Layout 2020: Move all Fragment code to the `fragment_tree` directory

This is a simple code organization change with no behavior change with
the idea of making Layout 2020 easier to understand by new folks to the
project. The idea is that we will have a cleaner separation between the
different parts of layout ie one directory for the fragment tree and one
(currently multiple) directory for the box tree.

20 files changed, 592 insertions, 558 deletions

diff --git a/components/layout_2020/display_list/mod.rs b/components/layout_2020/display_list/mod.rs
index b1201e26add..2103d62bb09 100644
--- a/components/layout_2020/display_list/mod.rs
+++ b/components/layout_2020/display_list/mod.rs
@@ -5,12 +5,10 @@
 use crate::context::LayoutContext;
 use crate::display_list::conversions::ToWebRender;
 use crate::display_list::stacking_context::StackingContextSection;
-use crate::fragment_tree::Tag;
-use crate::fragments::{BoxFragment, Fragment, TextFragment};
+use crate::fragment_tree::{BoxFragment, Fragment, FragmentTree, Tag, TextFragment};
 use crate::geom::{PhysicalPoint, PhysicalRect};
 use crate::replaced::IntrinsicSizes;
 use crate::style_ext::ComputedValuesExt;
-use crate::FragmentTree;
 use embedder_traits::Cursor;
 use euclid::{Point2D, SideOffsets2D, Size2D};
 use fnv::FnvHashMap;
diff --git a/components/layout_2020/display_list/stacking_context.rs b/components/layout_2020/display_list/stacking_context.rs
index 77421ea59a9..63fefd0c4f4 100644
--- a/components/layout_2020/display_list/stacking_context.rs
+++ b/components/layout_2020/display_list/stacking_context.rs
@@ -6,11 +6,11 @@ use super::DisplayList;
 use crate::cell::ArcRefCell;
 use crate::display_list::conversions::ToWebRender;
 use crate::display_list::DisplayListBuilder;
-use crate::fragment_tree::ContainingBlockManager;
-use crate::fragments::{AnonymousFragment, BoxFragment, Fragment};
+use crate::fragment_tree::{
+    AnonymousFragment, BoxFragment, ContainingBlockManager, Fragment, FragmentTree,
+};
 use crate::geom::PhysicalRect;
 use crate::style_ext::ComputedValuesExt;
-use crate::FragmentTree;
 use euclid::default::Rect;
 use script_traits::compositor::{ScrollTreeNodeId, ScrollableNodeInfo};
 use servo_arc::Arc as ServoArc;
diff --git a/components/layout_2020/flexbox/layout.rs b/components/layout_2020/flexbox/layout.rs
index 4a15af23005..e4e59379d1e 100644
--- a/components/layout_2020/flexbox/layout.rs
+++ b/components/layout_2020/flexbox/layout.rs
@@ -8,7 +8,7 @@ use super::geom::{
 use super::{FlexContainer, FlexLevelBox};
 use crate::context::LayoutContext;
 use crate::formatting_contexts::{IndependentFormattingContext, IndependentLayout};
-use crate::fragments::{BoxFragment, CollapsedBlockMargins, Fragment};
+use crate::fragment_tree::{BoxFragment, CollapsedBlockMargins, Fragment};
 use crate::geom::flow_relative::{Rect, Sides, Vec2};
 use crate::geom::LengthOrAuto;
 use crate::positioned::{AbsolutelyPositionedBox, PositioningContext};
diff --git a/components/layout_2020/flow/float.rs b/components/layout_2020/flow/float.rs
index 582ca9e1242..784c5d62722 100644
--- a/components/layout_2020/flow/float.rs
+++ b/components/layout_2020/flow/float.rs
@@ -10,7 +10,7 @@ use crate::context::LayoutContext;
 use crate::dom::NodeExt;
 use crate::dom_traversal::{Contents, NodeAndStyleInfo};
 use crate::formatting_contexts::IndependentFormattingContext;
-use crate::fragments::{
+use crate::fragment_tree::{
     BoxFragment, CollapsedBlockMargins, CollapsedMargin, FloatFragment, Fragment,
 };
 use crate::geom::flow_relative::{Rect, Vec2};
diff --git a/components/layout_2020/flow/inline.rs b/components/layout_2020/flow/inline.rs
index 962a90e9213..7d447c492db 100644
--- a/components/layout_2020/flow/inline.rs
+++ b/components/layout_2020/flow/inline.rs
@@ -7,9 +7,9 @@ use crate::context::LayoutContext;
 use crate::flow::float::{FloatBox, SequentialLayoutState};
 use crate::flow::FlowLayout;
 use crate::formatting_contexts::IndependentFormattingContext;
-use crate::fragment_tree::BaseFragmentInfo;
-use crate::fragments::{
-    AnonymousFragment, BoxFragment, CollapsedBlockMargins, FontMetrics, Fragment, TextFragment,
+use crate::fragment_tree::{
+    AnonymousFragment, BaseFragmentInfo, BoxFragment, CollapsedBlockMargins, FontMetrics, Fragment,
+    TextFragment,
 };
 use crate::geom::flow_relative::{Rect, Sides, Vec2};
 use crate::positioned::{
diff --git a/components/layout_2020/flow/mod.rs b/components/layout_2020/flow/mod.rs
index 22c23253be4..9d45bd2ff23 100644
--- a/components/layout_2020/flow/mod.rs
+++ b/components/layout_2020/flow/mod.rs
@@ -13,8 +13,9 @@ use crate::flow::inline::InlineFormattingContext;
 use crate::formatting_contexts::{
     IndependentFormattingContext, IndependentLayout, NonReplacedFormattingContext,
 };
-use crate::fragment_tree::BaseFragmentInfo;
-use crate::fragments::{BoxFragment, CollapsedBlockMargins, CollapsedMargin, Fragment};
+use crate::fragment_tree::{
+    BaseFragmentInfo, BoxFragment, CollapsedBlockMargins, CollapsedMargin, Fragment,
+};
 use crate::geom::flow_relative::{Rect, Sides, Vec2};
 use crate::positioned::{AbsolutelyPositionedBox, PositioningContext};
 use crate::replaced::ReplacedContent;
@@ -34,7 +35,7 @@ pub mod float;
 pub mod inline;
 mod root;
 
-pub use root::{BoxTree, FragmentTree};
+pub use root::{BoxTree, CanvasBackground};
 
 #[derive(Debug, Serialize)]
 pub(crate) struct BlockFormattingContext {
diff --git a/components/layout_2020/flow/root.rs b/components/layout_2020/flow/root.rs
index 39f82e85fa0..e30577c3c09 100644
--- a/components/layout_2020/flow/root.rs
+++ b/components/layout_2020/flow/root.rs
@@ -4,7 +4,6 @@
 
 use crate::cell::ArcRefCell;
 use crate::context::LayoutContext;
-use crate::display_list::StackingContext;
 use crate::dom::{LayoutBox, NodeExt};
 use crate::dom_traversal::{iter_child_nodes, Contents, NodeAndStyleInfo};
 use crate::flexbox::FlexLevelBox;
@@ -13,8 +12,7 @@ use crate::flow::float::FloatBox;
 use crate::flow::inline::InlineLevelBox;
 use crate::flow::{BlockContainer, BlockFormattingContext, BlockLevelBox};
 use crate::formatting_contexts::IndependentFormattingContext;
-use crate::fragment_tree::{ContainingBlockManager, Tag};
-use crate::fragments::Fragment;
+use crate::fragment_tree::FragmentTree;
 use crate::geom::flow_relative::Vec2;
 use crate::geom::{PhysicalPoint, PhysicalRect, PhysicalSize};
 use crate::positioned::AbsolutelyPositionedBox;
@@ -23,15 +21,10 @@ use crate::replaced::ReplacedContent;
 use crate::style_ext::ComputedValuesExt;
 use crate::style_ext::{Display, DisplayGeneratingBox, DisplayInside};
 use crate::DefiniteContainingBlock;
-use app_units::Au;
 use atomic_refcell::AtomicRef;
-use euclid::default::{Point2D, Rect, Size2D};
-use fxhash::FxHashSet;
-use gfx_traits::print_tree::PrintTree;
 use script_layout_interface::wrapper_traits::LayoutNode;
 use script_layout_interface::{LayoutElementType, LayoutNodeType};
 use servo_arc::Arc;
-use style::animation::AnimationSetKey;
 use style::dom::OpaqueNode;
 use style::properties::ComputedValues;
 use style::values::computed::Length;
@@ -47,30 +40,6 @@ pub struct BoxTree {
     canvas_background: CanvasBackground,
 }
 
-#[derive(Serialize)]
-pub struct FragmentTree {
-    /// Fragments at the top-level of the tree.
-    ///
-    /// If the root element has `display: none`, there are zero fragments.
-    /// Otherwise, there is at least one:
-    ///
-    /// * The first fragment is generated by the root element.
-    /// * There may be additional fragments generated by positioned boxes
-    ///   that have the initial containing block.
-    pub(crate) root_fragments: Vec<ArcRefCell<Fragment>>,
-
-    /// The scrollable overflow rectangle for the entire tree
-    /// https://drafts.csswg.org/css-overflow/#scrollable
-    pub(crate) scrollable_overflow: PhysicalRect<Length>,
-
-    /// The containing block used in the layout of this fragment tree.
-    pub(crate) initial_containing_block: PhysicalRect<Length>,
-
-    /// https://drafts.csswg.org/css-backgrounds/#special-backgrounds
-    #[serde(skip)]
-    pub(crate) canvas_background: CanvasBackground,
-}
-
 impl BoxTree {
     pub fn construct<'dom, Node>(context: &LayoutContext, root_element: Node) -> Self
     where
@@ -380,173 +349,9 @@ impl BoxTree {
     }
 }
 
-impl FragmentTree {
-    pub(crate) fn build_display_list(
-        &self,
-        builder: &mut crate::display_list::DisplayListBuilder,
-        root_stacking_context: &StackingContext,
-    ) {
-        // Paint the canvas’ background (if any) before/under everything else
-        root_stacking_context.build_canvas_background_display_list(
-            builder,
-            self,
-            &self.initial_containing_block,
-        );
-        root_stacking_context.build_display_list(builder);
-    }
-
-    pub fn print(&self) {
-        let mut print_tree = PrintTree::new("Fragment Tree".to_string());
-        for fragment in &self.root_fragments {
-            fragment.borrow().print(&mut print_tree);
-        }
-    }
-
-    pub fn scrollable_overflow(&self) -> webrender_api::units::LayoutSize {
-        webrender_api::units::LayoutSize::from_untyped(Size2D::new(
-            self.scrollable_overflow.size.width.px(),
-            self.scrollable_overflow.size.height.px(),
-        ))
-    }
-
-    pub(crate) fn find<T>(
-        &self,
-        mut process_func: impl FnMut(&Fragment, usize, &PhysicalRect<Length>) -> Option<T>,
-    ) -> Option<T> {
-        let info = ContainingBlockManager {
-            for_non_absolute_descendants: &self.initial_containing_block,
-            for_absolute_descendants: None,
-            for_absolute_and_fixed_descendants: &self.initial_containing_block,
-        };
-        self.root_fragments
-            .iter()
-            .find_map(|child| child.borrow().find(&info, 0, &mut process_func))
-    }
-
-    pub fn remove_nodes_in_fragment_tree_from_set(&self, set: &mut FxHashSet<AnimationSetKey>) {
-        self.find(|fragment, _, _| {
-            let tag = fragment.tag()?;
-            set.remove(&AnimationSetKey::new(tag.node, tag.pseudo));
-            None::<()>
-        });
-    }
-
-    pub fn get_content_box_for_node(&self, requested_node: OpaqueNode) -> Rect<Au> {
-        let mut bounding_box = PhysicalRect::zero();
-        let tag_to_find = Tag::new(requested_node);
-        self.find(|fragment, _, containing_block| {
-            if fragment.tag() != Some(tag_to_find) {
-                return None::<()>;
-            }
-
-            let fragment_relative_rect = match fragment {
-                Fragment::Box(fragment) | Fragment::Float(fragment) => fragment
-                    .border_rect()
-                    .to_physical(fragment.style.writing_mode, &containing_block),
-                Fragment::Text(fragment) => fragment
-                    .rect
-                    .to_physical(fragment.parent_style.writing_mode, &containing_block),
-                Fragment::AbsoluteOrFixedPositioned(_) |
-                Fragment::Image(_) |
-                Fragment::IFrame(_) |
-                Fragment::Anonymous(_) => return None,
-            };
-
-            bounding_box = fragment_relative_rect
-                .translate(containing_block.origin.to_vector())
-                .union(&bounding_box);
-            None::<()>
-        });
-
-        Rect::new(
-            Point2D::new(
-                Au::from_f32_px(bounding_box.origin.x.px()),
-                Au::from_f32_px(bounding_box.origin.y.px()),
-            ),
-            Size2D::new(
-                Au::from_f32_px(bounding_box.size.width.px()),
-                Au::from_f32_px(bounding_box.size.height.px()),
-            ),
-        )
-    }
-
-    pub fn get_border_dimensions_for_node(&self, requested_node: OpaqueNode) -> Rect<i32> {
-        let tag_to_find = Tag::new(requested_node);
-        self.find(|fragment, _, containing_block| {
-            if fragment.tag() != Some(tag_to_find) {
-                return None;
-            }
-
-            let (style, padding_rect) = match fragment {
-                Fragment::Box(fragment) => (&fragment.style, fragment.padding_rect()),
-                _ => return None,
-            };
-
-            // https://drafts.csswg.org/cssom-view/#dom-element-clienttop
-            // " If the element has no associated CSS layout box or if the
-            //   CSS layout box is inline, return zero." For this check we
-            // also explicitly ignore the list item portion of the display
-            // style.
-            let display = &style.get_box().display;
-            if display.inside() == style::values::specified::box_::DisplayInside::Flow &&
-                display.outside() == style::values::specified::box_::DisplayOutside::Inline
-            {
-                return Some(Rect::zero());
-            }
-
-            let padding_rect = padding_rect.to_physical(style.writing_mode, &containing_block);
-            let border = style.get_border();
-            Some(Rect::new(
-                Point2D::new(
-                    border.border_left_width.px() as i32,
-                    border.border_top_width.px() as i32,
-                ),
-                Size2D::new(
-                    padding_rect.size.width.px() as i32,
-                    padding_rect.size.height.px() as i32,
-                ),
-            ))
-        })
-        .unwrap_or_else(Rect::zero)
-    }
-
-    pub fn get_scroll_area_for_node(&self, requested_node: OpaqueNode) -> Rect<i32> {
-        let mut scroll_area: PhysicalRect<Length> = PhysicalRect::zero();
-        let tag_to_find = Tag::new(requested_node);
-        self.find(|fragment, _, containing_block| {
-            if fragment.tag() != Some(tag_to_find) {
-                return None::<()>;
-            }
-
-            scroll_area = match fragment {
-                Fragment::Box(fragment) | Fragment::Float(fragment) => fragment
-                    .scrollable_overflow(&containing_block)
-                    .translate(containing_block.origin.to_vector()),
-                Fragment::Text(_) |
-                Fragment::AbsoluteOrFixedPositioned(_) |
-                Fragment::Image(_) |
-                Fragment::IFrame(_) |
-                Fragment::Anonymous(_) => return None,
-            };
-            None::<()>
-        });
-
-        Rect::new(
-            Point2D::new(
-                scroll_area.origin.x.px() as i32,
-                scroll_area.origin.y.px() as i32,
-            ),
-            Size2D::new(
-                scroll_area.size.width.px() as i32,
-                scroll_area.size.height.px() as i32,
-            ),
-        )
-    }
-}
-
 /// https://drafts.csswg.org/css-backgrounds/#root-background
 #[derive(Clone, Serialize)]
-pub(crate) struct CanvasBackground {
+pub struct CanvasBackground {
     /// DOM node for the root element
     pub root_element: OpaqueNode,
 
diff --git a/components/layout_2020/formatting_contexts.rs b/components/layout_2020/formatting_contexts.rs
index d913ad7241a..494d7822fd9 100644
--- a/components/layout_2020/formatting_contexts.rs
+++ b/components/layout_2020/formatting_contexts.rs
@@ -7,8 +7,7 @@ use crate::dom::NodeExt;
 use crate::dom_traversal::{Contents, NodeAndStyleInfo};
 use crate::flexbox::FlexContainer;
 use crate::flow::BlockFormattingContext;
-use crate::fragment_tree::BaseFragmentInfo;
-use crate::fragments::Fragment;
+use crate::fragment_tree::{BaseFragmentInfo, Fragment};
 use crate::positioned::PositioningContext;
 use crate::replaced::ReplacedContent;
 use crate::sizing::{self, ContentSizes};
diff --git a/components/layout_2020/fragment_tree/base.rs b/components/layout_2020/fragment_tree/base_fragment.rs
index d9271a7a0f6..d9271a7a0f6 100644
--- a/components/layout_2020/fragment_tree/base.rs
+++ b/components/layout_2020/fragment_tree/base_fragment.rs
diff --git a/components/layout_2020/fragment_tree/box_fragment.rs b/components/layout_2020/fragment_tree/box_fragment.rs
new file mode 100644
index 00000000000..6f697ef23fd
--- /dev/null
+++ b/components/layout_2020/fragment_tree/box_fragment.rs
@@ -0,0 +1,280 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use super::{BaseFragment, BaseFragmentInfo, CollapsedBlockMargins, Fragment};
+use crate::cell::ArcRefCell;
+use crate::geom::flow_relative::{Rect, Sides};
+use crate::geom::{PhysicalPoint, PhysicalRect, PhysicalSides, PhysicalSize};
+use gfx_traits::print_tree::PrintTree;
+use servo_arc::Arc as ServoArc;
+use style::computed_values::overflow_x::T as ComputedOverflow;
+use style::computed_values::position::T as ComputedPosition;
+use style::properties::ComputedValues;
+use style::values::computed::{CSSPixelLength, Length, LengthPercentage, LengthPercentageOrAuto};
+use style::Zero;
+
+#[derive(Serialize)]
+pub(crate) struct BoxFragment {
+    pub base: BaseFragment,
+
+    #[serde(skip_serializing)]
+    pub style: ServoArc<ComputedValues>,
+    pub children: Vec<ArcRefCell<Fragment>>,
+
+    /// From the containing block’s start corner…?
+    /// This might be broken when the containing block is in a different writing mode:
+    /// https://drafts.csswg.org/css-writing-modes/#orthogonal-flows
+    pub content_rect: Rect<Length>,
+
+    pub padding: Sides<Length>,
+    pub border: Sides<Length>,
+    pub margin: Sides<Length>,
+
+    pub clearance: Length,
+
+    pub block_margins_collapsed_with_children: CollapsedBlockMargins,
+
+    /// The scrollable overflow of this box fragment.
+    pub scrollable_overflow_from_children: PhysicalRect<Length>,
+
+    /// Whether or not this box was overconstrained in the given dimension.
+    overconstrained: PhysicalSize<bool>,
+}
+
+impl BoxFragment {
+    pub fn new(
+        base_fragment_info: BaseFragmentInfo,
+        style: ServoArc<ComputedValues>,
+        children: Vec<Fragment>,
+        content_rect: Rect<Length>,
+        padding: Sides<Length>,
+        border: Sides<Length>,
+        margin: Sides<Length>,
+        clearance: Length,
+        block_margins_collapsed_with_children: CollapsedBlockMargins,
+    ) -> BoxFragment {
+        let position = style.get_box().position;
+        let insets = style.get_position();
+        let width_overconstrained = position == ComputedPosition::Relative &&
+            !insets.left.is_auto() &&
+            !insets.right.is_auto();
+        let height_overconstrained = position == ComputedPosition::Relative &&
+            !insets.left.is_auto() &&
+            !insets.bottom.is_auto();
+
+        Self::new_with_overconstrained(
+            base_fragment_info,
+            style,
+            children,
+            content_rect,
+            padding,
+            border,
+            margin,
+            clearance,
+            block_margins_collapsed_with_children,
+            PhysicalSize::new(width_overconstrained, height_overconstrained),
+        )
+    }
+
+    pub fn new_with_overconstrained(
+        base_fragment_info: BaseFragmentInfo,
+        style: ServoArc<ComputedValues>,
+        children: Vec<Fragment>,
+        content_rect: Rect<Length>,
+        padding: Sides<Length>,
+        border: Sides<Length>,
+        margin: Sides<Length>,
+        clearance: Length,
+        block_margins_collapsed_with_children: CollapsedBlockMargins,
+        overconstrained: PhysicalSize<bool>,
+    ) -> BoxFragment {
+        // FIXME(mrobinson, bug 25564): We should be using the containing block
+        // here to properly convert scrollable overflow to physical geometry.
+        let containing_block = PhysicalRect::zero();
+        let scrollable_overflow_from_children =
+            children.iter().fold(PhysicalRect::zero(), |acc, child| {
+                acc.union(&child.scrollable_overflow(&containing_block))
+            });
+
+        BoxFragment {
+            base: base_fragment_info.into(),
+            style,
+            children: children
+                .into_iter()
+                .map(|fragment| ArcRefCell::new(fragment))
+                .collect(),
+            content_rect,
+            padding,
+            border,
+            margin,
+            clearance,
+            block_margins_collapsed_with_children,
+            scrollable_overflow_from_children,
+            overconstrained,
+        }
+    }
+
+    pub fn scrollable_overflow(
+        &self,
+        containing_block: &PhysicalRect<Length>,
+    ) -> PhysicalRect<Length> {
+        let physical_padding_rect = self
+            .padding_rect()
+            .to_physical(self.style.writing_mode, containing_block);
+
+        let content_origin = self
+            .content_rect
+            .start_corner
+            .to_physical(self.style.writing_mode);
+        physical_padding_rect.union(
+            &self
+                .scrollable_overflow_from_children
+                .translate(content_origin.to_vector()),
+        )
+    }
+
+    pub fn padding_rect(&self) -> Rect<Length> {
+        self.content_rect.inflate(&self.padding)
+    }
+
+    pub fn border_rect(&self) -> Rect<Length> {
+        self.padding_rect().inflate(&self.border)
+    }
+
+    pub fn print(&self, tree: &mut PrintTree) {
+        tree.new_level(format!(
+            "Box\
+                \nbase={:?}\
+                \ncontent={:?}\
+                \npadding rect={:?}\
+                \nborder rect={:?}\
+                \nclearance={:?}\
+                \nscrollable_overflow={:?}\
+                \noverflow={:?} / {:?}",
+            self.base,
+            self.content_rect,
+            self.padding_rect(),
+            self.border_rect(),
+            self.clearance,
+            self.scrollable_overflow(&PhysicalRect::zero()),
+            self.style.get_box().overflow_x,
+            self.style.get_box().overflow_y,
+        ));
+
+        for child in &self.children {
+            child.borrow().print(tree);
+        }
+        tree.end_level();
+    }
+
+    pub fn scrollable_overflow_for_parent(
+        &self,
+        containing_block: &PhysicalRect<Length>,
+    ) -> PhysicalRect<Length> {
+        let mut overflow = self
+            .border_rect()
+            .to_physical(self.style.writing_mode, containing_block);
+
+        if self.style.get_box().overflow_y != ComputedOverflow::Visible &&
+            self.style.get_box().overflow_x != ComputedOverflow::Visible
+        {
+            return overflow;
+        }
+
+        // https://www.w3.org/TR/css-overflow-3/#scrollable
+        // Only include the scrollable overflow of a child box if it has overflow: visible.
+        let scrollable_overflow = self.scrollable_overflow(&containing_block);
+        let bottom_right = PhysicalPoint::new(
+            overflow.max_x().max(scrollable_overflow.max_x()),
+            overflow.max_y().max(scrollable_overflow.max_y()),
+        );
+
+        if self.style.get_box().overflow_y == ComputedOverflow::Visible {
+            overflow.origin.y = overflow.origin.y.min(scrollable_overflow.origin.y);
+            overflow.size.height = bottom_right.y - overflow.origin.y;
+        }
+
+        if self.style.get_box().overflow_x == ComputedOverflow::Visible {
+            overflow.origin.x = overflow.origin.x.min(scrollable_overflow.origin.x);
+            overflow.size.width = bottom_right.x - overflow.origin.x;
+        }
+
+        overflow
+    }
+
+    pub(crate) fn calculate_resolved_insets_if_positioned(
+        &self,
+        containing_block: &PhysicalRect<CSSPixelLength>,
+    ) -> PhysicalSides<CSSPixelLength> {
+        let position = self.style.get_box().position;
+        debug_assert_ne!(
+            position,
+            ComputedPosition::Static,
+            "Should not call this method on statically positioned box."
+        );
+
+        let (cb_width, cb_height) = (containing_block.width(), containing_block.height());
+        let content_rect = self
+            .content_rect
+            .to_physical(self.style.writing_mode, &containing_block);
+
+        // "A resolved value special case property like top defined in another
+        // specification If the property applies to a positioned element and the
+        // resolved value of the display property is not none or contents, and
+        // the property is not over-constrained, then the resolved value is the
+        // used value. Otherwise the resolved value is the computed value."
+        // https://drafts.csswg.org/cssom/#resolved-values
+        let insets = self.style.get_position();
+        if position == ComputedPosition::Relative {
+            let get_resolved_axis =
+                |start: &LengthPercentageOrAuto,
+                 end: &LengthPercentageOrAuto,
+                 container_length: CSSPixelLength| {
+                    let start = start.map(|v| v.percentage_relative_to(container_length));
+                    let end = end.map(|v| v.percentage_relative_to(container_length));
+                    match (start.non_auto(), end.non_auto()) {
+                        (None, None) => (Length::zero(), Length::zero()),
+                        (None, Some(end)) => (-end, end),
+                        (Some(start), None) => (start, -start),
+                        // This is the overconstrained case, for which the resolved insets will
+                        // simply be the computed insets.
+                        (Some(start), Some(end)) => (start, end),
+                    }
+                };
+            let (left, right) = get_resolved_axis(&insets.left, &insets.right, cb_width);
+            let (top, bottom) = get_resolved_axis(&insets.top, &insets.bottom, cb_height);
+            return PhysicalSides::new(top, right, bottom, left);
+        }
+
+        debug_assert!(
+            position == ComputedPosition::Fixed || position == ComputedPosition::Absolute,
+            "Got unknown position."
+        );
+
+        let resolve = |value: &LengthPercentageOrAuto, container_length| {
+            value
+                .auto_is(LengthPercentage::zero)
+                .percentage_relative_to(container_length)
+        };
+
+        let (top, bottom) = if self.overconstrained.height {
+            (
+                resolve(&insets.top, cb_height),
+                resolve(&insets.bottom, cb_height),
+            )
+        } else {
+            (content_rect.origin.y, cb_height - content_rect.max_y())
+        };
+        let (left, right) = if self.overconstrained.width {
+            (
+                resolve(&insets.left, cb_width),
+                resolve(&insets.right, cb_width),
+            )
+        } else {
+            (content_rect.origin.x, cb_width - content_rect.max_x())
+        };
+
+        PhysicalSides::new(top, right, bottom, left)
+    }
+}
diff --git a/components/layout_2020/fragment_tree/containing_block.rs b/components/layout_2020/fragment_tree/containing_block.rs
index febe52dfd6d..82a205fd2d5 100644
--- a/components/layout_2020/fragment_tree/containing_block.rs
+++ b/components/layout_2020/fragment_tree/containing_block.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 https://mozilla.org/MPL/2.0/. */
 
-use crate::fragments::Fragment;
+use crate::fragment_tree::Fragment;
 use style::computed_values::position::T as ComputedPosition;
 
 /// A data structure used to track the containing block when recursing
diff --git a/components/layout_2020/fragments.rs b/components/layout_2020/fragment_tree/fragment.rs
index f4b25ca2b46..782676be8bc 100644
--- a/components/layout_2020/fragments.rs
+++ b/components/layout_2020/fragment_tree/fragment.rs
@@ -2,11 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use super::{BaseFragment, BoxFragment, ContainingBlockManager, HoistedSharedFragment, Tag};
 use crate::cell::ArcRefCell;
-use crate::fragment_tree::{BaseFragment, BaseFragmentInfo, ContainingBlockManager, Tag};
 use crate::geom::flow_relative::{Rect, Sides};
-use crate::geom::{PhysicalPoint, PhysicalRect, PhysicalSides, PhysicalSize};
-use crate::positioned::HoistedSharedFragment;
+use crate::geom::PhysicalRect;
 use crate::style_ext::ComputedValuesExt;
 use gfx::font::FontMetrics as GfxFontMetrics;
 use gfx::text::glyph::GlyphStore;
@@ -14,11 +13,9 @@ use gfx_traits::print_tree::PrintTree;
 use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use servo_arc::Arc as ServoArc;
 use std::sync::Arc;
-use style::computed_values::overflow_x::T as ComputedOverflow;
-use style::computed_values::position::T as ComputedPosition;
 use style::logical_geometry::WritingMode;
 use style::properties::ComputedValues;
-use style::values::computed::{CSSPixelLength, Length, LengthPercentage, LengthPercentageOrAuto};
+use style::values::computed::Length;
 use style::values::specified::text::TextDecorationLine;
 use style::Zero;
 use webrender_api::{FontInstanceKey, ImageKey};
@@ -47,34 +44,6 @@ pub(crate) enum Fragment {
 }
 
 #[derive(Serialize)]
-pub(crate) struct BoxFragment {
-    pub base: BaseFragment,
-
-    #[serde(skip_serializing)]
-    pub style: ServoArc<ComputedValues>,
-    pub children: Vec<ArcRefCell<Fragment>>,
-
-    /// From the containing block’s start corner…?
-    /// This might be broken when the containing block is in a different writing mode:
-    /// https://drafts.csswg.org/css-writing-modes/#orthogonal-flows
-    pub content_rect: Rect<Length>,
-
-    pub padding: Sides<Length>,
-    pub border: Sides<Length>,
-    pub margin: Sides<Length>,
-
-    pub clearance: Length,
-
-    pub block_margins_collapsed_with_children: CollapsedBlockMargins,
-
-    /// The scrollable overflow of this box fragment.
-    pub scrollable_overflow_from_children: PhysicalRect<Length>,
-
-    /// Whether or not this box was overconstrained in the given dimension.
-    overconstrained: PhysicalSize<bool>,
-}
-
-#[derive(Serialize)]
 pub(crate) struct FloatFragment {
     pub box_fragment: BoxFragment,
 }
@@ -327,243 +296,6 @@ impl AnonymousFragment {
     }
 }
 
-impl BoxFragment {
-    pub fn new(
-        base_fragment_info: BaseFragmentInfo,
-        style: ServoArc<ComputedValues>,
-        children: Vec<Fragment>,
-        content_rect: Rect<Length>,
-        padding: Sides<Length>,
-        border: Sides<Length>,
-        margin: Sides<Length>,
-        clearance: Length,
-        block_margins_collapsed_with_children: CollapsedBlockMargins,
-    ) -> BoxFragment {
-        let position = style.get_box().position;
-        let insets = style.get_position();
-        let width_overconstrained = position == ComputedPosition::Relative &&
-            !insets.left.is_auto() &&
-            !insets.right.is_auto();
-        let height_overconstrained = position == ComputedPosition::Relative &&
-            !insets.left.is_auto() &&
-            !insets.bottom.is_auto();
-
-        Self::new_with_overconstrained(
-            base_fragment_info,
-            style,
-            children,
-            content_rect,
-            padding,
-            border,
-            margin,
-            clearance,
-            block_margins_collapsed_with_children,
-            PhysicalSize::new(width_overconstrained, height_overconstrained),
-        )
-    }
-
-    pub fn new_with_overconstrained(
-        base_fragment_info: BaseFragmentInfo,
-        style: ServoArc<ComputedValues>,
-        children: Vec<Fragment>,
-        content_rect: Rect<Length>,
-        padding: Sides<Length>,
-        border: Sides<Length>,
-        margin: Sides<Length>,
-        clearance: Length,
-        block_margins_collapsed_with_children: CollapsedBlockMargins,
-        overconstrained: PhysicalSize<bool>,
-    ) -> BoxFragment {
-        // FIXME(mrobinson, bug 25564): We should be using the containing block
-        // here to properly convert scrollable overflow to physical geometry.
-        let containing_block = PhysicalRect::zero();
-        let scrollable_overflow_from_children =
-            children.iter().fold(PhysicalRect::zero(), |acc, child| {
-                acc.union(&child.scrollable_overflow(&containing_block))
-            });
-
-        BoxFragment {
-            base: base_fragment_info.into(),
-            style,
-            children: children
-                .into_iter()
-                .map(|fragment| ArcRefCell::new(fragment))
-                .collect(),
-            content_rect,
-            padding,
-            border,
-            margin,
-            clearance,
-            block_margins_collapsed_with_children,
-            scrollable_overflow_from_children,
-            overconstrained,
-        }
-    }
-
-    pub fn scrollable_overflow(
-        &self,
-        containing_block: &PhysicalRect<Length>,
-    ) -> PhysicalRect<Length> {
-        let physical_padding_rect = self
-            .padding_rect()
-            .to_physical(self.style.writing_mode, containing_block);
-
-        let content_origin = self
-            .content_rect
-            .start_corner
-            .to_physical(self.style.writing_mode);
-        physical_padding_rect.union(
-            &self
-                .scrollable_overflow_from_children
-                .translate(content_origin.to_vector()),
-        )
-    }
-
-    pub fn padding_rect(&self) -> Rect<Length> {
-        self.content_rect.inflate(&self.padding)
-    }
-
-    pub fn border_rect(&self) -> Rect<Length> {
-        self.padding_rect().inflate(&self.border)
-    }
-
-    pub fn print(&self, tree: &mut PrintTree) {
-        tree.new_level(format!(
-            "Box\
-                \nbase={:?}\
-                \ncontent={:?}\
-                \npadding rect={:?}\
-                \nborder rect={:?}\
-                \nclearance={:?}\
-                \nscrollable_overflow={:?}\
-                \noverflow={:?} / {:?}",
-            self.base,
-            self.content_rect,
-            self.padding_rect(),
-            self.border_rect(),
-            self.clearance,
-            self.scrollable_overflow(&PhysicalRect::zero()),
-            self.style.get_box().overflow_x,
-            self.style.get_box().overflow_y,
-        ));
-
-        for child in &self.children {
-            child.borrow().print(tree);
-        }
-        tree.end_level();
-    }
-
-    pub fn scrollable_overflow_for_parent(
-        &self,
-        containing_block: &PhysicalRect<Length>,
-    ) -> PhysicalRect<Length> {
-        let mut overflow = self
-            .border_rect()
-            .to_physical(self.style.writing_mode, containing_block);
-
-        if self.style.get_box().overflow_y != ComputedOverflow::Visible &&
-            self.style.get_box().overflow_x != ComputedOverflow::Visible
-        {
-            return overflow;
-        }
-
-        // https://www.w3.org/TR/css-overflow-3/#scrollable
-        // Only include the scrollable overflow of a child box if it has overflow: visible.
-        let scrollable_overflow = self.scrollable_overflow(&containing_block);
-        let bottom_right = PhysicalPoint::new(
-            overflow.max_x().max(scrollable_overflow.max_x()),
-            overflow.max_y().max(scrollable_overflow.max_y()),
-        );
-
-        if self.style.get_box().overflow_y == ComputedOverflow::Visible {
-            overflow.origin.y = overflow.origin.y.min(scrollable_overflow.origin.y);
-            overflow.size.height = bottom_right.y - overflow.origin.y;
-        }
-
-        if self.style.get_box().overflow_x == ComputedOverflow::Visible {
-            overflow.origin.x = overflow.origin.x.min(scrollable_overflow.origin.x);
-            overflow.size.width = bottom_right.x - overflow.origin.x;
-        }
-
-        overflow
-    }
-
-    pub(crate) fn calculate_resolved_insets_if_positioned(
-        &self,
-        containing_block: &PhysicalRect<CSSPixelLength>,
-    ) -> PhysicalSides<CSSPixelLength> {
-        let position = self.style.get_box().position;
-        debug_assert_ne!(
-            position,
-            ComputedPosition::Static,
-            "Should not call this method on statically positioned box."
-        );
-
-        let (cb_width, cb_height) = (containing_block.width(), containing_block.height());
-        let content_rect = self
-            .content_rect
-            .to_physical(self.style.writing_mode, &containing_block);
-
-        // "A resolved value special case property like top defined in another
-        // specification If the property applies to a positioned element and the
-        // resolved value of the display property is not none or contents, and
-        // the property is not over-constrained, then the resolved value is the
-        // used value. Otherwise the resolved value is the computed value."
-        // https://drafts.csswg.org/cssom/#resolved-values
-        let insets = self.style.get_position();
-        if position == ComputedPosition::Relative {
-            let get_resolved_axis =
-                |start: &LengthPercentageOrAuto,
-                 end: &LengthPercentageOrAuto,
-                 container_length: CSSPixelLength| {
-                    let start = start.map(|v| v.percentage_relative_to(container_length));
-                    let end = end.map(|v| v.percentage_relative_to(container_length));
-                    match (start.non_auto(), end.non_auto()) {
-                        (None, None) => (Length::zero(), Length::zero()),
-                        (None, Some(end)) => (-end, end),
-                        (Some(start), None) => (start, -start),
-                        // This is the overconstrained case, for which the resolved insets will
-                        // simply be the computed insets.
-                        (Some(start), Some(end)) => (start, end),
-                    }
-                };
-            let (left, right) = get_resolved_axis(&insets.left, &insets.right, cb_width);
-            let (top, bottom) = get_resolved_axis(&insets.top, &insets.bottom, cb_height);
-            return PhysicalSides::new(top, right, bottom, left);
-        }
-
-        debug_assert!(
-            position == ComputedPosition::Fixed || position == ComputedPosition::Absolute,
-            "Got unknown position."
-        );
-
-        let resolve = |value: &LengthPercentageOrAuto, container_length| {
-            value
-                .auto_is(LengthPercentage::zero)
-                .percentage_relative_to(container_length)
-        };
-
-        let (top, bottom) = if self.overconstrained.height {
-            (
-                resolve(&insets.top, cb_height),
-                resolve(&insets.bottom, cb_height),
-            )
-        } else {
-            (content_rect.origin.y, cb_height - content_rect.max_y())
-        };
-        let (left, right) = if self.overconstrained.width {
-            (
-                resolve(&insets.left, cb_width),
-                resolve(&insets.right, cb_width),
-            )
-        } else {
-            (content_rect.origin.x, cb_width - content_rect.max_x())
-        };
-
-        PhysicalSides::new(top, right, bottom, left)
-    }
-}
-
 impl TextFragment {
     pub fn print(&self, tree: &mut PrintTree) {
         tree.add_item(format!(
diff --git a/components/layout_2020/fragment_tree/fragment_tree.rs b/components/layout_2020/fragment_tree/fragment_tree.rs
new file mode 100644
index 00000000000..6234d9e3390
--- /dev/null
+++ b/components/layout_2020/fragment_tree/fragment_tree.rs
@@ -0,0 +1,204 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use super::{ContainingBlockManager, Fragment, Tag};
+use crate::cell::ArcRefCell;
+use crate::display_list::StackingContext;
+use crate::flow::CanvasBackground;
+use crate::geom::PhysicalRect;
+use app_units::Au;
+use euclid::default::{Point2D, Rect, Size2D};
+use fxhash::FxHashSet;
+use gfx_traits::print_tree::PrintTree;
+use style::animation::AnimationSetKey;
+use style::dom::OpaqueNode;
+use style::values::computed::Length;
+
+#[derive(Serialize)]
+pub struct FragmentTree {
+    /// Fragments at the top-level of the tree.
+    ///
+    /// If the root element has `display: none`, there are zero fragments.
+    /// Otherwise, there is at least one:
+    ///
+    /// * The first fragment is generated by the root element.
+    /// * There may be additional fragments generated by positioned boxes
+    ///   that have the initial containing block.
+    pub(crate) root_fragments: Vec<ArcRefCell<Fragment>>,
+
+    /// The scrollable overflow rectangle for the entire tree
+    /// https://drafts.csswg.org/css-overflow/#scrollable
+    pub(crate) scrollable_overflow: PhysicalRect<Length>,
+
+    /// The containing block used in the layout of this fragment tree.
+    pub(crate) initial_containing_block: PhysicalRect<Length>,
+
+    /// https://drafts.csswg.org/css-backgrounds/#special-backgrounds
+    #[serde(skip)]
+    pub(crate) canvas_background: CanvasBackground,
+}
+
+impl FragmentTree {
+    pub(crate) fn build_display_list(
+        &self,
+        builder: &mut crate::display_list::DisplayListBuilder,
+        root_stacking_context: &StackingContext,
+    ) {
+        // Paint the canvas’ background (if any) before/under everything else
+        root_stacking_context.build_canvas_background_display_list(
+            builder,
+            self,
+            &self.initial_containing_block,
+        );
+        root_stacking_context.build_display_list(builder);
+    }
+
+    pub fn print(&self) {
+        let mut print_tree = PrintTree::new("Fragment Tree".to_string());
+        for fragment in &self.root_fragments {
+            fragment.borrow().print(&mut print_tree);
+        }
+    }
+
+    pub fn scrollable_overflow(&self) -> webrender_api::units::LayoutSize {
+        webrender_api::units::LayoutSize::from_untyped(Size2D::new(
+            self.scrollable_overflow.size.width.px(),
+            self.scrollable_overflow.size.height.px(),
+        ))
+    }
+
+    pub(crate) fn find<T>(
+        &self,
+        mut process_func: impl FnMut(&Fragment, usize, &PhysicalRect<Length>) -> Option<T>,
+    ) -> Option<T> {
+        let info = ContainingBlockManager {
+            for_non_absolute_descendants: &self.initial_containing_block,
+            for_absolute_descendants: None,
+            for_absolute_and_fixed_descendants: &self.initial_containing_block,
+        };
+        self.root_fragments
+            .iter()
+            .find_map(|child| child.borrow().find(&info, 0, &mut process_func))
+    }
+
+    pub fn remove_nodes_in_fragment_tree_from_set(&self, set: &mut FxHashSet<AnimationSetKey>) {
+        self.find(|fragment, _, _| {
+            let tag = fragment.tag()?;
+            set.remove(&AnimationSetKey::new(tag.node, tag.pseudo));
+            None::<()>
+        });
+    }
+
+    pub fn get_content_box_for_node(&self, requested_node: OpaqueNode) -> Rect<Au> {
+        let mut bounding_box = PhysicalRect::zero();
+        let tag_to_find = Tag::new(requested_node);
+        self.find(|fragment, _, containing_block| {
+            if fragment.tag() != Some(tag_to_find) {
+                return None::<()>;
+            }
+
+            let fragment_relative_rect = match fragment {
+                Fragment::Box(fragment) | Fragment::Float(fragment) => fragment
+                    .border_rect()
+                    .to_physical(fragment.style.writing_mode, &containing_block),
+                Fragment::Text(fragment) => fragment
+                    .rect
+                    .to_physical(fragment.parent_style.writing_mode, &containing_block),
+                Fragment::AbsoluteOrFixedPositioned(_) |
+                Fragment::Image(_) |
+                Fragment::IFrame(_) |
+                Fragment::Anonymous(_) => return None,
+            };
+
+            bounding_box = fragment_relative_rect
+                .translate(containing_block.origin.to_vector())
+                .union(&bounding_box);
+            None::<()>
+        });
+
+        Rect::new(
+            Point2D::new(
+                Au::from_f32_px(bounding_box.origin.x.px()),
+                Au::from_f32_px(bounding_box.origin.y.px()),
+            ),
+            Size2D::new(
+                Au::from_f32_px(bounding_box.size.width.px()),
+                Au::from_f32_px(bounding_box.size.height.px()),
+            ),
+        )
+    }
+
+    pub fn get_border_dimensions_for_node(&self, requested_node: OpaqueNode) -> Rect<i32> {
+        let tag_to_find = Tag::new(requested_node);
+        self.find(|fragment, _, containing_block| {
+            if fragment.tag() != Some(tag_to_find) {
+                return None;
+            }
+
+            let (style, padding_rect) = match fragment {
+                Fragment::Box(fragment) => (&fragment.style, fragment.padding_rect()),
+                _ => return None,
+            };
+
+            // https://drafts.csswg.org/cssom-view/#dom-element-clienttop
+            // " If the element has no associated CSS layout box or if the
+            //   CSS layout box is inline, return zero." For this check we
+            // also explicitly ignore the list item portion of the display
+            // style.
+            let display = &style.get_box().display;
+            if display.inside() == style::values::specified::box_::DisplayInside::Flow &&
+                display.outside() == style::values::specified::box_::DisplayOutside::Inline
+            {
+                return Some(Rect::zero());
+            }
+
+            let padding_rect = padding_rect.to_physical(style.writing_mode, &containing_block);
+            let border = style.get_border();
+            Some(Rect::new(
+                Point2D::new(
+                    border.border_left_width.px() as i32,
+                    border.border_top_width.px() as i32,
+                ),
+                Size2D::new(
+                    padding_rect.size.width.px() as i32,
+                    padding_rect.size.height.px() as i32,
+                ),
+            ))
+        })
+        .unwrap_or_else(Rect::zero)
+    }
+
+    pub fn get_scroll_area_for_node(&self, requested_node: OpaqueNode) -> Rect<i32> {
+        let mut scroll_area: PhysicalRect<Length> = PhysicalRect::zero();
+        let tag_to_find = Tag::new(requested_node);
+        self.find(|fragment, _, containing_block| {
+            if fragment.tag() != Some(tag_to_find) {
+                return None::<()>;
+            }
+
+            scroll_area = match fragment {
+                Fragment::Box(fragment) | Fragment::Float(fragment) => fragment
+                    .scrollable_overflow(&containing_block)
+                    .translate(containing_block.origin.to_vector()),
+                Fragment::Text(_) |
+                Fragment::AbsoluteOrFixedPositioned(_) |
+                Fragment::Image(_) |
+                Fragment::IFrame(_) |
+                Fragment::Anonymous(_) => return None,
+            };
+            None::<()>
+        });
+
+        Rect::new(
+            Point2D::new(
+                scroll_area.origin.x.px() as i32,
+                scroll_area.origin.y.px() as i32,
+            ),
+            Size2D::new(
+                scroll_area.size.width.px() as i32,
+                scroll_area.size.height.px() as i32,
+            ),
+        )
+    }
+}
diff --git a/components/layout_2020/fragment_tree/hoisted_shared_fragment.rs b/components/layout_2020/fragment_tree/hoisted_shared_fragment.rs
new file mode 100644
index 00000000000..a9ccbd64e71
--- /dev/null
+++ b/components/layout_2020/fragment_tree/hoisted_shared_fragment.rs
@@ -0,0 +1,68 @@
+/* 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 https://mozilla.org/MPL/2.0/. */
+
+use super::Fragment;
+use crate::{cell::ArcRefCell, geom::flow_relative::Vec2};
+use style::values::computed::{Length, LengthPercentage};
+
+/// A reference to a Fragment which is shared between `HoistedAbsolutelyPositionedBox`
+/// and its placeholder `AbsoluteOrFixedPositionedFragment` in the original tree position.
+/// This will be used later in order to paint this hoisted box in tree order.
+#[derive(Serialize)]
+pub(crate) struct HoistedSharedFragment {
+    pub fragment: Option<ArcRefCell<Fragment>>,
+    pub box_offsets: Vec2<AbsoluteBoxOffsets>,
+}
+
+impl HoistedSharedFragment {
+    pub(crate) fn new(box_offsets: Vec2<AbsoluteBoxOffsets>) -> Self {
+        HoistedSharedFragment {
+            fragment: None,
+            box_offsets,
+        }
+    }
+}
+
+impl HoistedSharedFragment {
+    /// In some cases `inset: auto`-positioned elements do not know their precise
+    /// position until after they're hoisted. This lets us adjust auto values
+    /// after the fact.
+    pub(crate) fn adjust_offsets(&mut self, offsets: Vec2<Length>) {
+        self.box_offsets.inline.adjust_offset(offsets.inline);
+        self.box_offsets.block.adjust_offset(offsets.block);
+    }
+}
+
+#[derive(Clone, Debug, Serialize)]
+pub(crate) enum AbsoluteBoxOffsets {
+    StaticStart {
+        start: Length,
+    },
+    Start {
+        start: LengthPercentage,
+    },
+    End {
+        end: LengthPercentage,
+    },
+    Both {
+        start: LengthPercentage,
+        end: LengthPercentage,
+    },
+}
+
+impl AbsoluteBoxOffsets {
+    pub(crate) fn both_specified(&self) -> bool {
+        match self {
+            AbsoluteBoxOffsets::Both { .. } => return true,
+            _ => return false,
+        }
+    }
+
+    pub(crate) fn adjust_offset(&mut self, new_offset: Length) {
+        match *self {
+            AbsoluteBoxOffsets::StaticStart { ref mut start } => *start = new_offset,
+            _ => (),
+        }
+    }
+}
diff --git a/components/layout_2020/fragment_tree/mod.rs b/components/layout_2020/fragment_tree/mod.rs
index 96f07217ac4..03e112d7adf 100644
--- a/components/layout_2020/fragment_tree/mod.rs
+++ b/components/layout_2020/fragment_tree/mod.rs
@@ -2,8 +2,16 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-mod base;
+mod base_fragment;
+mod box_fragment;
 mod containing_block;
+mod fragment;
+mod fragment_tree;
+mod hoisted_shared_fragment;
 
-pub(crate) use base::*;
+pub(crate) use base_fragment::*;
+pub(crate) use box_fragment::*;
 pub(crate) use containing_block::*;
+pub(crate) use fragment::*;
+pub use fragment_tree::*;
+pub(crate) use hoisted_shared_fragment::*;
diff --git a/components/layout_2020/layout_debug.rs b/components/layout_2020/layout_debug.rs
index 21636f32a89..549075c7595 100644
--- a/components/layout_2020/layout_debug.rs
+++ b/components/layout_2020/layout_debug.rs
@@ -5,7 +5,8 @@
 //! Supports writing a trace file created during each layout scope
 //! that can be viewed by an external tool to make layout debugging easier.
 
-use crate::flow::{BoxTree, FragmentTree};
+use crate::flow::BoxTree;
+use crate::fragment_tree::FragmentTree;
 #[cfg(not(debug_assertions))]
 use serde::ser::{Serialize, Serializer};
 use serde_json::{to_string, to_value, Value};
diff --git a/components/layout_2020/lib.rs b/components/layout_2020/lib.rs
index ce737c89258..91f4bf5ffc9 100644
--- a/components/layout_2020/lib.rs
+++ b/components/layout_2020/lib.rs
@@ -18,7 +18,6 @@ mod flexbox;
 pub mod flow;
 mod formatting_contexts;
 mod fragment_tree;
-mod fragments;
 pub mod geom;
 #[macro_use]
 pub mod layout_debug;
@@ -30,7 +29,8 @@ mod sizing;
 mod style_ext;
 pub mod traversal;
 
-pub use flow::{BoxTree, FragmentTree};
+pub use flow::BoxTree;
+pub use fragment_tree::FragmentTree;
 
 use crate::geom::flow_relative::Vec2;
 use style::properties::ComputedValues;
diff --git a/components/layout_2020/positioned.rs b/components/layout_2020/positioned.rs
index 1b1c931477a..282046456e2 100644
--- a/components/layout_2020/positioned.rs
+++ b/components/layout_2020/positioned.rs
@@ -7,7 +7,9 @@ use crate::context::LayoutContext;
 use crate::dom::NodeExt;
 use crate::dom_traversal::{Contents, NodeAndStyleInfo};
 use crate::formatting_contexts::IndependentFormattingContext;
-use crate::fragments::{BoxFragment, CollapsedBlockMargins, Fragment};
+use crate::fragment_tree::{
+    AbsoluteBoxOffsets, BoxFragment, CollapsedBlockMargins, Fragment, HoistedSharedFragment,
+};
 use crate::geom::flow_relative::{Rect, Sides, Vec2};
 use crate::geom::{LengthOrAuto, LengthPercentageOrAuto};
 use crate::style_ext::{ComputedValuesExt, DisplayInside};
@@ -16,7 +18,7 @@ use rayon::iter::{IntoParallelRefMutIterator, ParallelExtend};
 use rayon_croissant::ParallelIteratorExt;
 use style::computed_values::position::T as Position;
 use style::properties::ComputedValues;
-use style::values::computed::{CSSPixelLength, Length, LengthPercentage};
+use style::values::computed::{CSSPixelLength, Length};
 use style::values::specified::text::TextDecorationLine;
 use style::Zero;
 
@@ -48,67 +50,6 @@ pub(crate) struct HoistedAbsolutelyPositionedBox {
     pub fragment: ArcRefCell<HoistedSharedFragment>,
 }
 
-/// A reference to a Fragment which is shared between `HoistedAbsolutelyPositionedBox`
-/// and its placeholder `AbsoluteOrFixedPositionedFragment` in the original tree position.
-/// This will be used later in order to paint this hoisted box in tree order.
-#[derive(Serialize)]
-pub(crate) struct HoistedSharedFragment {
-    pub(crate) fragment: Option<ArcRefCell<Fragment>>,
-    pub(crate) box_offsets: Vec2<AbsoluteBoxOffsets>,
-}
-
-impl HoistedSharedFragment {
-    pub(crate) fn new(box_offsets: Vec2<AbsoluteBoxOffsets>) -> Self {
-        HoistedSharedFragment {
-            fragment: None,
-            box_offsets,
-        }
-    }
-}
-
-impl HoistedSharedFragment {
-    /// In some cases `inset: auto`-positioned elements do not know their precise
-    /// position until after they're hoisted. This lets us adjust auto values
-    /// after the fact.
-    pub(crate) fn adjust_offsets(&mut self, offsets: Vec2<Length>) {
-        self.box_offsets.inline.adjust_offset(offsets.inline);
-        self.box_offsets.block.adjust_offset(offsets.block);
-    }
-}
-
-#[derive(Clone, Debug, Serialize)]
-pub(crate) enum AbsoluteBoxOffsets {
-    StaticStart {
-        start: Length,
-    },
-    Start {
-        start: LengthPercentage,
-    },
-    End {
-        end: LengthPercentage,
-    },
-    Both {
-        start: LengthPercentage,
-        end: LengthPercentage,
-    },
-}
-
-impl AbsoluteBoxOffsets {
-    fn both_specified(&self) -> bool {
-        match self {
-            AbsoluteBoxOffsets::Both { .. } => return true,
-            _ => return false,
-        }
-    }
-
-    fn adjust_offset(&mut self, new_offset: Length) {
-        match *self {
-            AbsoluteBoxOffsets::StaticStart { ref mut start } => *start = new_offset,
-            _ => (),
-        }
-    }
-}
-
 impl AbsolutelyPositionedBox {
     pub fn construct<'dom>(
         context: &LayoutContext,
diff --git a/components/layout_2020/query.rs b/components/layout_2020/query.rs
index 7d19963b951..ed5e7f1b54c 100644
--- a/components/layout_2020/query.rs
+++ b/components/layout_2020/query.rs
@@ -4,9 +4,7 @@
 
 //! Utilities for querying the layout, as needed by the layout thread.
 use crate::context::LayoutContext;
-use crate::flow::FragmentTree;
-use crate::fragment_tree::{FragmentFlags, Tag};
-use crate::fragments::Fragment;
+use crate::fragment_tree::{Fragment, FragmentFlags, FragmentTree, Tag};
 use app_units::Au;
 use euclid::default::{Point2D, Rect};
 use euclid::Size2D;
diff --git a/components/layout_2020/replaced.rs b/components/layout_2020/replaced.rs
index 48d06802edf..d350086c63a 100644
--- a/components/layout_2020/replaced.rs
+++ b/components/layout_2020/replaced.rs
@@ -4,8 +4,7 @@
 
 use crate::context::LayoutContext;
 use crate::dom::NodeExt;
-use crate::fragment_tree::BaseFragmentInfo;
-use crate::fragments::{Fragment, IFrameFragment, ImageFragment};
+use crate::fragment_tree::{BaseFragmentInfo, Fragment, IFrameFragment, ImageFragment};
 use crate::geom::flow_relative::{Rect, Vec2};
 use crate::geom::PhysicalSize;
 use crate::sizing::ContentSizes;