Split layout code into a separate crate.

src/components/layout/layout_task.rs

@@ -0,0 +1,916 @@
+/* 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/. */
+
+//! The layout task. Performs layout on the DOM, builds display lists and sends them to be
+/// rendered.
+
+use css::matching::{ApplicableDeclarations, ApplicableDeclarationsCache, MatchMethods};
+use css::matching::{StyleSharingCandidateCache};
+use css::select::new_stylist;
+use css::node_style::StyledNode;
+use construct::{FlowConstructionResult, NoConstructionResult};
+use context::LayoutContext;
+use flow::{Flow, ImmutableFlowUtils, MutableFlowUtils, MutableOwnedFlowUtils};
+use flow::{PreorderFlowTraversal, PostorderFlowTraversal};
+use flow;
+use flow_ref::FlowRef;
+use incremental::RestyleDamage;
+use parallel::UnsafeFlow;
+use parallel;
+use util::{LayoutDataAccess, LayoutDataWrapper, OpaqueNodeMethods, ToGfxColor};
+use wrapper::{LayoutNode, TLayoutNode, ThreadSafeLayoutNode};
+
+use collections::dlist::DList;
+use geom::point::Point2D;
+use geom::rect::Rect;
+use geom::size::Size2D;
+use gfx::display_list::{ClipDisplayItemClass, ContentStackingLevel, DisplayItem};
+use gfx::display_list::{DisplayItemIterator, DisplayList, OpaqueNode};
+use gfx::font_context::{FontContext, FontContextInfo};
+use gfx::render_task::{RenderMsg, RenderChan, RenderLayer};
+use gfx::{render_task, color};
+use script::dom::bindings::js::JS;
+use script::dom::event::ReflowEvent;
+use script::dom::node::{ElementNodeTypeId, LayoutDataRef, Node};
+use script::dom::element::{HTMLBodyElementTypeId, HTMLHtmlElementTypeId};
+use script::layout_interface::{AddStylesheetMsg, ContentBoxQuery};
+use script::layout_interface::{ContentBoxesQuery, ContentBoxesResponse, ExitNowMsg, LayoutQuery};
+use script::layout_interface::{HitTestQuery, ContentBoxResponse, HitTestResponse, MouseOverQuery, MouseOverResponse};
+use script::layout_interface::{ContentChangedDocumentDamage, LayoutChan, Msg, PrepareToExitMsg};
+use script::layout_interface::{QueryMsg, ReapLayoutDataMsg, Reflow, UntrustedNodeAddress};
+use script::layout_interface::{ReflowForDisplay, ReflowMsg};
+use script::script_task::{ReflowCompleteMsg, ScriptChan, SendEventMsg};
+use servo_msg::compositor_msg::Scrollable;
+use servo_msg::constellation_msg::{ConstellationChan, PipelineId, Failure, FailureMsg};
+use servo_net::image_cache_task::{ImageCacheTask, ImageResponseMsg};
+use servo_net::local_image_cache::{ImageResponder, LocalImageCache};
+use servo_util::geometry::Au;
+use servo_util::geometry;
+use servo_util::opts::Opts;
+use servo_util::smallvec::{SmallVec, SmallVec1};
+use servo_util::time::{TimeProfilerChan, profile};
+use servo_util::time;
+use servo_util::task::send_on_failure;
+use servo_util::workqueue::WorkQueue;
+use std::comm::{channel, Sender, Receiver};
+use std::mem;
+use std::ptr;
+use std::task::TaskBuilder;
+use style::{AuthorOrigin, Stylesheet, Stylist};
+use sync::{Arc, Mutex};
+use url::Url;
+
+/// Information needed by the layout task.
+pub struct LayoutTask {
+    /// The ID of the pipeline that we belong to.
+    pub id: PipelineId,
+
+    /// The port on which we receive messages.
+    pub port: Receiver<Msg>,
+
+    //// The channel to send messages to ourself.
+    pub chan: LayoutChan,
+
+    /// The channel on which messages can be sent to the constellation.
+    pub constellation_chan: ConstellationChan,
+
+    /// The channel on which messages can be sent to the script task.
+    pub script_chan: ScriptChan,
+
+    /// The channel on which messages can be sent to the painting task.
+    pub render_chan: RenderChan,
+
+    /// The channel on which messages can be sent to the image cache.
+    pub image_cache_task: ImageCacheTask,
+
+    /// The local image cache.
+    pub local_image_cache: Arc<Mutex<LocalImageCache>>,
+
+    /// The size of the viewport.
+    pub screen_size: Size2D<Au>,
+
+    /// A cached display list.
+    pub display_list: Option<Arc<DisplayList>>,
+
+    pub stylist: Box<Stylist>,
+
+    /// The workers that we use for parallel operation.
+    pub parallel_traversal: Option<WorkQueue<*mut LayoutContext,UnsafeFlow>>,
+
+    /// The channel on which messages can be sent to the time profiler.
+    pub time_profiler_chan: TimeProfilerChan,
+
+    /// The command-line options.
+    pub opts: Opts,
+
+    /// The dirty rect. Used during display list construction.
+    pub dirty: Rect<Au>,
+}
+
+/// The damage computation traversal.
+#[deriving(Clone)]
+struct ComputeDamageTraversal;
+
+impl PostorderFlowTraversal for ComputeDamageTraversal {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        let mut damage = flow::base(flow).restyle_damage;
+        for child in flow::child_iter(flow) {
+            damage.insert(flow::base(child).restyle_damage.propagate_up())
+        }
+        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 {
+    all_style_damage: bool,
+}
+
+impl PreorderFlowTraversal for PropagateDamageTraversal {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        if self.all_style_damage {
+            flow::mut_base(flow).restyle_damage.insert(RestyleDamage::all())
+        }
+        debug!("restyle damage = {:?}", flow::base(flow).restyle_damage);
+
+        let prop = flow::base(flow).restyle_damage.propagate_down();
+        if !prop.is_empty() {
+            for kid_ctx in flow::child_iter(flow) {
+                flow::mut_base(kid_ctx).restyle_damage.insert(prop)
+            }
+        }
+        true
+    }
+}
+
+/// The flow tree verification traversal. This is only on in debug builds.
+#[cfg(debug)]
+struct FlowTreeVerificationTraversal;
+
+#[cfg(debug)]
+impl PreorderFlowTraversal for FlowTreeVerificationTraversal {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        let base = flow::base(flow);
+        if !base.flags.is_leaf() && !base.flags.is_nonleaf() {
+            println("flow tree verification failed: flow wasn't a leaf or a nonleaf!");
+            flow.dump();
+            fail!("flow tree verification failed")
+        }
+        true
+    }
+}
+
+/// The bubble-widths traversal, the first part of layout computation. This computes preferred
+/// and intrinsic widths and bubbles them up the tree.
+pub struct BubbleWidthsTraversal<'a> {
+    pub layout_context: &'a mut LayoutContext,
+}
+
+impl<'a> PostorderFlowTraversal for BubbleWidthsTraversal<'a> {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        flow.bubble_widths(self.layout_context);
+        true
+    }
+
+    // FIXME: We can't prune until we start reusing flows
+    /*
+    #[inline]
+    fn should_prune(&mut self, flow: &mut Flow) -> bool {
+        flow::mut_base(flow).restyle_damage.lacks(BubbleWidths)
+    }
+    */
+}
+
+/// The assign-widths traversal. In Gecko this corresponds to `Reflow`.
+pub struct AssignWidthsTraversal<'a> {
+    pub layout_context: &'a mut LayoutContext,
+}
+
+impl<'a> PreorderFlowTraversal for AssignWidthsTraversal<'a> {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        flow.assign_widths(self.layout_context);
+        true
+    }
+}
+
+/// The assign-heights-and-store-overflow traversal, the last (and most expensive) part of layout
+/// computation. Determines the final heights for all layout objects, computes positions, and
+/// computes overflow regions. In Gecko this corresponds to `FinishAndStoreOverflow`.
+pub struct AssignHeightsAndStoreOverflowTraversal<'a> {
+    pub layout_context: &'a mut LayoutContext,
+}
+
+impl<'a> PostorderFlowTraversal for AssignHeightsAndStoreOverflowTraversal<'a> {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) -> bool {
+        flow.assign_height(self.layout_context);
+        // Skip store-overflow for absolutely positioned flows. That will be
+        // done in a separate traversal.
+        if !flow.is_store_overflow_delayed() {
+            flow.store_overflow(self.layout_context);
+        }
+        true
+    }
+
+    #[inline]
+    fn should_process(&mut self, flow: &mut Flow) -> bool {
+        !flow::base(flow).flags.impacted_by_floats()
+    }
+}
+
+/// The display list construction traversal.
+pub struct BuildDisplayListTraversal<'a> {
+    layout_context: &'a LayoutContext,
+}
+
+impl<'a> BuildDisplayListTraversal<'a> {
+    #[inline]
+    fn process(&mut self, flow: &mut Flow) {
+        flow.compute_absolute_position();
+
+        for kid in flow::mut_base(flow).child_iter() {
+            if !kid.is_absolutely_positioned() {
+                self.process(kid)
+            }
+        }
+
+        for absolute_descendant_link in flow::mut_base(flow).abs_descendants.iter() {
+            self.process(absolute_descendant_link)
+        }
+
+        flow.build_display_list(self.layout_context)
+    }
+}
+
+struct LayoutImageResponder {
+    id: PipelineId,
+    script_chan: ScriptChan,
+}
+
+impl ImageResponder for LayoutImageResponder {
+    fn respond(&self) -> proc(ImageResponseMsg):Send {
+        let id = self.id.clone();
+        let script_chan = self.script_chan.clone();
+        let f: proc(ImageResponseMsg):Send = proc(_) {
+            let ScriptChan(chan) = script_chan;
+            drop(chan.send_opt(SendEventMsg(id.clone(), ReflowEvent)))
+        };
+        f
+    }
+}
+
+impl LayoutTask {
+    /// Spawns a new layout task.
+    pub fn create(id: PipelineId,
+                  port: Receiver<Msg>,
+                  chan: LayoutChan,
+                  constellation_chan: ConstellationChan,
+                  failure_msg: Failure,
+                  script_chan: ScriptChan,
+                  render_chan: RenderChan,
+                  img_cache_task: ImageCacheTask,
+                  opts: Opts,
+                  time_profiler_chan: TimeProfilerChan,
+                  shutdown_chan: Sender<()>) {
+        let mut builder = TaskBuilder::new().named("LayoutTask");
+        let ConstellationChan(con_chan) = constellation_chan.clone();
+        send_on_failure(&mut builder, FailureMsg(failure_msg), con_chan);
+        builder.spawn(proc() {
+            { // Ensures layout task is destroyed before we send shutdown message
+                let mut layout = LayoutTask::new(id,
+                                                 port,
+                                                 chan,
+                                                 constellation_chan,
+                                                 script_chan,
+                                                 render_chan,
+                                                 img_cache_task,
+                                                 &opts,
+                                                 time_profiler_chan);
+                layout.start();
+            }
+            shutdown_chan.send(());
+        });
+    }
+
+    /// Creates a new `LayoutTask` structure.
+    fn new(id: PipelineId,
+           port: Receiver<Msg>,
+           chan: LayoutChan,
+           constellation_chan: ConstellationChan,
+           script_chan: ScriptChan,
+           render_chan: RenderChan,
+           image_cache_task: ImageCacheTask,
+           opts: &Opts,
+           time_profiler_chan: TimeProfilerChan)
+           -> LayoutTask {
+        let local_image_cache = Arc::new(Mutex::new(LocalImageCache(image_cache_task.clone())));
+        let screen_size = Size2D(Au(0), Au(0));
+        let parallel_traversal = if opts.layout_threads != 1 {
+            Some(WorkQueue::new("LayoutWorker", opts.layout_threads, ptr::mut_null()))
+        } else {
+            None
+        };
+
+        LayoutTask {
+            id: id,
+            port: port,
+            chan: chan,
+            constellation_chan: constellation_chan,
+            script_chan: script_chan,
+            render_chan: render_chan,
+            image_cache_task: image_cache_task.clone(),
+            local_image_cache: local_image_cache,
+            screen_size: screen_size,
+
+            display_list: None,
+            stylist: box new_stylist(),
+            parallel_traversal: parallel_traversal,
+            time_profiler_chan: time_profiler_chan,
+            opts: opts.clone(),
+            dirty: Rect::zero(),
+        }
+    }
+
+    /// Starts listening on the port.
+    fn start(&mut self) {
+        while self.handle_request() {
+            // Loop indefinitely.
+        }
+    }
+
+    // Create a layout context for use in building display lists, hit testing, &c.
+    fn build_layout_context(&self, reflow_root: &LayoutNode, url: &Url) -> LayoutContext {
+        let font_context_info = FontContextInfo {
+            backend: self.opts.render_backend,
+            needs_font_list: true,
+            time_profiler_chan: self.time_profiler_chan.clone(),
+        };
+
+        LayoutContext {
+            image_cache: self.local_image_cache.clone(),
+            screen_size: self.screen_size.clone(),
+            constellation_chan: self.constellation_chan.clone(),
+            layout_chan: self.chan.clone(),
+            font_context_info: font_context_info,
+            stylist: &*self.stylist,
+            url: (*url).clone(),
+            reflow_root: OpaqueNodeMethods::from_layout_node(reflow_root),
+            opts: self.opts.clone(),
+            dirty: Rect::zero(),
+        }
+    }
+
+    /// Receives and dispatches messages from the port.
+    fn handle_request(&mut self) -> bool {
+        match self.port.recv() {
+            AddStylesheetMsg(sheet) => self.handle_add_stylesheet(sheet),
+            ReflowMsg(data) => {
+                profile(time::LayoutPerformCategory, self.time_profiler_chan.clone(), || {
+                    self.handle_reflow(data);
+                });
+            }
+            QueryMsg(query) => {
+                let mut query = Some(query);
+                profile(time::LayoutQueryCategory, self.time_profiler_chan.clone(), || {
+                    self.handle_query(query.take_unwrap());
+                });
+            }
+            ReapLayoutDataMsg(dead_layout_data) => {
+                unsafe {
+                    self.handle_reap_layout_data(dead_layout_data)
+                }
+            }
+            PrepareToExitMsg(response_chan) => {
+                debug!("layout: PrepareToExitMsg received");
+                self.prepare_to_exit(response_chan);
+                return false
+            }
+            ExitNowMsg => {
+                debug!("layout: ExitNowMsg received");
+                self.exit_now();
+                return false
+            }
+        }
+
+        true
+    }
+
+    /// Enters a quiescent state in which no new messages except for `ReapLayoutDataMsg` will be
+    /// processed until an `ExitNowMsg` is received. A pong is immediately sent on the given
+    /// response channel.
+    fn prepare_to_exit(&mut self, response_chan: Sender<()>) {
+        response_chan.send(());
+        loop {
+            match self.port.recv() {
+                ReapLayoutDataMsg(dead_layout_data) => {
+                    unsafe {
+                        self.handle_reap_layout_data(dead_layout_data)
+                    }
+                }
+                ExitNowMsg => {
+                    debug!("layout task is exiting...");
+                    self.exit_now();
+                    break
+                }
+                _ => {
+                    fail!("layout: message that wasn't `ExitNowMsg` received after \
+                           `PrepareToExitMsg`")
+                }
+            }
+        }
+    }
+
+    /// Shuts down the layout task now. If there are any DOM nodes left, layout will now (safely)
+    /// crash.
+    fn exit_now(&mut self) {
+        let (response_chan, response_port) = channel();
+
+        match self.parallel_traversal {
+            None => {}
+            Some(ref mut traversal) => traversal.shutdown(),
+        }
+
+        self.render_chan.send(render_task::ExitMsg(Some(response_chan)));
+        response_port.recv()
+    }
+
+    fn handle_add_stylesheet(&mut self, sheet: Stylesheet) {
+        self.stylist.add_stylesheet(sheet, AuthorOrigin)
+    }
+
+    /// Retrieves the flow tree root from the root node.
+    fn get_layout_root(&self, node: LayoutNode) -> FlowRef {
+        let mut layout_data_ref = node.mutate_layout_data();
+        let result = match &mut *layout_data_ref {
+            &Some(ref mut layout_data) => {
+                mem::replace(&mut layout_data.data.flow_construction_result, NoConstructionResult)
+            }
+            &None => fail!("no layout data for root node"),
+        };
+        let mut flow = match result {
+            FlowConstructionResult(mut flow, abs_descendants) => {
+                // Note: Assuming that the root has display 'static' (as per
+                // CSS Section 9.3.1). Otherwise, if it were absolutely
+                // positioned, it would return a reference to itself in
+                // `abs_descendants` and would lead to a circular reference.
+                // Set Root as CB for any remaining absolute descendants.
+                flow.set_abs_descendants(abs_descendants);
+                flow
+            }
+            _ => fail!("Flow construction didn't result in a flow at the root of the tree!"),
+        };
+        flow.get_mut().mark_as_root();
+        flow
+    }
+
+    /// Performs layout constraint solving.
+    ///
+    /// This corresponds to `Reflow()` in Gecko and `layout()` in WebKit/Blink and should be
+    /// benchmarked against those two. It is marked `#[inline(never)]` to aid profiling.
+    #[inline(never)]
+    fn solve_constraints(&mut self,
+                         layout_root: &mut Flow,
+                         layout_context: &mut LayoutContext) {
+        if layout_context.opts.bubble_widths_separately {
+            let mut traversal = BubbleWidthsTraversal {
+                layout_context: layout_context,
+            };
+            layout_root.traverse_postorder(&mut traversal);
+        }
+
+        // FIXME(kmc): We want to prune nodes without the Reflow restyle damage
+        // bit, 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.
+        {
+            let mut traversal = AssignWidthsTraversal {
+                layout_context: layout_context,
+            };
+            layout_root.traverse_preorder(&mut traversal);
+        }
+
+        // FIXME(pcwalton): Prune this pass as well.
+        {
+            let mut traversal = AssignHeightsAndStoreOverflowTraversal {
+                layout_context: layout_context,
+            };
+            layout_root.traverse_postorder(&mut traversal);
+        }
+    }
+
+    /// Performs layout constraint solving in parallel.
+    ///
+    /// This corresponds to `Reflow()` in Gecko and `layout()` in WebKit/Blink and should be
+    /// benchmarked against those two. It is marked `#[inline(never)]` to aid profiling.
+    #[inline(never)]
+    fn solve_constraints_parallel(&mut self,
+                                  layout_root: &mut FlowRef,
+                                  layout_context: &mut LayoutContext) {
+        if layout_context.opts.bubble_widths_separately {
+            let mut traversal = BubbleWidthsTraversal {
+                layout_context: layout_context,
+            };
+            layout_root.get_mut().traverse_postorder(&mut traversal);
+        }
+
+        match self.parallel_traversal {
+            None => fail!("solve_contraints_parallel() called with no parallel traversal ready"),
+            Some(ref mut traversal) => {
+                // NOTE: this currently computes borders, so any pruning should separate that
+                // operation out.
+                parallel::traverse_flow_tree_preorder(layout_root,
+                                                      self.time_profiler_chan.clone(),
+                                                      layout_context,
+                                                      traversal);
+            }
+        }
+    }
+
+    /// Verifies that every node was either marked as a leaf or as a nonleaf in the flow tree.
+    /// This is only on in debug builds.
+    #[inline(never)]
+    #[cfg(debug)]
+    fn verify_flow_tree(&mut self, layout_root: &mut FlowRef) {
+        let mut traversal = FlowTreeVerificationTraversal;
+        layout_root.traverse_preorder(&mut traversal);
+    }
+
+    #[cfg(not(debug))]
+    fn verify_flow_tree(&mut self, _: &mut FlowRef) {
+    }
+
+    /// The high-level routine that performs layout tasks.
+    fn handle_reflow(&mut self, data: &Reflow) {
+        // FIXME: Isolate this transmutation into a "bridge" module.
+        let node: &mut LayoutNode = unsafe {
+            let mut node: JS<Node> = JS::from_trusted_node_address(data.document_root);
+            mem::transmute(&mut node)
+        };
+
+        debug!("layout: received layout request for: {:s}", data.url.to_str());
+        debug!("layout: damage is {:?}", data.damage);
+        debug!("layout: parsed Node tree");
+        debug!("{:?}", node.dump());
+
+        {
+            // Reset the image cache.
+            let mut local_image_cache = self.local_image_cache.lock();
+            local_image_cache.next_round(self.make_on_image_available_cb());
+        }
+
+        // true => Do the reflow with full style damage, because content
+        // changed or the window was resized.
+        let mut all_style_damage = match data.damage.level {
+            ContentChangedDocumentDamage => true,
+            _ => false
+        };
+
+        // TODO: Calculate the "actual viewport":
+        // http://www.w3.org/TR/css-device-adapt/#actual-viewport
+        let viewport_size = data.window_size.initial_viewport;
+
+        let current_screen_size = Size2D(Au::from_frac32_px(viewport_size.width.get()),
+                                         Au::from_frac32_px(viewport_size.height.get()));
+        if self.screen_size != current_screen_size {
+            all_style_damage = true
+        }
+        self.screen_size = current_screen_size;
+
+        // Create a layout context for use throughout the following passes.
+        let mut layout_ctx = self.build_layout_context(node, &data.url);
+
+        // Create a font context, if this is sequential.
+        //
+        // FIXME(pcwalton): This is a pretty bogus thing to do. Essentially this is a workaround
+        // for libgreen having slow TLS.
+        let mut font_context_opt = if self.parallel_traversal.is_none() {
+            Some(box FontContext::new(layout_ctx.font_context_info.clone()))
+        } else {
+            None
+        };
+
+        let mut layout_root = profile(time::LayoutStyleRecalcCategory,
+                                      self.time_profiler_chan.clone(),
+                                      || {
+            // Perform CSS selector matching and flow construction.
+            match self.parallel_traversal {
+                None => {
+                    let mut applicable_declarations = ApplicableDeclarations::new();
+                    let mut applicable_declarations_cache = ApplicableDeclarationsCache::new();
+                    let mut style_sharing_candidate_cache = StyleSharingCandidateCache::new();
+                    drop(node.recalc_style_for_subtree(self.stylist,
+                                                       &mut layout_ctx,
+                                                       font_context_opt.take_unwrap(),
+                                                       &mut applicable_declarations,
+                                                       &mut applicable_declarations_cache,
+                                                       &mut style_sharing_candidate_cache,
+                                                       None))
+                }
+                Some(ref mut traversal) => {
+                    parallel::recalc_style_for_subtree(node, &mut layout_ctx, traversal)
+                }
+            }
+
+            self.get_layout_root((*node).clone())
+        });
+
+        // Verification of the flow tree, which ensures that all nodes were either marked as leaves
+        // or as non-leaves. This becomes a no-op in release builds. (It is inconsequential to
+        // memory safety but is a useful debugging tool.)
+        self.verify_flow_tree(&mut layout_root);
+
+        // Propagate damage.
+        profile(time::LayoutDamagePropagateCategory, self.time_profiler_chan.clone(), || {
+            layout_root.get_mut().traverse_preorder(&mut PropagateDamageTraversal {
+                all_style_damage: all_style_damage
+            });
+            layout_root.get_mut().traverse_postorder(&mut ComputeDamageTraversal.clone());
+        });
+
+        // Perform the primary layout passes over the flow tree to compute the locations of all
+        // the boxes.
+        profile(time::LayoutMainCategory, self.time_profiler_chan.clone(), || {
+            match self.parallel_traversal {
+                None => {
+                    // Sequential mode.
+                    self.solve_constraints(layout_root.get_mut(), &mut layout_ctx)
+                }
+                Some(_) => {
+                    // Parallel mode.
+                    self.solve_constraints_parallel(&mut layout_root, &mut layout_ctx)
+                }
+            }
+        });
+
+        // Build the display list if necessary, and send it to the renderer.
+        if data.goal == ReflowForDisplay {
+            profile(time::LayoutDispListBuildCategory, self.time_profiler_chan.clone(), || {
+                layout_ctx.dirty = flow::base(layout_root.get()).position.clone();
+
+                match self.parallel_traversal {
+                    None => {
+                        let mut traversal = BuildDisplayListTraversal {
+                            layout_context: &layout_ctx,
+                        };
+                        traversal.process(layout_root.get_mut());
+                    }
+                    Some(ref mut traversal) => {
+                        parallel::build_display_list_for_subtree(&mut layout_root,
+                                                                 self.time_profiler_chan.clone(),
+                                                                 &mut layout_ctx,
+                                                                 traversal);
+                    }
+                }
+
+                let root_display_list =
+                    mem::replace(&mut flow::mut_base(layout_root.get_mut()).display_list,
+                                 DisplayList::new());
+                let display_list = Arc::new(root_display_list.flatten(ContentStackingLevel));
+
+                // FIXME(pcwalton): This is really ugly and can't handle overflow: scroll. Refactor
+                // it with extreme prejudice.
+                let mut color = color::rgba(255.0, 255.0, 255.0, 255.0);
+                for child in node.traverse_preorder() {
+                    if child.type_id() == Some(ElementNodeTypeId(HTMLHtmlElementTypeId)) ||
+                            child.type_id() == Some(ElementNodeTypeId(HTMLBodyElementTypeId)) {
+                        let element_bg_color = {
+                            let thread_safe_child = ThreadSafeLayoutNode::new(&child);
+                            thread_safe_child.style()
+                                             .resolve_color(thread_safe_child.style()
+                                                                             .get_background()
+                                                                             .background_color)
+                                             .to_gfx_color()
+                        };
+                        match element_bg_color {
+                            color::rgba(0., 0., 0., 0.) => {}
+                            _ => {
+                                color = element_bg_color;
+                                break;
+                           }
+                        }
+                    }
+                }
+
+                let root_size = flow::base(layout_root.get()).position.size;
+                let root_size = Size2D(root_size.width.to_nearest_px() as uint,
+                                       root_size.height.to_nearest_px() as uint);
+                let render_layer = RenderLayer {
+                    id: layout_root.get().layer_id(0),
+                    display_list: display_list.clone(),
+                    position: Rect(Point2D(0u, 0u), root_size),
+                    background_color: color,
+                    scroll_policy: Scrollable,
+                };
+
+                self.display_list = Some(display_list.clone());
+
+                // TODO(pcwalton): Eventually, when we have incremental reflow, this will have to
+                // be smarter in order to handle retained layer contents properly from reflow to
+                // reflow.
+                let mut layers = SmallVec1::new();
+                layers.push(render_layer);
+                for layer in mem::replace(&mut flow::mut_base(layout_root.get_mut()).layers,
+                                          DList::new()).move_iter() {
+                    layers.push(layer)
+                }
+
+                debug!("Layout done!");
+
+                self.render_chan.send(RenderMsg(layers));
+            });
+        }
+
+        // Tell script that we're done.
+        //
+        // FIXME(pcwalton): This should probably be *one* channel, but we can't fix this without
+        // either select or a filtered recv() that only looks for messages of a given type.
+        data.script_join_chan.send(());
+        let ScriptChan(ref chan) = data.script_chan;
+        chan.send(ReflowCompleteMsg(self.id, data.id));
+    }
+
+    /// Handles a query from the script task. This is the main routine that DOM functions like
+    /// `getClientRects()` or `getBoundingClientRect()` ultimately invoke.
+    fn handle_query(&self, query: LayoutQuery) {
+        match query {
+            // The neat thing here is that in order to answer the following two queries we only
+            // need to compare nodes for equality. Thus we can safely work only with `OpaqueNode`.
+            ContentBoxQuery(node, reply_chan) => {
+                let node: OpaqueNode = OpaqueNodeMethods::from_script_node(node);
+                fn union_boxes_for_node(accumulator: &mut Option<Rect<Au>>,
+                                        mut iter: DisplayItemIterator,
+                                        node: OpaqueNode) {
+                    for item in iter {
+                        union_boxes_for_node(accumulator, item.children(), node);
+                        if item.base().node == node {
+                            match *accumulator {
+                                None => *accumulator = Some(item.base().bounds),
+                                Some(ref mut acc) => *acc = acc.union(&item.base().bounds),
+                            }
+                        }
+                    }
+                }
+
+                let mut rect = None;
+                match self.display_list {
+                    None => fail!("no display list!"),
+                    Some(ref display_list) => {
+                        union_boxes_for_node(&mut rect, display_list.iter(), node)
+                    }
+                }
+                reply_chan.send(ContentBoxResponse(rect.unwrap_or(Rect::zero())))
+            }
+            ContentBoxesQuery(node, reply_chan) => {
+                let node: OpaqueNode = OpaqueNodeMethods::from_script_node(node);
+
+                fn add_boxes_for_node(accumulator: &mut Vec<Rect<Au>>,
+                                      mut iter: DisplayItemIterator,
+                                      node: OpaqueNode) {
+                    for item in iter {
+                        add_boxes_for_node(accumulator, item.children(), node);
+                        if item.base().node == node {
+                            accumulator.push(item.base().bounds)
+                        }
+                    }
+                }
+
+                let mut boxes = vec!();
+                match self.display_list {
+                    None => fail!("no display list!"),
+                    Some(ref display_list) => {
+                        add_boxes_for_node(&mut boxes, display_list.iter(), node)
+                    }
+                }
+                reply_chan.send(ContentBoxesResponse(boxes))
+            }
+            HitTestQuery(_, point, reply_chan) => {
+                fn hit_test<'a,I:Iterator<&'a DisplayItem>>(x: Au, y: Au, mut iterator: I)
+                            -> Option<HitTestResponse> {
+                    for item in iterator {
+                        match *item {
+                            ClipDisplayItemClass(ref cc) => {
+                                if geometry::rect_contains_point(cc.base.bounds, Point2D(x, y)) {
+                                    let ret = hit_test(x, y, cc.children.list.iter().rev());
+                                    if !ret.is_none() {
+                                        return ret
+                                    }
+                                }
+                                continue
+                            }
+                            _ => {}
+                        }
+
+                        let bounds = item.bounds();
+
+                        // TODO(tikue): This check should really be performed by a method of
+                        // DisplayItem.
+                        if x < bounds.origin.x + bounds.size.width &&
+                                bounds.origin.x <= x &&
+                                y < bounds.origin.y + bounds.size.height &&
+                                bounds.origin.y <= y {
+                            return Some(HitTestResponse(item.base()
+                                                            .node
+                                                            .to_untrusted_node_address()))
+                        }
+                    }
+                    let ret: Option<HitTestResponse> = None;
+                    ret
+                }
+                let (x, y) = (Au::from_frac_px(point.x as f64),
+                              Au::from_frac_px(point.y as f64));
+                let resp = match self.display_list {
+                    None => fail!("no display list!"),
+                    Some(ref display_list) => hit_test(x, y, display_list.list.iter().rev()),
+                };
+                if resp.is_some() {
+                    reply_chan.send(Ok(resp.unwrap()));
+                    return
+                }
+                reply_chan.send(Err(()));
+
+            }
+            MouseOverQuery(_, point, reply_chan) => {
+                fn mouse_over_test<'a,
+                                   I:Iterator<&'a DisplayItem>>(
+                                   x: Au,
+                                   y: Au,
+                                   mut iterator: I,
+                                   result: &mut Vec<UntrustedNodeAddress>) {
+                    for item in iterator {
+                        match *item {
+                            ClipDisplayItemClass(ref cc) => {
+                                mouse_over_test(x, y, cc.children.list.iter().rev(), result);
+                            }
+                            _ => {
+                                let bounds = item.bounds();
+
+                                // TODO(tikue): This check should really be performed by a method
+                                // of DisplayItem.
+                                if x < bounds.origin.x + bounds.size.width &&
+                                        bounds.origin.x <= x &&
+                                        y < bounds.origin.y + bounds.size.height &&
+                                        bounds.origin.y <= y {
+                                    result.push(item.base()
+                                                    .node
+                                                    .to_untrusted_node_address());
+                                }
+                            }
+                        }
+                    }
+                }
+
+                let mut mouse_over_list: Vec<UntrustedNodeAddress> = vec!();
+                let (x, y) = (Au::from_frac_px(point.x as f64), Au::from_frac_px(point.y as f64));
+                match self.display_list {
+                    None => fail!("no display list!"),
+                    Some(ref display_list) => {
+                        mouse_over_test(x,
+                                        y,
+                                        display_list.list.iter().rev(),
+                                        &mut mouse_over_list);
+                    }
+                };
+
+                if mouse_over_list.is_empty() {
+                    reply_chan.send(Err(()));
+                } else {
+                    reply_chan.send(Ok(MouseOverResponse(mouse_over_list)));
+                }
+            }
+        }
+    }
+
+    // When images can't be loaded in time to display they trigger
+    // this callback in some task somewhere. This will send a message
+    // 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) -> Box<ImageResponder+Send> {
+        // 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
+        box LayoutImageResponder {
+            id: self.id.clone(),
+            script_chan: self.script_chan.clone(),
+        } as Box<ImageResponder+Send>
+    }
+
+    /// Handles a message to destroy layout data. Layout data must be destroyed on *this* task
+    /// because it contains local managed pointers.
+    unsafe fn handle_reap_layout_data(&self, layout_data: LayoutDataRef) {
+        let mut layout_data_ref = layout_data.borrow_mut();
+        let _: Option<LayoutDataWrapper> = mem::transmute(
+            mem::replace(&mut *layout_data_ref, None));
+    }
+}