list.rev_iter() {
@@ -481,21 +561,15 @@ impl LayoutTask {
     // to the script task, and ultimately cause the image to be
     // re-requested. We probably don't need to go all the way back to
     // the script task for this.
-    fn make_on_image_available_cb(&self, script_chan: ScriptChan)
-                                  -> ~fn() -> ~fn(ImageResponseMsg) {
+    fn make_on_image_available_cb(&self) -> @ImageResponder {
         // This has a crazy signature because the image cache needs to
         // make multiple copies of the callback, and the dom event
         // channel is not a copyable type, so this is actually a
         // little factory to produce callbacks
-        let id = self.id.clone();
-        let f: ~fn() -> ~fn(ImageResponseMsg) = || {
-            let script_chan = script_chan.clone();
-            let f: ~fn(ImageResponseMsg) = |_| {
-                script_chan.send(SendEventMsg(id.clone(), ReflowEvent))
-            };
-            f
-        };
-        return f;
+        @LayoutImageResponder {
+            id: self.id.clone(),
+            script_chan: self.script_chan.clone(),
+        } as @ImageResponder
     }
 }
 
diff --git a/src/components/main/layout/model.rs b/src/components/main/layout/model.rs
index 9225c8408fd..cc66ec1f70d 100644
--- a/src/components/main/layout/model.rs
+++ b/src/components/main/layout/model.rs
@@ -11,6 +11,7 @@ use style::ComputedValues;
 use style::properties::common_types::computed;
 
 /// Encapsulates the borders, padding, and margins, which we collectively call the "box model".
+#[deriving(Clone)]
 pub struct BoxModel {
     border: SideOffsets2D<Au>,
     padding: SideOffsets2D<Au>,
@@ -26,7 +27,9 @@ pub enum MaybeAuto {
 }
 
 impl MaybeAuto {
-    pub fn from_style(length: computed::LengthOrPercentageOrAuto, containing_length: Au) -> MaybeAuto {
+    #[inline]
+    pub fn from_style(length: computed::LengthOrPercentageOrAuto, containing_length: Au)
+                      -> MaybeAuto {
         match length {
             computed::LPA_Auto => Auto,
             computed::LPA_Percentage(percent) => Specified(containing_length.scale_by(percent)),
@@ -34,6 +37,7 @@ impl MaybeAuto {
         }
     }
 
+    #[inline]
     pub fn specified_or_default(&self, default: Au) -> Au {
         match *self {
             Auto => default,
@@ -41,8 +45,9 @@ impl MaybeAuto {
         }
     }
 
+    #[inline]
     pub fn specified_or_zero(&self) -> Au {
-        self.specified_or_default(Au(0))
+        self.specified_or_default(Au::new(0))
     }
 }
 
@@ -62,20 +67,24 @@ impl Zero for BoxModel {
 }
 
 impl BoxModel {
-    /// Populates the box model parameters from the given computed style.
+    /// Populates the box model border parameters from the given computed style.
     pub fn compute_borders(&mut self, style: &ComputedValues) {
-        // Compute the borders.
         self.border.top = style.Border.border_top_width;
         self.border.right = style.Border.border_right_width;
         self.border.bottom = style.Border.border_bottom_width;
         self.border.left = style.Border.border_left_width;
     }
 
+    /// Populates the box model padding parameters from the given computed style.
     pub fn compute_padding(&mut self, style: &ComputedValues, containing_width: Au) {
-        self.padding.top = self.compute_padding_length(style.Padding.padding_top, containing_width);
-        self.padding.right = self.compute_padding_length(style.Padding.padding_right, containing_width);
-        self.padding.bottom = self.compute_padding_length(style.Padding.padding_bottom, containing_width);
-        self.padding.left = self.compute_padding_length(style.Padding.padding_left, containing_width);
+        self.padding.top = self.compute_padding_length(style.Padding.padding_top,
+                                                       containing_width);
+        self.padding.right = self.compute_padding_length(style.Padding.padding_right,
+                                                         containing_width);
+        self.padding.bottom = self.compute_padding_length(style.Padding.padding_bottom,
+                                                          containing_width);
+        self.padding.left = self.compute_padding_length(style.Padding.padding_left,
+                                                        containing_width);
     }
 
     pub fn noncontent_width(&self) -> Au {
diff --git a/src/components/main/layout/text.rs b/src/components/main/layout/text.rs
index a14dd2bf74a..77a8831d137 100644
--- a/src/components/main/layout/text.rs
+++ b/src/components/main/layout/text.rs
@@ -8,51 +8,11 @@ use std::vec;
 
 use gfx::text::text_run::TextRun;
 use gfx::text::util::{CompressWhitespaceNewline, transform_text};
-use layout::box::{RenderBox, RenderBoxBase, TextRenderBox};
-use layout::box::{TextRenderBoxClass, UnscannedTextRenderBoxClass};
+use layout::box::{RenderBox, RenderBoxUtils, TextRenderBox, UnscannedTextRenderBoxClass};
 use layout::context::LayoutContext;
 use layout::flow::FlowContext;
 use servo_util::range::Range;
 
-
-/// Creates a TextRenderBox from a range and a text run.
-pub fn adapt_textbox_with_range(mut base: RenderBoxBase, run: @TextRun, range: Range)
-                                -> TextRenderBox {
-    debug!("Creating textbox with span: (strlen=%u, off=%u, len=%u) of textrun (%s) (len=%u)",
-           run.char_len(),
-           range.begin(),
-           range.length(),
-           run.text,
-           run.char_len());
-
-    assert!(range.begin() < run.char_len());
-    assert!(range.end() <= run.char_len());
-    assert!(range.length() > 0);
-
-    let metrics = run.metrics_for_range(&range);
-    base.position.size = metrics.bounding_box.size;
-
-    TextRenderBox {
-        base: base,
-        run: run,
-        range: range,
-    }
-}
-
-pub trait UnscannedMethods {
-    /// Copies out the text from an unscanned text box. Fails if this is not an unscanned text box.
-    fn raw_text(&self) -> ~str;
-}
-
-impl UnscannedMethods for RenderBox {
-    fn raw_text(&self) -> ~str {
-        match *self {
-            UnscannedTextRenderBoxClass(text_box) => text_box.text.clone(),
-            _ => fail!(~"unsupported operation: box.raw_text() on non-unscanned text box."),
-        }
-    }
-}
-
 /// A stack-allocated object for scanning an inline flow into `TextRun`-containing `TextBox`es.
 struct TextRunScanner {
     clump: Range,
@@ -67,7 +27,7 @@ impl TextRunScanner {
 
     pub fn scan_for_runs(&mut self, ctx: &LayoutContext, flow: &mut FlowContext) {
         {
-            let inline = flow.imm_inline();
+            let inline = flow.as_immutable_inline();
             // FIXME: this assertion fails on wikipedia, but doesn't seem
             // to cause problems.
             // assert!(inline.boxes.len() > 0);
@@ -76,9 +36,11 @@ impl TextRunScanner {
 
         let mut last_whitespace = true;
         let mut out_boxes = ~[];
-        for box_i in range(0, flow.imm_inline().boxes.len()) {
-            debug!("TextRunScanner: considering box: %?", flow.imm_inline().boxes[box_i].debug_str());
-            if box_i > 0 && !can_coalesce_text_nodes(flow.imm_inline().boxes, box_i-1, box_i) {
+        for box_i in range(0, flow.as_immutable_inline().boxes.len()) {
+            debug!("TextRunScanner: considering box: %u", box_i);
+            if box_i > 0 && !can_coalesce_text_nodes(flow.as_immutable_inline().boxes,
+                                                     box_i - 1,
+                                                     box_i) {
                 last_whitespace = self.flush_clump_to_list(ctx, flow, last_whitespace, &mut out_boxes);
             }
             self.clump.extend_by(1);
@@ -91,17 +53,17 @@ impl TextRunScanner {
         debug!("TextRunScanner: swapping out boxes.");
 
         // Swap out the old and new box list of the flow.
-        flow.inline().boxes = out_boxes;
+        flow.as_inline().boxes = out_boxes;
 
         // A helper function.
-        fn can_coalesce_text_nodes(boxes: &[RenderBox], left_i: uint, right_i: uint) -> bool {
+        fn can_coalesce_text_nodes(boxes: &[@RenderBox], left_i: uint, right_i: uint) -> bool {
             assert!(left_i < boxes.len());
             assert!(right_i > 0 && right_i < boxes.len());
             assert!(left_i != right_i);
 
             let (left, right) = (boxes[left_i], boxes[right_i]);
-            match (left, right) {
-                (UnscannedTextRenderBoxClass(*), UnscannedTextRenderBoxClass(*)) => {
+            match (left.class(), right.class()) {
+                (UnscannedTextRenderBoxClass, UnscannedTextRenderBoxClass) => {
                     left.can_merge_with_box(right)
                 }
                 (_, _) => false
@@ -123,18 +85,17 @@ impl TextRunScanner {
                                ctx: &LayoutContext,
                                flow: &mut FlowContext,
                                last_whitespace: bool,
-                               out_boxes: &mut ~[RenderBox]) -> bool {
-        let inline = flow.inline();
+                               out_boxes: &mut ~[@RenderBox])
+                               -> bool {
+        let inline = flow.as_inline();
         let in_boxes = &inline.boxes;
 
         assert!(self.clump.length() > 0);
 
         debug!("TextRunScanner: flushing boxes in range=%?", self.clump);
         let is_singleton = self.clump.length() == 1;
-        let is_text_clump = match in_boxes[self.clump.begin()] {
-            UnscannedTextRenderBoxClass(*) => true,
-            _ => false
-        };
+        let possible_text_clump = in_boxes[self.clump.begin()]; // FIXME(pcwalton): Rust bug
+        let is_text_clump = possible_text_clump.class() == UnscannedTextRenderBoxClass;
 
         let mut new_whitespace = last_whitespace;
 
@@ -148,9 +109,9 @@ impl TextRunScanner {
             },
             (true, true)  => {
                 let old_box = in_boxes[self.clump.begin()];
-                let text = old_box.raw_text();
-                let font_style = old_box.font_style();
-                let decoration = old_box.text_decoration();
+                let text = old_box.as_unscanned_text_render_box().raw_text();
+                let font_style = old_box.base().font_style();
+                let decoration = old_box.base().text_decoration();
 
                 // TODO(#115): Use the actual CSS `white-space` property of the relevant style.
                 let compression = CompressWhitespaceNewline;
@@ -166,12 +127,10 @@ impl TextRunScanner {
                     let run = @fontgroup.create_textrun(transformed_text, decoration);
 
                     debug!("TextRunScanner: pushing single text box in range: %? (%?)", self.clump, text);
-                    let new_box = do old_box.with_base |old_box_base| {
-                        let range = Range::new(0, run.char_len());
-                        @mut adapt_textbox_with_range(*old_box_base, run, range)
-                    };
+                    let range = Range::new(0, run.char_len());
+                    let new_box = @TextRenderBox::new((*old_box.base()).clone(), run, range);
 
-                    out_boxes.push(TextRenderBoxClass(new_box));
+                    out_boxes.push(new_box as @RenderBox);
                 }
             },
             (false, true) => {
@@ -185,7 +144,8 @@ impl TextRunScanner {
                     // `transform_text`, so that boxes starting and/or ending with whitespace can
                     // be compressed correctly with respect to the text run.
                     let idx = i + self.clump.begin();
-                    let (new_str, new_whitespace) = transform_text(in_boxes[idx].raw_text(),
+                    let in_box = in_boxes[idx].as_unscanned_text_render_box().raw_text();
+                    let (new_str, new_whitespace) = transform_text(in_box,
                                                                    compression,
                                                                    last_whitespace_in_clump);
                     last_whitespace_in_clump = new_whitespace;
@@ -210,9 +170,10 @@ impl TextRunScanner {
                 // TODO(#177): Text run creation must account for the renderability of text by
                 // font group fonts. This is probably achieved by creating the font group above
                 // and then letting `FontGroup` decide which `Font` to stick into the text run.
-                let font_style = in_boxes[self.clump.begin()].font_style();
+                let in_box = in_boxes[self.clump.begin()];
+                let font_style = in_box.base().font_style();
                 let fontgroup = ctx.font_ctx.get_resolved_font_for_style(&font_style);
-                let decoration = in_boxes[self.clump.begin()].text_decoration();
+                let decoration = in_box.base().text_decoration();
 
                 // TextRuns contain a cycle which is usually resolved by the teardown
                 // sequence. If no clump takes ownership, however, it will leak.
@@ -234,10 +195,10 @@ impl TextRunScanner {
                         continue
                     }
 
-                    do in_boxes[i].with_base |base| {
-                        let new_box = @mut adapt_textbox_with_range(*base, run.unwrap(), range);
-                        out_boxes.push(TextRenderBoxClass(new_box));
-                    }
+                    let new_box = @TextRenderBox::new((*in_boxes[i].base()).clone(),
+                                                      run.unwrap(),
+                                                      range);
+                    out_boxes.push(new_box as @RenderBox);
                 }
             }
         } // End of match.
diff --git a/src/components/main/layout/util.rs b/src/components/main/layout/util.rs
index c45ca1f3631..7701aa2bd66 100644
--- a/src/components/main/layout/util.rs
+++ b/src/components/main/layout/util.rs
@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use layout::box::{RenderBox};
+use layout::box::{RenderBox, RenderBoxUtils};
 use script::dom::node::{AbstractNode, LayoutView};
 use servo_util::range::Range;
 
@@ -46,7 +46,7 @@ impl ElementMapping {
         self.entries.iter().enumerate()
     }
 
-    pub fn repair_for_box_changes(&mut self, old_boxes: &[RenderBox], new_boxes: &[RenderBox]) {
+    pub fn repair_for_box_changes(&mut self, old_boxes: &[@RenderBox], new_boxes: &[@RenderBox]) {
         let entries = &mut self.entries;
 
         debug!("--- Old boxes: ---");
@@ -88,18 +88,8 @@ impl ElementMapping {
                     repair_stack.push(item);
                     entries_k += 1;
                 }
-                // XXX: the following loop form causes segfaults; assigning to locals doesn't.
-                // while new_j < new_boxes.len() && old_boxes[old_i].d().node != new_boxes[new_j].d().node {
-                while new_j < new_boxes.len() {
-                    let should_leave = do old_boxes[old_i].with_base |old_box_base| {
-                        do new_boxes[new_j].with_base |new_box_base| {
-                            old_box_base.node != new_box_base.node
-                        }
-                    };
-                    if should_leave {
-                        break
-                    }
-
+                while new_j < new_boxes.len() &&
+                        old_boxes[old_i].base().node != new_boxes[new_j].base().node {
                     debug!("repair_for_box_changes: Slide through new box %u", new_j);
                     new_j += 1;
                 }
diff --git a/src/components/main/pipeline.rs b/src/components/main/pipeline.rs
index fc34ecd9f29..9c89a07fb4e 100644
--- a/src/components/main/pipeline.rs
+++ b/src/components/main/pipeline.rs
@@ -44,7 +44,6 @@ impl Pipeline {
                        opts: Opts,
                        script_pipeline: &Pipeline,
                        size_future: Future<Size2D<uint>>) -> Pipeline {
-        
         let (layout_port, layout_chan) = special_stream!(LayoutChan);
         let (render_port, render_chan) = special_stream!(RenderChan);
 
@@ -78,7 +77,6 @@ impl Pipeline {
                       script_pipeline.script_chan.clone(),
                       layout_chan,
                       render_chan)
-
     }
 
     pub fn create(id: PipelineId,
diff --git a/src/components/main/platform/common/glfw_windowing.rs b/src/components/main/platform/common/glfw_windowing.rs
index 6dee5c7f97b..88032a221b4 100644
--- a/src/components/main/platform/common/glfw_windowing.rs
+++ b/src/components/main/platform/common/glfw_windowing.rs
@@ -11,6 +11,8 @@ use windowing::{QuitWindowEvent, MouseWindowClickEvent, MouseWindowMouseDownEven
 use windowing::{Forward, Back};
 
 use alert::{Alert, AlertMethods};
+use extra::time::Timespec;
+use extra::time;
 use std::libc::c_int;
 use std::local_data;
 use geom::point::Point2D;
@@ -20,8 +22,6 @@ use servo_msg::compositor_msg::{FinishedLoading, Blank, Loading, PerformingLayou
 
 use glfw;
 
-static THROBBER: [char, ..8] = [ '⣾', '⣽', '⣻', '⢿', '⡿', '⣟', '⣯', '⣷' ];
-
 /// A structure responsible for setting up and tearing down the entire windowing system.
 pub struct Application;
 
@@ -57,7 +57,8 @@ pub struct Window {
 
     ready_state: ReadyState,
     render_state: RenderState,
-    throbber_frame: u8,
+
+    last_title_set_time: Timespec,
 }
 
 impl WindowMethods<Application> for Window {
@@ -81,7 +82,8 @@ impl WindowMethods<Application> for Window {
 
             ready_state: Blank,
             render_state: IdleRenderState,
-            throbber_frame: 0,
+
+            last_title_set_time: Timespec::new(0, 0),
         };
 
         install_local_window(window);
@@ -141,8 +143,7 @@ impl WindowMethods<Application> for Window {
             return self.event_queue.shift()
         }
         glfw::poll_events();
-        self.throbber_frame = (self.throbber_frame + 1) % (THROBBER.len() as u8);
-        self.update_window_title(false);
+
         if self.glfw_window.should_close() {
             QuitWindowEvent
         } else if !self.event_queue.is_empty() {
@@ -155,7 +156,7 @@ impl WindowMethods<Application> for Window {
     /// Sets the ready state.
     fn set_ready_state(@mut self, ready_state: ReadyState) {
         self.ready_state = ready_state;
-        self.update_window_title(true)
+        self.update_window_title()
     }
 
     /// Sets the render state.
@@ -168,7 +169,7 @@ impl WindowMethods<Application> for Window {
         }
 
         self.render_state = render_state;
-        self.update_window_title(true)
+        self.update_window_title()
     }
 
     fn hidpi_factor(@mut self) -> f32 {
@@ -180,25 +181,30 @@ impl WindowMethods<Application> for Window {
 
 impl Window {
     /// Helper function to set the window title in accordance with the ready state.
-    fn update_window_title(&self, state_change: bool) {
-        let throbber = THROBBER[self.throbber_frame];
+    fn update_window_title(&mut self) {
+        let now = time::get_time();
+        if now.sec == self.last_title_set_time.sec {
+            return
+        }
+        self.last_title_set_time = now;
+
         match self.ready_state {
             Blank => {
-                if state_change { self.glfw_window.set_title(fmt!("blank — Servo")) }
+                self.glfw_window.set_title(fmt!("blank — Servo"))
             }
             Loading => {
-                self.glfw_window.set_title(fmt!("%c Loading — Servo", throbber))
+                self.glfw_window.set_title(fmt!("Loading — Servo"))
             }
             PerformingLayout => {
-                self.glfw_window.set_title(fmt!("%c Performing Layout — Servo", throbber))
+                self.glfw_window.set_title(fmt!("Performing Layout — Servo"))
             }
             FinishedLoading => {
                 match self.render_state {
                     RenderingRenderState => {
-                        self.glfw_window.set_title(fmt!("%c Rendering — Servo", throbber))
+                        self.glfw_window.set_title(fmt!("Rendering — Servo"))
                     }
                     IdleRenderState => {
-                        if state_change { self.glfw_window.set_title("Servo") }
+                        self.glfw_window.set_title("Servo")
                     }
                 }
             }
diff --git a/src/components/net/local_image_cache.rs b/src/components/net/local_image_cache.rs
index c7c922e6910..2970c16bb5f 100644
--- a/src/components/net/local_image_cache.rs
+++ b/src/components/net/local_image_cache.rs
@@ -17,6 +17,10 @@ use std::task;
 use servo_util::url::{UrlMap, url_map};
 use extra::url::Url;
 
+pub trait ImageResponder {
+    fn respond(&self) -> ~fn(ImageResponseMsg);
+}
+
 pub fn LocalImageCache(image_cache_task: ImageCacheTask) -> LocalImageCache {
     LocalImageCache {
         image_cache_task: image_cache_task,
@@ -29,7 +33,7 @@ pub fn LocalImageCache(image_cache_task: ImageCacheTask) -> LocalImageCache {
 pub struct LocalImageCache {
     priv image_cache_task: ImageCacheTask,
     priv round_number: uint,
-    priv on_image_available: Option<~fn() -> ~fn(ImageResponseMsg)>,
+    priv on_image_available: Option<@ImageResponder>,
     priv state_map: UrlMap<@mut ImageState>
 }
 
@@ -43,7 +47,7 @@ struct ImageState {
 impl LocalImageCache {
     /// The local cache will only do a single remote request for a given
     /// URL in each 'round'. Layout should call this each time it begins
-    pub fn next_round(&mut self, on_image_available: ~fn() -> ~fn(ImageResponseMsg)) {
+    pub fn next_round(&mut self, on_image_available: @ImageResponder) {
         self.round_number += 1;
         self.on_image_available = Some(on_image_available);
     }
@@ -109,7 +113,7 @@ impl LocalImageCache {
                 // on the image to load and triggering layout
                 let image_cache_task = self.image_cache_task.clone();
                 assert!(self.on_image_available.is_some());
-                let on_image_available = (*self.on_image_available.get_ref())();
+                let on_image_available = self.on_image_available.unwrap().respond();
                 let url = (*url).clone();
                 do task::spawn {
                     let (response_port, response_chan) = comm::stream();
diff --git a/src/components/script/dom/node.rs b/src/components/script/dom/node.rs
index 9b8f7ea1b07..8b5478fefe2 100644
--- a/src/components/script/dom/node.rs
+++ b/src/components/script/dom/node.rs
@@ -882,7 +882,7 @@ pub struct DisplayBoxes {
 /// Data that layout associates with a node.
 pub struct LayoutData {
     /// The results of CSS matching for this node.
-    applicable_declarations: ~[@[PropertyDeclaration]],
+    applicable_declarations: ~[Arc<~[PropertyDeclaration]>],
 
     /// The results of CSS styling for this node.
     style: Option<ComputedValues>,
@@ -902,11 +902,21 @@ impl LayoutData {
             applicable_declarations: ~[],
             style: None,
             restyle_damage: None,
-            boxes: DisplayBoxes { display_list: None, range: None },
+            boxes: DisplayBoxes {
+                display_list: None,
+                range: None,
+            },
         }
     }
 }
 
+// This serves as a static assertion that layout data remains sendable. If this is not done, then
+// we can have memory unsafety, which usually manifests as shutdown crashes.
+fn assert_is_sendable<T:Send>(_: T) {}
+fn assert_layout_data_is_sendable() {
+    assert_is_sendable(LayoutData::new())
+}
+
 impl AbstractNode<LayoutView> {
     // These accessors take a continuation rather than returning a reference, because
     // an AbstractNode doesn't have a lifetime parameter relating to the underlying
diff --git a/src/components/script/html/hubbub_html_parser.rs b/src/components/script/html/hubbub_html_parser.rs
index c6dca22ee21..460053d9686 100644
--- a/src/components/script/html/hubbub_html_parser.rs
+++ b/src/components/script/html/hubbub_html_parser.rs
@@ -28,8 +28,8 @@ use servo_net::image_cache_task::ImageCacheTask;
 use servo_net::resource_task::{Load, Payload, Done, ResourceTask, load_whole_resource};
 use servo_util::tree::{TreeNodeRef, ElementLike};
 use servo_util::url::make_url;
-use extra::url::Url;
 use extra::future::Future;
+use extra::url::Url;
 use geom::size::Size2D;
 
 macro_rules! handle_element(
diff --git a/src/components/style/properties.rs.mako b/src/components/style/properties.rs.mako
index 5585e0ba764..e2f968d432f 100644
--- a/src/components/style/properties.rs.mako
+++ b/src/components/style/properties.rs.mako
@@ -5,6 +5,7 @@
 // This file is a Mako template: http://www.makotemplates.org/
 
 use std::ascii::StrAsciiExt;
+pub use extra::arc::Arc;
 pub use std::iter;
 pub use cssparser::*;
 pub use errors::{ErrorLoggerIterator, log_css_error};
@@ -979,7 +980,7 @@ fn get_initial_values() -> ComputedValues {
 
 
 // Most specific/important declarations last
-pub fn cascade(applicable_declarations: &[@[PropertyDeclaration]],
+pub fn cascade(applicable_declarations: &[Arc<~[PropertyDeclaration]>],
                parent_style: Option< &ComputedValues>)
             -> ComputedValues {
     let initial_keep_alive;
@@ -1002,7 +1003,7 @@ pub fn cascade(applicable_declarations: &[@[PropertyDeclaration]],
         % endfor
     };
     for sub_list in applicable_declarations.iter() {
-        for declaration in sub_list.iter() {
+        for declaration in sub_list.get().iter() {
             match declaration {
                 % for property in LONGHANDS:
                     &${property.ident}_declaration(ref value) => {
diff --git a/src/components/style/selector_matching.rs b/src/components/style/selector_matching.rs
index a203df620e1..d6c7d75fdd0 100644
--- a/src/components/style/selector_matching.rs
+++ b/src/components/style/selector_matching.rs
@@ -2,8 +2,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use std::at_vec;
 use std::ascii::StrAsciiExt;
+use extra::arc::Arc;
 use extra::sort::tim_sort;
 
 use selectors::*;
@@ -54,7 +54,7 @@ impl Stylist {
                         // TODO: avoid copying?
                         rules.$priority.push(Rule {
                             selector: @(*selector).clone(),
-                            declarations:at_vec::to_managed(style_rule.declarations.$priority),
+                            declarations: Arc::new(style_rule.declarations.$priority.clone()),
                         })
                     }
                 }
@@ -79,7 +79,7 @@ impl Stylist {
 
     pub fn get_applicable_declarations<N: TreeNode<T>, T: TreeNodeRefAsElement<N, E>, E: ElementLike>(
             &self, element: &T, style_attribute: Option<&PropertyDeclarationBlock>,
-            pseudo_element: Option<PseudoElement>) -> ~[@[PropertyDeclaration]] {
+            pseudo_element: Option<PseudoElement>) -> ~[Arc<~[PropertyDeclaration]>] {
         assert!(element.is_element())
         assert!(style_attribute.is_none() || pseudo_element.is_none(),
                 "Style attributes do not apply to pseudo-elements")
@@ -89,7 +89,7 @@ impl Stylist {
             ($rules: expr) => {
                 for rule in $rules.iter() {
                     if matches_selector::<N, T, E>(rule.selector, element, pseudo_element) {
-                        applicable_declarations.push(rule.declarations)
+                        applicable_declarations.push(rule.declarations.clone())
                     }
                 }
             };
@@ -102,11 +102,11 @@ impl Stylist {
         // Style attributes have author origin but higher specificity than style rules.
         append!(self.author_rules.normal);
         // TODO: avoid copying?
-        style_attribute.map(|sa| applicable_declarations.push(at_vec::to_managed(sa.normal)));
+        style_attribute.map(|sa| applicable_declarations.push(Arc::new(sa.normal.clone())));
 
         append!(self.author_rules.important);
         // TODO: avoid copying?
-        style_attribute.map(|sa| applicable_declarations.push(at_vec::to_managed(sa.important)));
+        style_attribute.map(|sa| applicable_declarations.push(Arc::new(sa.important.clone())));
 
         append!(self.user_rules.important);
         append!(self.ua_rules.important);
@@ -131,7 +131,7 @@ impl PerOriginRules {
 #[deriving(Clone)]
 struct Rule {
     selector: @Selector,
-    declarations: @[PropertyDeclaration],
+    declarations: Arc<~[PropertyDeclaration]>,
 }
 
 
diff --git a/src/components/util/geometry.rs b/src/components/util/geometry.rs
index 4acc8d8dd72..d67fae0c003 100644
--- a/src/components/util/geometry.rs
+++ b/src/components/util/geometry.rs
@@ -8,52 +8,78 @@ use geom::size::Size2D;
 
 use std::num::{NumCast, One, Zero};
 
-#[deriving(Clone,Eq)]
+#[deriving(Clone)]
 pub struct Au(i32);
 
+impl Eq for Au {
+    #[inline]
+    fn eq(&self, other: &Au) -> bool {
+        **self == **other
+    }
+    #[inline]
+    fn ne(&self, other: &Au) -> bool {
+        **self != **other
+    }
+}
+
 impl Add<Au,Au> for Au {
+    #[inline]
     fn add(&self, other: &Au) -> Au { Au(**self + **other) }
 }
 
 impl Sub<Au,Au> for Au {
+    #[inline]
     fn sub(&self, other: &Au) -> Au { Au(**self - **other) }
 }
 
 impl Mul<Au,Au> for Au {
+    #[inline]
     fn mul(&self, other: &Au) -> Au { Au(**self * **other) }
 }
 
 impl Div<Au,Au> for Au {
+    #[inline]
     fn div(&self, other: &Au) -> Au { Au(**self / **other) }
 }
 
 impl Rem<Au,Au> for Au {
+    #[inline]
     fn rem(&self, other: &Au) -> Au { Au(**self % **other) }
 }
 
 impl Neg<Au> for Au {
+    #[inline]
     fn neg(&self) -> Au { Au(-**self) }
 }
 
 impl Ord for Au {
+    #[inline]
     fn lt(&self, other: &Au) -> bool { **self <  **other }
+    #[inline]
     fn le(&self, other: &Au) -> bool { **self <= **other }
+    #[inline]
     fn ge(&self, other: &Au) -> bool { **self >= **other }
+    #[inline]
     fn gt(&self, other: &Au) -> bool { **self >  **other }
 }
 
 impl One for Au {
+    #[inline]
     fn one() -> Au { Au(1) }
 }
 
 impl Zero for Au {
+    #[inline]
     fn zero() -> Au { Au(0) }
+    #[inline]
     fn is_zero(&self) -> bool { **self == 0 }
 }
 
 impl Num for Au {}
 
+#[inline]
 pub fn min(x: Au, y: Au) -> Au { if x < y { x } else { y } }
+#[inline]
 pub fn max(x: Au, y: Au) -> Au { if x > y { x } else { y } }
 
 impl NumCast for Au {
@@ -70,6 +96,14 @@ impl ToPrimitive for Au {
     fn to_u64(&self) -> Option<u64> {
         Some(**self as u64)
     }
+
+    fn to_f32(&self) -> Option<f32> {
+        (**self).to_f32()
+    }
+
+    fn to_f64(&self) -> Option<f64> {
+        (**self).to_f64()
+    }
 }
 
 pub fn box<T:Clone + Ord + Add<T,T> + Sub<T,T>>(x: T, y: T, w: T, h: T) -> Rect<T> {
@@ -77,42 +111,58 @@ pub fn box<T:Clone + Ord + Add<T,T> + Sub<T,T>>(x: T, y: T, w: T, h: T) -> Rect<
 }
 
 impl Au {
+    /// FIXME(pcwalton): Workaround for lack of cross crate inlining of newtype structs!
+    #[inline]
+    pub fn new(value: i32) -> Au {
+        Au(value)
+    }
+
+    #[inline]
     pub fn scale_by(self, factor: f64) -> Au {
-        Au(((*self as f64) * factor).round() as i32)
+        Au(((*self as f64) * factor) as i32)
     }
 
+    #[inline]
     pub fn from_px(px: int) -> Au {
         NumCast::from(px * 60).unwrap()
     }
 
+    #[inline]
     pub fn to_nearest_px(&self) -> int {
         ((**self as f64) / 60f64).round() as int
     }
 
+    #[inline]
     pub fn to_snapped(&self) -> Au {
         let res = **self % 60i32;
         return if res >= 30i32 { return Au(**self - res + 60i32) }
                        else { return Au(**self - res) };
     }
 
+    #[inline]
     pub fn zero_point() -> Point2D<Au> {
         Point2D(Au(0), Au(0))
     }
 
+    #[inline]
     pub fn zero_rect() -> Rect<Au> {
         let z = Au(0);
         Rect(Point2D(z, z), Size2D(z, z))
     }
 
+    #[inline]
     pub fn from_pt(pt: f64) -> Au {
         from_px(pt_to_px(pt) as int)
     }
 
+    #[inline]
     pub fn from_frac_px(px: f64) -> Au {
         Au((px * 60f64) as i32)
     }
 
+    #[inline]
     pub fn min(x: Au, y: Au) -> Au { if *x < *y { x } else { y } }
+    #[inline]
     pub fn max(x: Au, y: Au) -> Au { if *x > *y { x } else { y } }
 }
 
@@ -159,3 +209,4 @@ pub fn to_frac_px(au: Au) -> f64 {
 pub fn from_pt(pt: f64) -> Au {
     from_px((pt / 72f64 * 96f64) as int)
 }
+
diff --git a/src/components/util/range.rs b/src/components/util/range.rs
index a0943728a77..e52dbae02ab 100644
--- a/src/components/util/range.rs
+++ b/src/components/util/range.rs
@@ -22,58 +22,76 @@ pub struct Range {
 }
 
 impl Range {
+    #[inline]
     pub fn new(off: uint, len: uint) -> Range {
-        Range { off: off, len: len }
+        Range {
+            off: off,
+            len: len,
+        }
     }
 
+    #[inline]
     pub fn empty() -> Range {
         Range::new(0, 0)
     }
 }
 
 impl Range {
+    #[inline]
     pub fn begin(&self) -> uint { self.off  }
+    #[inline]
     pub fn length(&self) -> uint { self.len }
+    #[inline]
     pub fn end(&self) -> uint { self.off + self.len }
 
+    #[inline]
     pub fn eachi(&self) -> iter::Range<uint> {
         range(self.off, self.off + self.len)
     }
 
+    #[inline]
     pub fn contains(&self, i: uint) -> bool {
         i >= self.begin() && i < self.end()
     }
 
+    #[inline]
     pub fn is_valid_for_string(&self, s: &str) -> bool {
         self.begin() < s.len() && self.end() <= s.len() && self.length() <= s.len()
     }
 
+    #[inline]
     pub fn is_empty(&self) -> bool {
         self.len == 0
     }
 
+    #[inline]
     pub fn shift_by(&mut self, i: int) { 
         self.off = ((self.off as int) + i) as uint;
     }
 
+    #[inline]
     pub fn extend_by(&mut self, i: int) { 
         self.len = ((self.len as int) + i) as uint;
     }
 
+    #[inline]
     pub fn extend_to(&mut self, i: uint) { 
         self.len = i - self.off;
     }
 
+    #[inline]
     pub fn adjust_by(&mut self, off_i: int, len_i: int) {
         self.off = ((self.off as int) + off_i) as uint;
         self.len = ((self.len as int) + len_i) as uint;
     }
 
+    #[inline]
     pub fn reset(&mut self, off_i: uint, len_i: uint) {
         self.off = off_i;
         self.len = len_i;
     }
 
+    #[inline]
     pub fn intersect(&self, other: &Range) -> Range {
         let begin = max(self.begin(), other.begin());
         let end = min(self.end(), other.end());
@@ -88,6 +106,7 @@ impl Range {
     /// Computes the relationship between two ranges (`self` and `other`),
     /// from the point of view of `self`. So, 'EntirelyBefore' means
     /// that the `self` range is entirely before `other` range.
+    #[inline]
     pub fn relation_to_range(&self, other: &Range) -> RangeRelation {
         if other.begin() > self.end() {
             return EntirelyBefore;
@@ -116,6 +135,7 @@ impl Range {
                   self, other));
     }
 
+    #[inline]
     pub fn repair_after_coalesced_range(&mut self, other: &Range) {
         let relation = self.relation_to_range(other);
         debug!("repair_after_coalesced_range: possibly repairing range %?", self);
diff --git a/src/components/util/slot.rs b/src/components/util/slot.rs
new file mode 100644
index 00000000000..6c8eca9068f
--- /dev/null
+++ b/src/components/util/slot.rs
@@ -0,0 +1,127 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! An in-place, dynamically borrowable slot. Useful for "mutable fields". Assuming this works out
+//! well, this type should be upstreamed to the Rust standard library.
+
+use std::cast;
+
+#[unsafe_no_drop_flag]
+#[no_freeze]
+pub struct Slot<T> {
+    // NB: Must be priv, or else someone could borrow it.
+    priv value: T,
+    priv immutable_borrow_count: u8,
+    priv mutably_borrowed: bool,
+}
+
+impl<T:Clone> Clone for Slot<T> {
+    #[inline]
+    fn clone(&self) -> Slot<T> {
+        Slot {
+            value: self.value.clone(),
+            immutable_borrow_count: 0,
+            mutably_borrowed: false,
+        }
+    }
+}
+
+#[unsafe_destructor]
+impl<T> Drop for Slot<T> {
+    fn drop(&mut self) {
+        // Noncopyable.
+    }
+}
+
+pub struct SlotRef<'self,T> {
+    ptr: &'self T,
+    priv immutable_borrow_count: *mut u8,
+}
+
+#[unsafe_destructor]
+impl<'self,T> Drop for SlotRef<'self,T> {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe {
+            *self.immutable_borrow_count -= 1
+        }
+    }
+}
+
+pub struct MutSlotRef<'self,T> {
+    ptr: &'self mut T,
+    priv mutably_borrowed: *mut bool,
+}
+
+#[unsafe_destructor]
+impl<'self,T> Drop for MutSlotRef<'self,T> {
+    #[inline]
+    fn drop(&mut self) {
+        unsafe {
+            *self.mutably_borrowed = false
+        }
+    }
+}
+
+impl<T> Slot<T> {
+    #[inline]
+    pub fn init(value: T) -> Slot<T> {
+        Slot {
+            value: value,
+            immutable_borrow_count: 0,
+            mutably_borrowed: false,
+        }
+    }
+
+    #[inline]
+    pub fn borrow<'a>(&'a self) -> SlotRef<'a,T> {
+        unsafe {
+            if self.immutable_borrow_count == 255 || self.mutably_borrowed {
+                self.fail()
+            }
+            let immutable_borrow_count = cast::transmute_mut(&self.immutable_borrow_count);
+            *immutable_borrow_count += 1;
+            SlotRef {
+                ptr: &self.value,
+                immutable_borrow_count: immutable_borrow_count,
+            }
+        }
+    }
+
+    #[inline]
+    pub fn mutate<'a>(&'a self) -> MutSlotRef<'a,T> {
+        unsafe {
+            if self.immutable_borrow_count > 0 || self.mutably_borrowed {
+                self.fail()
+            }
+            let mutably_borrowed = cast::transmute_mut(&self.mutably_borrowed);
+            *mutably_borrowed = true;
+            MutSlotRef {
+                ptr: cast::transmute_mut(&self.value),
+                mutably_borrowed: mutably_borrowed,
+            }
+        }
+    }
+
+    #[inline]
+    pub fn set(&self, value: T) {
+        *self.mutate().ptr = value
+    }
+
+    #[inline(never)]
+    pub fn fail(&self) {
+        fail!("slot is borrowed")
+    }
+}
+
+impl<T:Clone> Slot<T> {
+    #[inline]
+    pub fn get(&self) -> T {
+        self.value.clone()
+    }
+}
+
+
+
+
diff --git a/src/components/util/time.rs b/src/components/util/time.rs
index 35186ef3fa5..9937dbd40e0 100644
--- a/src/components/util/time.rs
+++ b/src/components/util/time.rs
@@ -2,23 +2,28 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-// Timing functions.
-use std::comm::{Port, SharedChan};
-use std::iter::AdditiveIterator;
-use std::rt::io::timer::Timer;
-use std::task::spawn_with;
+//! Timing functions.
 
 use extra::sort::tim_sort;
 use extra::time::precise_time_ns;
 use extra::treemap::TreeMap;
+use std::comm::{Port, SendDeferred, SharedChan};
+use std::iter::AdditiveIterator;
+use std::rt::io::timer::Timer;
+use std::task::spawn_with;
 
 // front-end representation of the profiler used to communicate with the profiler
 #[deriving(Clone)]
 pub struct ProfilerChan(SharedChan<ProfilerMsg>);
+
 impl ProfilerChan {
     pub fn new(chan: Chan<ProfilerMsg>) -> ProfilerChan {
         ProfilerChan(SharedChan::new(chan))
     }
+
+    pub fn send_deferred(&self, msg: ProfilerMsg) {
+        (**self).send_deferred(msg);
+    }
 }
 
 pub enum ProfilerMsg {
@@ -185,7 +190,7 @@ pub fn profile<T>(category: ProfilerCategory,
     let val = callback();
     let end_time = precise_time_ns();
     let ms = ((end_time - start_time) as f64 / 1000000f64);
-    profiler_chan.send(TimeMsg(category, ms));
+    profiler_chan.send_deferred(TimeMsg(category, ms));
     return val;
 }
 
diff --git a/src/components/util/util.rc b/src/components/util/util.rc
index bbcf7203740..c84a87b13e5 100644
--- a/src/components/util/util.rc
+++ b/src/components/util/util.rc
@@ -16,6 +16,7 @@ extern mod geom;
 pub mod cache;
 pub mod geometry;
 pub mod range;
+pub mod slot;
 pub mod time;
 pub mod tree;
 pub mod url;
diff --git a/src/support/spidermonkey/mozjs b/src/support/spidermonkey/mozjs
-Subproject 81aeed0b6159acdedf7dd530fb6c3bde5ddbb70
+Subproject d57edb998865f9616d9164d561f26d54ade4964
diff --git a/src/test/html/longcatbot.png b/src/test/html/longcatbot.png
index 10b897e88a8..e1a4bc00fcf 100644
--- a/src/test/html/longcatbot.png
+++ b/src/test/html/longcatbot.png
diff --git a/src/test/html/longcatmid.png b/src/test/html/longcatmid.png
index bd9a921ac64..e1b6f83eea5 100644
--- a/src/test/html/longcatmid.png
+++ b/src/test/html/longcatmid.png
diff --git a/src/test/html/longcattop.png b/src/test/html/longcattop.png
index beb85663d3a..1d786dc1950 100644
--- a/src/test/html/longcattop.png
+++ b/src/test/html/longcattop.png
diff --git a/src/test/html/rust-0.png b/src/test/html/rust-0.png
index dd0f5c102fc..20d93badf5e 100644
--- a/src/test/html/rust-0.png
+++ b/src/test/html/rust-0.png
diff --git a/src/test/html/rust-135.png b/src/test/html/rust-135.png
index 1f4f8500432..d5a26f62139 100644
--- a/src/test/html/rust-135.png
+++ b/src/test/html/rust-135.png
diff --git a/src/test/html/rust-180.png b/src/test/html/rust-180.png
index d3230266ce1..af2a0631b13 100644
--- a/src/test/html/rust-180.png
+++ b/src/test/html/rust-180.png
diff --git a/src/test/html/rust-225.png b/src/test/html/rust-225.png
index 03252755fc6..b0a671ea3b1 100644
--- a/src/test/html/rust-225.png
+++ b/src/test/html/rust-225.png
diff --git a/src/test/html/rust-270.png b/src/test/html/rust-270.png
index 2fc953bb363..0c7d1d7f002 100644
--- a/src/test/html/rust-270.png
+++ b/src/test/html/rust-270.png
diff --git a/src/test/html/rust-315.png b/src/test/html/rust-315.png
index 4c41fad4c0c..5d15acd0cfb 100644
--- a/src/test/html/rust-315.png
+++ b/src/test/html/rust-315.png
diff --git a/src/test/html/rust-45.png b/src/test/html/rust-45.png
index d1bc4bbc05d..b7dd1fd9c5a 100644
--- a/src/test/html/rust-45.png
+++ b/src/test/html/rust-45.png
diff --git a/src/test/html/rust-90.png b/src/test/html/rust-90.png
index 0c956fce847..25fd8a66251 100644
--- a/src/test/html/rust-90.png
+++ b/src/test/html/rust-90.png
diff --git a/src/test/html/test_sandboxed_iframe.html b/src/test/html/test_sandboxed_iframe.html
index d4dbb045ea7..061ea8c09a5 100644
--- a/src/test/html/test_sandboxed_iframe.html
+++ b/src/test/html/test_sandboxed_iframe.html
@@ -37,11 +37,11 @@ function run() {
   var now = new Date();
   var div = document.getElementsByTagName('div')[0];
   var elems = [];
-  for (var i = 0; i < 900; i++) {
+  for (var i = 0; i < 600; i++) {
     elems.push(i);
   }
   var outer = [];
-  for (var i = 0; i < 900; i++) {
+  for (var i = 0; i < 600; i++) {
     outer.push(elems);
   }
   var a = new Matrix(outer);
@@ -56,10 +56,10 @@ function run() {
 
 setTimeout(function forever() {
   run();
-  setTimeout(forever, 3000);
+  setTimeout(forever, 1000);
 }, 0);
   </script>
-<p>Time required to multiply two 900x900 matrices:</p>
+<p>Time required to multiply two 600x600 matrices:</p>
 <div></div>
 </body>
 </html>