forward/back navigation with shift+backspace and backspace

script caches last loaded url -- currently no caching policy
naive caching of render layers for near-instant forward/back
handling evicted pipelines is currently broken

16 files changed, 213 insertions, 97 deletions

diff --git a/src/components/gfx/render_task.rs b/src/components/gfx/render_task.rs
index db0083d5876..b0fed806ef2 100644
--- a/src/components/gfx/render_task.rs
+++ b/src/components/gfx/render_task.rs
@@ -21,7 +21,6 @@ use render_context::RenderContext;
 use std::cell::Cell;
 use std::comm::{Chan, Port, SharedChan};
 use std::uint;
-use std::util::replace;
 
 use servo_util::time::{ProfilerChan, profile};
 use servo_util::time;
@@ -73,7 +72,7 @@ priv struct RenderTask<C> {
     /// A token that grants permission to send paint messages to compositor
     compositor_token: Option<~CompositorToken>,
     /// Cached copy of last layers rendered
-    next_paint_msg: Option<(LayerBufferSet, Size2D<uint>)>,
+    last_paint_msg: Option<(LayerBufferSet, Size2D<uint>)>,
 }
 
 impl<C: RenderListener + Owned> RenderTask<C> {
@@ -108,7 +107,7 @@ impl<C: RenderListener + Owned> RenderTask<C> {
 
                 constellation_chan: constellation_chan.take(),
                 compositor_token: None,
-                next_paint_msg: None,
+                last_paint_msg: None,
             };
 
             render_task.start();
@@ -129,11 +128,9 @@ impl<C: RenderListener + Owned> RenderTask<C> {
                 }
                 TokenBestowMsg(token) => {
                     self.compositor_token = Some(token);
-                    let next_paint_msg = replace(&mut self.next_paint_msg, None);
-                    match next_paint_msg {
-                        Some((layer_buffer_set, layer_size)) => {
-                            println("retrieving cached paint msg");
-                            self.compositor.paint(layer_buffer_set, layer_size);
+                    match self.last_paint_msg {
+                        Some((ref layer_buffer_set, ref layer_size)) => {
+                            self.compositor.paint(layer_buffer_set.clone(), *layer_size);
                             self.compositor.set_render_state(IdleRenderState);
                         }
                         None => {}
@@ -162,7 +159,7 @@ impl<C: RenderListener + Owned> RenderTask<C> {
         }
 
         self.compositor.set_render_state(RenderingRenderState);
-        do profile(time::RenderingCategory, self.profiler_chan.clone()) {
+        do time::profile(time::RenderingCategory, self.profiler_chan.clone()) {
             let tile_size = self.opts.tile_size;
 
             // FIXME: Try not to create a new array here.
@@ -235,12 +232,10 @@ impl<C: RenderListener + Owned> RenderTask<C> {
 
             debug!("render_task: returning surface");
             if self.compositor_token.is_some() {
-                self.compositor.paint(layer_buffer_set, render_layer.size);
-            }
-            else {
-                println("caching paint msg");
-                self.next_paint_msg = Some((layer_buffer_set, render_layer.size));
+                self.compositor.paint(layer_buffer_set.clone(), render_layer.size);
             }
+            debug!("caching paint msg");
+            self.last_paint_msg = Some((layer_buffer_set, render_layer.size));
             self.compositor.set_render_state(IdleRenderState);
         }
     }
diff --git a/src/components/main/compositing/mod.rs b/src/components/main/compositing/mod.rs
index 873c46a645a..e8a847e5c30 100644
--- a/src/components/main/compositing/mod.rs
+++ b/src/components/main/compositing/mod.rs
@@ -4,12 +4,14 @@
 
 use platform::{Application, Window};
 use script::dom::event::{Event, ClickEvent, MouseDownEvent, MouseUpEvent, ResizeEvent};
-use script::script_task::{LoadMsg, SendEventMsg};
+use script::script_task::{LoadMsg, NavigateMsg, SendEventMsg};
 use script::layout_interface::{LayoutChan, RouteScriptMsg};
 use windowing::{ApplicationMethods, WindowMethods, WindowMouseEvent, WindowClickEvent};
 use windowing::{WindowMouseDownEvent, WindowMouseUpEvent};
+
 use servo_msg::compositor::{RenderListener, LayerBufferSet, RenderState};
 use servo_msg::compositor::{ReadyState, ScriptListener};
+use servo_msg::constellation;
 use gfx::render_task::{RenderChan, ReRenderMsg};
 
 use azure::azure_hl::{DataSourceSurface, DrawTarget, SourceSurfaceMethods, current_gl_context};
@@ -32,6 +34,7 @@ use servo_util::{time, url};
 use servo_util::time::profile;
 use servo_util::time::ProfilerChan;
 
+pub use windowing;
 
 /// The implementation of the layers-based compositor.
 #[deriving(Clone)]
@@ -180,13 +183,22 @@ impl CompositorTask {
 
         let update_layout_callbacks: @fn(LayoutChan) = |layout_chan: LayoutChan| {
             let layout_chan_clone = layout_chan.clone();
+            do window.set_navigation_callback |direction| {
+                let direction = match direction {
+                    windowing::Forward => constellation::Forward,
+                    windowing::Back => constellation::Back,
+                };
+                layout_chan_clone.send(RouteScriptMsg(NavigateMsg(direction)));
+            }
+
+            let layout_chan_clone = layout_chan.clone();
             // Hook the windowing system's resize callback up to the resize rate limiter.
             do window.set_resize_callback |width, height| {
                 let new_size = Size2D(width as int, height as int);
                 if *window_size != new_size {
                     debug!("osmain: window resized to %ux%u", width, height);
                     *window_size = new_size;
-                    layout_chan_clone.chan.send(RouteScriptMsg(SendEventMsg(ResizeEvent(width, height))));
+                    layout_chan_clone.send(RouteScriptMsg(SendEventMsg(ResizeEvent(width, height))));
                 } else {
                     debug!("osmain: dropping window resize since size is still %ux%u", width, height);
                 }
@@ -197,7 +209,7 @@ impl CompositorTask {
             // When the user enters a new URL, load it.
             do window.set_load_url_callback |url_string| {
                 debug!("osmain: loading URL `%s`", url_string);
-                layout_chan_clone.chan.send(RouteScriptMsg(LoadMsg(url::make_url(url_string.to_str(), None))));
+                layout_chan_clone.send(RouteScriptMsg(LoadMsg(url::make_url(url_string.to_str(), None))));
             }
 
             let layout_chan_clone = layout_chan.clone();
@@ -229,7 +241,7 @@ impl CompositorTask {
                         event = MouseUpEvent(button, world_mouse_point(layer_mouse_point));
                     }
                 }
-                layout_chan_clone.chan.send(RouteScriptMsg(SendEventMsg(event)));
+                layout_chan_clone.send(RouteScriptMsg(SendEventMsg(event)));
             }
         };
 
@@ -410,6 +422,7 @@ impl CompositorTask {
             
             window.set_needs_display()
         }
+
         // Enter the main event loop.
         while !*done {
             // Check for new messages coming from the rendering task.
diff --git a/src/components/main/constellation.rs b/src/components/main/constellation.rs
index bed38b8739f..c522bb033ef 100644
--- a/src/components/main/constellation.rs
+++ b/src/components/main/constellation.rs
@@ -12,14 +12,15 @@ use gfx::opts::Opts;
 use gfx::render_task::{TokenBestowMsg, TokenProcureMsg};
 use pipeline::Pipeline;
 use servo_msg::compositor::{CompositorToken};
-use servo_msg::constellation::{ConstellationChan, ExitMsg, LoadUrlMsg, Msg, RendererReadyMsg};
-use servo_msg::constellation::TokenSurrenderMsg;
-use script::script_task::{ExecuteMsg, LoadMsg};
+use servo_msg::constellation::{ConstellationChan, ExitMsg, LoadUrlMsg, Msg, NavigateMsg};
+use servo_msg::constellation::{Forward, Back, RendererReadyMsg, TokenSurrenderMsg};
+use script::script_task::ExecuteMsg;
 use servo_net::image_cache_task::{ImageCacheTask, ImageCacheTaskClient};
 use servo_net::resource_task::ResourceTask;
 use servo_net::resource_task;
 use servo_util::time::ProfilerChan;
 use std::hashmap::HashMap;
+use std::util::replace;
 
 pub struct Constellation {
     chan: ConstellationChan,
@@ -30,12 +31,20 @@ pub struct Constellation {
     pipelines: HashMap<uint, Pipeline>,
     navigation_context: NavigationContext,
     next_id: uint,
-    current_token_holder: Option<uint>,
-    loading: Option<uint>,
+    current_token_bearer: Option<uint>,
+    next_token_bearer: Option<(uint, NavigationType)>,
+    compositor_token: Option<~CompositorToken>,
     profiler_chan: ProfilerChan,
     opts: Opts,
 }
 
+/// Represents the two different ways to which a page can be navigated
+enum NavigationType {
+    Load,               // entered or clicked on a url
+    Navigate,           // browser forward/back buttons
+}
+
+/// Stores the ID's of the pipelines previous and next in the browser's history
 pub struct NavigationContext {
     previous: ~[uint],
     next: ~[uint],
@@ -52,18 +61,16 @@ impl NavigationContext {
     }
 
     pub fn back(&mut self) -> uint {
-        do self.current.mutate |id| {
-            self.next.push(id);
-            self.previous.pop()
-        }
+        self.next.push(self.current.get());
+        self.current = Some(self.previous.pop());
+        debug!("previous: %? next: %? current: %?", self.previous, self.next, self.current);
         self.current.get()
     }
 
     pub fn forward(&mut self) -> uint {
-        do self.current.mutate |id| {
-            self.previous.push(id);
-            self.next.pop()
-        }
+        self.previous.push(self.current.get());
+        self.current = Some(self.next.pop());
+        debug!("previous: %? next: %? current: %?", self.previous, self.next, self.current);
         self.current.get()
     }
 
@@ -102,8 +109,9 @@ impl Constellation {
                     pipelines: HashMap::new(),
                     navigation_context: NavigationContext::new(),
                     next_id: 0,
-                    current_token_holder: None,
-                    loading: None,
+                    current_token_bearer: None,
+                    next_token_bearer: None,
+                    compositor_token: Some(~CompositorToken::new()),
                     profiler_chan: profiler_chan.clone(),
                     opts: opts.take(),
                 };
@@ -122,17 +130,19 @@ impl Constellation {
         }
     }
 
+    /// Helper function for getting a unique pipeline ID
     fn get_next_id(&mut self) -> uint {
         let id = self.next_id;
         self.next_id = id + 1;
         id
     }
 
+    /// Handles loading pages, navigation, and granting access to the compositor
     fn handle_request(&mut self, request: Msg) -> bool {
         match request {
             LoadUrlMsg(url) => {
                 let pipeline_id = self.get_next_id();
-                let pipeline = Pipeline::create(pipeline_id,
+                let mut pipeline = Pipeline::create(pipeline_id,
                                                 self.chan.clone(),
                                                 self.compositor_chan.clone(),
                                                 self.image_cache_task.clone(),
@@ -142,23 +152,41 @@ impl Constellation {
                 if url.path.ends_with(".js") {
                     pipeline.script_chan.send(ExecuteMsg(url));
                 } else {
-                    pipeline.script_chan.send(LoadMsg(url));
-                    self.loading = Some(pipeline_id);
+                    pipeline.load(url);
+                    self.next_token_bearer = Some((pipeline_id, Load));
                 }
                 self.pipelines.insert(pipeline_id, pipeline);
             }
 
+            NavigateMsg(direction) => {
+                debug!("received message to navigate %?", direction);
+                let destination_id = match direction {
+                    Forward => {
+                        if self.navigation_context.next.is_empty() {
+                            debug!("no next page to navigate to");
+                            return true
+                        }
+                        self.navigation_context.forward()
+                    }
+                    Back => {
+                        if self.navigation_context.previous.is_empty() {
+                            debug!("no previous page to navigate to");
+                            return true
+                        }
+                        self.navigation_context.back()
+                    }
+                };
+                debug!("navigating to pipeline %u", destination_id);
+                self.pipelines.get(&destination_id).reload();
+                self.next_token_bearer = Some((destination_id, Navigate));
+                self.procure_or_bestow();
+            }
+
             RendererReadyMsg(pipeline_id) => {
-                let loading = self.loading.clone();
-                do loading.map() |&id| {
+                let next_token_bearer = self.next_token_bearer;
+                for next_token_bearer.iter().advance |&(id, _)| {
                     if pipeline_id == id {
-                        match self.current_token_holder {
-                            Some(ref id) => {
-                                let current_holder = self.pipelines.get(id);
-                                current_holder.render_chan.send(TokenProcureMsg);
-                            }
-                            None => self.bestow_compositor_token(id, ~CompositorToken::new())
-                        }
+                        self.procure_or_bestow();
                     }
                 };
             }
@@ -166,9 +194,9 @@ impl Constellation {
             TokenSurrenderMsg(token) => {
                 self.remove_active_pipeline();
                 let token = Cell::new(token);
-                let loading = self.loading;
-                do loading.map |&id| {
-                    self.bestow_compositor_token(id, token.take());
+                let next_token_bearer = self.next_token_bearer;
+                for next_token_bearer.iter().advance |&(id, nav_type)| {
+                    self.bestow_compositor_token(id, token.take(), nav_type);
                 };
             }
 
@@ -186,23 +214,45 @@ impl Constellation {
         true
     }
     
+    /// Either procures the token, sends the token to next bearer, or does nothing if waiting for token surrender.
+    fn procure_or_bestow(&mut self) {
+        let current_token_bearer = replace(&mut self.current_token_bearer, None);
+        match current_token_bearer {
+            Some(ref id) => {
+                let pipeline = self.pipelines.get(id);
+                pipeline.render_chan.send(TokenProcureMsg);
+            }
+            None => {
+                let compositor_token = replace(&mut self.compositor_token, None);
+                for compositor_token.iter().advance |&token| {
+                    let (id, nav_type) = self.next_token_bearer.get();
+                    self.bestow_compositor_token(id, token, nav_type);
+                }
+            }
+        };
+    }
+
     fn remove_active_pipeline(&mut self) {
 // FIXME(tkuehn): currently, pipelines are not removed at all
-//        do self.current_token_holder.map |id| {
+//        do self.current_token_bearer.map |id| {
 //            self.pipelines.pop(id).unwrap().exit();
 //        };
 
-        self.current_token_holder = None;
+        self.current_token_bearer = None;
     }
 
-    fn bestow_compositor_token(&mut self, id: uint, compositor_token: ~CompositorToken) {
+    fn bestow_compositor_token(&mut self, id: uint, compositor_token: ~CompositorToken, navigation_type: NavigationType) {
         let pipeline = self.pipelines.get(&id);
         pipeline.render_chan.send(TokenBestowMsg(compositor_token));
         self.compositor_chan.send(SetLayoutChan(pipeline.layout_chan.clone()));
         self.compositor_chan.send(SetRenderChan(pipeline.render_chan.clone()));
-        self.current_token_holder = Some(id);
-        self.loading = None;
-        self.navigation_context.navigate(id);
+        self.current_token_bearer = Some(id);
+        self.next_token_bearer = None;
+        // Don't navigate on Navigate type, because that is handled by forward/back
+        match navigation_type {
+            Load => self.navigation_context.navigate(id),
+            _ => {}
+        }
     }
 }
 
diff --git a/src/components/main/layout/layout_task.rs b/src/components/main/layout/layout_task.rs
index cab29204559..0f93798af10 100644
--- a/src/components/main/layout/layout_task.rs
+++ b/src/components/main/layout/layout_task.rs
@@ -141,6 +141,7 @@ impl LayoutTask {
                 }
             }
             RouteScriptMsg(script_msg) => {
+                debug!("layout: routing %? to script task", script_msg);
                 self.route_script_msg(script_msg);
             }
             ExitMsg => {
diff --git a/src/components/main/layout/text.rs b/src/components/main/layout/text.rs
index b6b3e43eb99..59093ed70ff 100644
--- a/src/components/main/layout/text.rs
+++ b/src/components/main/layout/text.rs
@@ -234,7 +234,7 @@ impl TextRunScanner {
                 for clump.eachi |i| {
                     let range = new_ranges[i - self.clump.begin()];
                     if range.length() == 0 {
-                        error!("Elided an `UnscannedTextbox` because it was zero-length after \
+                        debug!("Elided an `UnscannedTextbox` because it was zero-length after \
                                 compression; %s",
                                in_boxes[i].debug_str());
                         loop
diff --git a/src/components/main/pipeline.rs b/src/components/main/pipeline.rs
index 0a801d7d577..dcb14569c4c 100644
--- a/src/components/main/pipeline.rs
+++ b/src/components/main/pipeline.rs
@@ -2,13 +2,14 @@
  * 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 extra::net::url::Url;
 use compositing::CompositorChan;
 use gfx::render_task::{RenderChan, RenderTask};
 use gfx::render_task;
 use gfx::opts::Opts;
 use layout::layout_task::LayoutTask;
 use script::layout_interface::LayoutChan;
-use script::layout_interface;
+use script::script_task::LoadMsg;
 use servo_msg::constellation::{ConstellationChan};
 use script::script_task::{ScriptTask, ScriptChan, ScriptMsg};
 use script::script_task;
@@ -23,6 +24,8 @@ pub struct Pipeline {
     script_chan: ScriptChan,
     layout_chan: LayoutChan,
     render_chan: RenderChan,
+    /// The most recently loaded url
+    url: Option<Url>,
 }
 
 impl Pipeline {
@@ -90,12 +93,24 @@ impl Pipeline {
             script_chan: script_chan,
             layout_chan: layout_chan,
             render_chan: render_chan,
+            url: None,
+        }
+    }
+
+    pub fn load(&mut self, url: Url) {
+        self.url = Some(url.clone());
+        self.script_chan.send(LoadMsg(url));
+    }
+
+    pub fn reload(&self) {
+        for self.url.iter().advance |&url| {
+            self.script_chan.send(LoadMsg(url));
         }
     }
 
     pub fn exit(&self) {
+        // Script task handles shutting down layout, as well
         self.script_chan.send(script_task::ExitMsg);
-        self.layout_chan.send(layout_interface::ExitMsg);
 
         let (response_port, response_chan) = comm::stream();
         self.render_chan.send(render_task::ExitMsg(response_chan));
diff --git a/src/components/main/platform/common/glut_windowing.rs b/src/components/main/platform/common/glut_windowing.rs
index a47fab68acc..da07fa99ef0 100644
--- a/src/components/main/platform/common/glut_windowing.rs
+++ b/src/components/main/platform/common/glut_windowing.rs
@@ -9,7 +9,7 @@
 
 use windowing::{ApplicationMethods, CompositeCallback, LoadUrlCallback, MouseCallback};
 use windowing::{ResizeCallback, ScrollCallback, WindowMethods, WindowMouseEvent, WindowClickEvent};
-use windowing::{WindowMouseDownEvent, WindowMouseUpEvent, ZoomCallback};
+use windowing::{WindowMouseDownEvent, WindowMouseUpEvent, ZoomCallback, Forward, Back, NavigationCallback};
 
 use alert::{Alert, AlertMethods};
 use std::libc::c_int;
@@ -17,7 +17,8 @@ use geom::point::Point2D;
 use geom::size::Size2D;
 use servo_msg::compositor::{IdleRenderState, RenderState, RenderingRenderState};
 use servo_msg::compositor::{FinishedLoading, Loading, PerformingLayout, ReadyState};
-use glut::glut::{ACTIVE_CTRL, DOUBLE, HAVE_PRECISE_MOUSE_WHEEL, WindowHeight, WindowWidth};
+use glut::glut::{ACTIVE_CTRL, ACTIVE_SHIFT, DOUBLE, HAVE_PRECISE_MOUSE_WHEEL, WindowHeight};
+use glut::glut::WindowWidth;
 use glut::glut;
 use glut::machack;
 
@@ -44,6 +45,7 @@ pub struct Window {
     mouse_callback: Option<MouseCallback>,
     scroll_callback: Option<ScrollCallback>,
     zoom_callback: Option<ZoomCallback>,
+    navigation_callback: Option<NavigationCallback>,
 
     drag_origin: Point2D<c_int>,
 
@@ -72,6 +74,7 @@ impl WindowMethods<Application> for Window {
             mouse_callback: None,
             scroll_callback: None,
             zoom_callback: None,
+            navigation_callback: None,
 
             drag_origin: Point2D(0 as c_int, 0),
 
@@ -177,6 +180,11 @@ impl WindowMethods<Application> for Window {
         self.zoom_callback = Some(new_zoom_callback)
     }
 
+    /// Registers a callback to be run when backspace or shift-backspace is pressed.
+    pub fn set_navigation_callback(&mut self, new_navigation_callback: NavigationCallback) {
+        self.navigation_callback = Some(new_navigation_callback)
+    }
+
     /// Spins the event loop.
     pub fn check_loop(@mut self) {
         glut::check_loop()
@@ -226,20 +234,33 @@ impl Window {
 
     /// Helper function to handle keyboard events.
     fn handle_key(&self, key: u8) {
-        debug!("got key: %d", key as int);
+        debug!("got key: %?", key);
+        let modifiers = glut::get_modifiers();
         match key {
-            12 => self.load_url(),                                                      // Ctrl+L
-            k if k == ('=' as u8) && (glut::get_modifiers() & ACTIVE_CTRL) != 0 => {    // Ctrl++
+            12 => self.load_url(),                                              // Ctrl+L
+            31 if (modifiers & ACTIVE_CTRL) != 0 => {                           // Ctrl+-
                 for self.zoom_callback.iter().advance |&callback| {
-                    callback(0.1);
+                    callback(-0.1);
                 }
             }
-            k if k == 31 && (glut::get_modifiers() & ACTIVE_CTRL) != 0 => {             // Ctrl+-
-                for self.zoom_callback.iter().advance |&callback| {
-                    callback(-0.1);
+            127 => {
+                for self.navigation_callback.iter().advance |&callback| {
+                    if (modifiers & ACTIVE_SHIFT) != 0 {                            // Shift+Backspace
+                        callback(Forward);
+                    }
+                    else {
+                        callback(Back);
+                    }
+                }
+            }
+            c => match c as char {
+                '=' if (modifiers & ACTIVE_CTRL) != 0 => {                      // Ctrl++
+                    for self.zoom_callback.iter().advance |&callback| {
+                        callback(0.1);
+                    }
                 }
+                _ => {}
             }
-            _ => {}
         }
     }
 
diff --git a/src/components/main/windowing.rs b/src/components/main/windowing.rs
index 56a6cbcb0ab..dc9f5472b6c 100644
--- a/src/components/main/windowing.rs
+++ b/src/components/main/windowing.rs
@@ -14,6 +14,11 @@ pub enum WindowMouseEvent {
     WindowMouseUpEvent(uint, Point2D<f32>),
 }
 
+pub enum WindowNavigateMsg {
+    Forward,
+    Back,
+}
+
 /// Type of the function that is called when the screen is to be redisplayed.
 pub type CompositeCallback = @fn();
 
@@ -29,9 +34,12 @@ pub type MouseCallback = @fn(WindowMouseEvent);
 /// Type of the function that is called when the user scrolls.
 pub type ScrollCallback = @fn(Point2D<f32>);
 
-///Type of the function that is called when the user zooms.
+/// Type of the function that is called when the user zooms.
 pub type ZoomCallback = @fn(f32);
 
+/// Type of the function that is called when the user clicks backspace or shift-backspace
+pub type NavigationCallback = @fn(WindowNavigateMsg);
+
 /// Methods for an abstract Application.
 pub trait ApplicationMethods {
     fn new() -> Self;
@@ -57,6 +65,8 @@ pub trait WindowMethods<A> {
     pub fn set_scroll_callback(&mut self, new_scroll_callback: ScrollCallback);
     /// Registers a callback to run when the user zooms.
     pub fn set_zoom_callback(&mut self, new_zoom_callback: ZoomCallback);
+    /// Registers a callback to run when the user presses backspace or shift-backspace.
+    pub fn set_navigation_callback(&mut self, new_navigation_callback: NavigationCallback);
 
     /// Spins the event loop.
     pub fn check_loop(@mut self);
diff --git a/src/components/msg/compositor.rs b/src/components/msg/compositor.rs
index 6d8fe2ecc50..d677cdfdaf1 100644
--- a/src/components/msg/compositor.rs
+++ b/src/components/msg/compositor.rs
@@ -8,6 +8,7 @@ use geom::rect::Rect;
 use geom::size::Size2D;
 use std::util::NonCopyable;
 
+#[deriving(Clone)]
 pub struct LayerBuffer {
     draw_target: DrawTarget,
 
@@ -23,6 +24,7 @@ pub struct LayerBuffer {
 
 /// A set of layer buffers. This is an atomic unit used to switch between the front and back
 /// buffers.
+#[deriving(Clone)]
 pub struct LayerBufferSet {
     buffers: ~[LayerBuffer]
 }
diff --git a/src/components/msg/constellation.rs b/src/components/msg/constellation.rs
index b0b97ba528a..aa6442a525d 100644
--- a/src/components/msg/constellation.rs
+++ b/src/components/msg/constellation.rs
@@ -27,8 +27,13 @@ impl ConstellationChan {
 
 pub enum Msg {
     LoadUrlMsg(Url),
+    NavigateMsg(NavigationDirection),
     ExitMsg(Chan<()>),
     RendererReadyMsg(uint),
     TokenSurrenderMsg(~CompositorToken),
 }
 
+pub enum NavigationDirection {
+    Forward,
+    Back,
+}
diff --git a/src/components/script/dom/characterdata.rs b/src/components/script/dom/characterdata.rs
index 91838432f6b..9d77d3b64ee 100644
--- a/src/components/script/dom/characterdata.rs
+++ b/src/components/script/dom/characterdata.rs
@@ -7,8 +7,6 @@
 use dom::bindings::utils::{DOMString, null_string, str};
 use dom::node::{Node, NodeTypeId, ScriptView};
 
-use std::str;
-
 pub struct CharacterData {
     parent: Node<ScriptView>,
     data: DOMString
diff --git a/src/components/script/dom/node.rs b/src/components/script/dom/node.rs
index d415288bfa2..c573f9912fb 100644
--- a/src/components/script/dom/node.rs
+++ b/src/components/script/dom/node.rs
@@ -367,7 +367,7 @@ impl<View> AbstractNode<View> {
     pub fn dump_indent(&self, indent: uint) {
         let mut s = ~"";
         for uint::range(0u, indent) |_i| {
-            s += ~"    ";
+            s += "    ";
         }
 
         s += self.debug_str();
diff --git a/src/components/script/html/cssparse.rs b/src/components/script/html/cssparse.rs
index 1278ba47f6c..95a600682ac 100644
--- a/src/components/script/html/cssparse.rs
+++ b/src/components/script/html/cssparse.rs
@@ -8,7 +8,6 @@ use std::cell::Cell;
 use std::comm;
 use std::comm::Port;
 use std::task;
-use std::str;
 use newcss::stylesheet::Stylesheet;
 use newcss::util::DataStream;
 use servo_net::resource_task::{ResourceTask, ProgressMsg, Load, Payload, Done};
diff --git a/src/components/script/script_task.rs b/src/components/script/script_task.rs
index 00b5611caba..8acee44e891 100644
--- a/src/components/script/script_task.rs
+++ b/src/components/script/script_task.rs
@@ -19,7 +19,9 @@ use layout_interface::{LayoutChan, MatchSelectorsDocumentDamage, QueryMsg, Reflo
 use layout_interface::{ReflowDocumentDamage, ReflowForDisplay, ReflowForScriptQuery, ReflowGoal};
 use layout_interface::ReflowMsg;
 use layout_interface;
-use servo_msg::constellation::{ConstellationChan, LoadUrlMsg, RendererReadyMsg};
+use servo_msg::constellation::{ConstellationChan, LoadUrlMsg, NavigationDirection};
+use servo_msg::constellation::RendererReadyMsg;
+use servo_msg::constellation;
 
 use std::cast::transmute;
 use std::cell::Cell;
@@ -53,6 +55,8 @@ pub enum ScriptMsg {
     LoadMsg(Url),
     /// Executes a standalone script.
     ExecuteMsg(Url),
+    /// Instructs the script task to send a navigate message to the constellation.
+    NavigateMsg(NavigationDirection),
     /// Sends a DOM event.
     SendEventMsg(Event),
     /// Fires a JavaScript timeout.
@@ -136,6 +140,11 @@ pub struct ScriptTask {
     window_size: Size2D<uint>,
     /// What parts of the document are dirty, if any.
     damage: Option<DocumentDamage>,
+
+    /// Cached copy of the most recent url loaded by the script
+    /// TODO(tkuehn): this currently does not follow any particular caching policy
+    /// and simply caches pages forever (!).
+    last_loaded_url: Option<Url>,
 }
 
 fn global_script_context_key(_: @ScriptTask) {}
@@ -212,6 +221,8 @@ impl ScriptTask {
 
             window_size: Size2D(800u, 600),
             damage: None,
+
+            last_loaded_url: None,
         };
         // Indirection for Rust Issue #6248, dynamic freeze scope artifically extended
         let script_task_ptr = {
@@ -264,31 +275,18 @@ impl ScriptTask {
     /// Handles an incoming control message.
     fn handle_msg(&mut self) -> bool {
         match self.script_port.recv() {
-            LoadMsg(url) => {
-                self.load(url);
-                true
-            }
-            ExecuteMsg(url) => {
-                self.handle_execute_msg(url);
-                true
-            }
-            SendEventMsg(event) => {
-                self.handle_event(event);
-                true
-            }
-            FireTimerMsg(timer_data) => {
-                self.handle_fire_timer_msg(timer_data);
-                true
-            }
-            ReflowCompleteMsg => {
-                self.handle_reflow_complete_msg();
-                true
-            }
+            LoadMsg(url) => self.load(url),
+            ExecuteMsg(url) => self.handle_execute_msg(url),
+            SendEventMsg(event) => self.handle_event(event),
+            FireTimerMsg(timer_data) => self.handle_fire_timer_msg(timer_data),
+            NavigateMsg(direction) => self.handle_navigate_msg(direction),
+            ReflowCompleteMsg => self.handle_reflow_complete_msg(),
             ExitMsg => {
                 self.handle_exit_msg();
-                false
+                return false
             }
         }
+        true
     }
 
     /// Handles a request to execute a script.
@@ -337,6 +335,11 @@ impl ScriptTask {
         self.compositor.set_ready_state(FinishedLoading);
     }
 
+    /// Handles a navigate forward or backward message.
+    fn handle_navigate_msg(&self, direction: NavigationDirection) {
+        self.constellation_chan.send(constellation::NavigateMsg(direction));
+    }
+
     /// Handles a request to exit the script task and shut down layout.
     fn handle_exit_msg(&mut self) {
         self.join_layout();
@@ -350,6 +353,9 @@ impl ScriptTask {
     /// The entry point to document loading. Defines bindings, sets up the window and document
     /// objects, parses HTML and CSS, and kicks off initial layout.
     fn load(&mut self, url: Url) {
+        for self.last_loaded_url.iter().advance |&last_loaded_url| {
+            if url == last_loaded_url { return; }
+        }
         // Define the script DOM bindings.
         //
         // FIXME: Can this be done earlier, to save the flag?
@@ -362,7 +368,7 @@ impl ScriptTask {
         // Parse HTML.
         //
         // Note: We can parse the next document in parallel with any previous documents.
-        let html_parsing_result = hubbub_html_parser::parse_html(copy url,
+        let html_parsing_result = hubbub_html_parser::parse_html(url.clone(),
                                                                  self.resource_task.clone(),
                                                                  self.image_cache_task.clone());
 
@@ -396,7 +402,7 @@ impl ScriptTask {
         self.root_frame = Some(Frame {
             document: document,
             window: window,
-            url: url
+            url: url.clone(),
         });
 
         // Perform the initial reflow.
@@ -416,6 +422,7 @@ impl ScriptTask {
                                                     ~"???",
                                                     1);
         }
+        self.last_loaded_url = Some(url);
     }
 
     /// Sends a ping to layout and waits for the response. The response will arrive when the
diff --git a/src/components/util/time.rs b/src/components/util/time.rs
index 00e222baaeb..ac94abcfb7a 100644
--- a/src/components/util/time.rs
+++ b/src/components/util/time.rs
@@ -152,7 +152,7 @@ impl Profiler {
 
     priv fn print_buckets(&mut self) {
         println(fmt!("%31s %15s %15s %15s %15s %15s",
-                         "_category (ms)_", "_mean (ms)_", "_median (ms)_",
+                         "_category_", "_mean (ms)_", "_median (ms)_",
                          "_min (ms)_", "_max (ms)_", "_bucket size_"));
         for self.buckets.mut_iter().advance |bucket| {
             match *bucket {
diff --git a/src/support/alert/rust-alert b/src/support/alert/rust-alert
-Subproject 502ec156da38860a4dff911cf8f33f388a0a188
+Subproject 68875af396cb583e670dd5caad99431dac62f8d