layout: Largely move display list building out to a separate file.

`layout::fragment` and `layout::block` were getting too big.

components/layout/display_list_builder.rs

@@ -0,0 +1,710 @@
+/* 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/. */
+
+//! Builds display lists from flows and fragments.
+//!
+//! Other browser engines sometimes call this "painting", but it is more accurately called display
+//! list building, as the actual painting does not happen here—only deciding *what* we're going to
+//! paint.
+
+#![deny(unsafe_block)]
+
+use block::BlockFlow;
+use context::LayoutContext;
+use flow::{mod, Flow};
+use fragment::{Fragment, GenericFragment, IframeFragment, IframeFragmentInfo, ImageFragment};
+use fragment::{ImageFragmentInfo, InlineAbsoluteHypotheticalFragment, InlineBlockFragment};
+use fragment::{InputFragment, ScannedTextFragment, ScannedTextFragmentInfo, TableFragment};
+use fragment::{TableCellFragment, TableColumnFragment, TableRowFragment, TableWrapperFragment};
+use fragment::{UnscannedTextFragment};
+use model;
+use util::{OpaqueNodeMethods, ToGfxColor};
+
+use collections::dlist::DList;
+use geom::approxeq::ApproxEq;
+use geom::{Point2D, Rect, Size2D, SideOffsets2D};
+use gfx::color;
+use gfx::display_list::{BackgroundAndBorderLevel, BaseDisplayItem, BorderDisplayItem};
+use gfx::display_list::{BorderDisplayItemClass, ContentStackingLevel, DisplayList};
+use gfx::display_list::{FloatStackingLevel, ImageDisplayItem, ImageDisplayItemClass};
+use gfx::display_list::{LineDisplayItem, LineDisplayItemClass, PositionedDescendantStackingLevel};
+use gfx::display_list::{PseudoDisplayItemClass, RootOfStackingContextLevel, SidewaysLeft};
+use gfx::display_list::{SidewaysRight, SolidColorDisplayItem, SolidColorDisplayItemClass};
+use gfx::display_list::{StackingLevel, TextDisplayItem, TextDisplayItemClass, Upright};
+use gfx::render_task::RenderLayer;
+use servo_msg::compositor_msg::{FixedPosition, Scrollable};
+use servo_msg::constellation_msg::{ConstellationChan, FrameRectMsg};
+use servo_net::image::holder::ImageHolder;
+use servo_util::geometry::{mod, Au, ZERO_RECT};
+use servo_util::logical_geometry::{LogicalRect, WritingMode};
+use servo_util::opts;
+use std::mem;
+use style::{ComputedValues, RGBA};
+use style::computed_values::{background_attachment, background_repeat, border_style, overflow};
+use style::computed_values::{visibility};
+use sync::Arc;
+
+pub trait FragmentDisplayListBuilding {
+    /// Adds the display items necessary to paint the background of this fragment to the display
+    /// list if necessary.
+    fn build_display_list_for_background_if_applicable(&self,
+                                                       style: &ComputedValues,
+                                                       list: &mut DisplayList,
+                                                       layout_context: &LayoutContext,
+                                                       level: StackingLevel,
+                                                       absolute_bounds: &Rect<Au>,
+                                                       clip_rect: &Rect<Au>);
+
+    /// Adds the display items necessary to paint the borders of this fragment to a display list if
+    /// necessary.
+    fn build_display_list_for_borders_if_applicable(&self,
+                                                    style: &ComputedValues,
+                                                    list: &mut DisplayList,
+                                                    abs_bounds: &Rect<Au>,
+                                                    level: StackingLevel,
+                                                    clip_rect: &Rect<Au>);
+
+    fn build_debug_borders_around_text_fragments(&self,
+                                                 display_list: &mut DisplayList,
+                                                 flow_origin: Point2D<Au>,
+                                                 text_fragment: &ScannedTextFragmentInfo,
+                                                 clip_rect: &Rect<Au>);
+
+    fn build_debug_borders_around_fragment(&self,
+                                           display_list: &mut DisplayList,
+                                           flow_origin: Point2D<Au>,
+                                           clip_rect: &Rect<Au>);
+
+    /// Adds the display items for this fragment to the given stacking context.
+    ///
+    /// Arguments:
+    ///
+    /// * `display_list`: The unflattened display list to add display items to.
+    /// * `layout_context`: The layout context.
+    /// * `dirty`: The dirty rectangle in the coordinate system of the owning flow.
+    /// * `flow_origin`: Position of the origin of the owning flow wrt the display list root flow.
+    /// * `clip_rect`: The rectangle to clip the display items to.
+    fn build_display_list(&mut self,
+                          display_list: &mut DisplayList,
+                          layout_context: &LayoutContext,
+                          flow_origin: Point2D<Au>,
+                          background_and_border_level: BackgroundAndBorderLevel,
+                          clip_rect: &Rect<Au>);
+
+    /// Sends the size and position of this iframe fragment to the constellation. This is out of
+    /// line to guide inlining.
+    fn finalize_position_and_size_of_iframe(&self,
+                                            iframe_fragment: &IframeFragmentInfo,
+                                            offset: Point2D<Au>,
+                                            layout_context: &LayoutContext);
+
+    fn clip_rect_for_children(&self, current_clip_rect: Rect<Au>, flow_origin: Point2D<Au>)
+                              -> Rect<Au>;
+}
+
+impl FragmentDisplayListBuilding for Fragment {
+    fn build_display_list_for_background_if_applicable(&self,
+                                                       style: &ComputedValues,
+                                                       list: &mut DisplayList,
+                                                       layout_context: &LayoutContext,
+                                                       level: StackingLevel,
+                                                       absolute_bounds: &Rect<Au>,
+                                                       clip_rect: &Rect<Au>) {
+        // FIXME: This causes a lot of background colors to be displayed when they are clearly not
+        // needed. We could use display list optimization to clean this up, but it still seems
+        // inefficient. What we really want is something like "nearest ancestor element that
+        // doesn't have a fragment".
+        let background_color = style.resolve_color(style.get_background().background_color);
+        if !background_color.alpha.approx_eq(&0.0) {
+            let display_item = box SolidColorDisplayItem {
+                base: BaseDisplayItem::new(*absolute_bounds, self.node, level, *clip_rect),
+                color: background_color.to_gfx_color(),
+            };
+
+            list.push(SolidColorDisplayItemClass(display_item))
+        }
+
+        // The background image is painted on top of the background color.
+        // Implements background image, per spec:
+        // http://www.w3.org/TR/CSS21/colors.html#background
+        let background = style.get_background();
+        let image_url = match background.background_image {
+            None => return,
+            Some(ref image_url) => image_url,
+        };
+
+        let mut holder = ImageHolder::new(image_url.clone(),
+                                          layout_context.shared.image_cache.clone());
+        let image = match holder.get_image(self.node.to_untrusted_node_address()) {
+            None => {
+                // No image data at all? Do nothing.
+                //
+                // TODO: Add some kind of placeholder background image.
+                debug!("(building display list) no background image :(");
+                return
+            }
+            Some(image) => image,
+        };
+        debug!("(building display list) building background image");
+
+        let image_width = Au::from_px(image.width as int);
+        let image_height = Au::from_px(image.height as int);
+        let mut bounds = *absolute_bounds;
+
+        // Clip.
+        //
+        // TODO: Check the bounds to see if a clip item is actually required.
+        let clip_rect = clip_rect.intersection(&bounds).unwrap_or(ZERO_RECT);
+
+        // Use background-attachment to get the initial virtual origin
+        let (virtual_origin_x, virtual_origin_y) = match background.background_attachment {
+            background_attachment::scroll => {
+                (absolute_bounds.origin.x, absolute_bounds.origin.y)
+            }
+            background_attachment::fixed => {
+                (Au(0), Au(0))
+            }
+        };
+
+        // Use background-position to get the offset
+        let horizontal_position = model::specified(background.background_position.horizontal,
+                                                   bounds.size.width - image_width);
+        let vertical_position = model::specified(background.background_position.vertical,
+                                                 bounds.size.height - image_height);
+
+        let abs_x = virtual_origin_x + horizontal_position;
+        let abs_y = virtual_origin_y + vertical_position;
+
+        // Adjust origin and size based on background-repeat
+        match background.background_repeat {
+            background_repeat::no_repeat => {
+                bounds.origin.x = abs_x;
+                bounds.origin.y = abs_y;
+                bounds.size.width = image_width;
+                bounds.size.height = image_height;
+            }
+            background_repeat::repeat_x => {
+                bounds.origin.y = abs_y;
+                bounds.size.height = image_height;
+                ImageFragmentInfo::tile_image(&mut bounds.origin.x, &mut bounds.size.width,
+                                                abs_x, image.width);
+            }
+            background_repeat::repeat_y => {
+                bounds.origin.x = abs_x;
+                bounds.size.width = image_width;
+                ImageFragmentInfo::tile_image(&mut bounds.origin.y, &mut bounds.size.height,
+                                                abs_y, image.height);
+            }
+            background_repeat::repeat => {
+                ImageFragmentInfo::tile_image(&mut bounds.origin.x, &mut bounds.size.width,
+                                                abs_x, image.width);
+                ImageFragmentInfo::tile_image(&mut bounds.origin.y, &mut bounds.size.height,
+                                                abs_y, image.height);
+            }
+        };
+
+        // Create the image display item.
+        let image_display_item = ImageDisplayItemClass(box ImageDisplayItem {
+            base: BaseDisplayItem::new(bounds, self.node, level, clip_rect),
+            image: image.clone(),
+            stretch_size: Size2D(Au::from_px(image.width as int),
+                                 Au::from_px(image.height as int)),
+        });
+        list.push(image_display_item)
+    }
+
+    /// Adds the display items necessary to paint the borders of this fragment to a display list if
+    /// necessary.
+    fn build_display_list_for_borders_if_applicable(&self,
+                                                    style: &ComputedValues,
+                                                    list: &mut DisplayList,
+                                                    abs_bounds: &Rect<Au>,
+                                                    level: StackingLevel,
+                                                    clip_rect: &Rect<Au>) {
+        let border = style.logical_border_width();
+        if border.is_zero() {
+            return
+        }
+
+        let top_color = style.resolve_color(style.get_border().border_top_color);
+        let right_color = style.resolve_color(style.get_border().border_right_color);
+        let bottom_color = style.resolve_color(style.get_border().border_bottom_color);
+        let left_color = style.resolve_color(style.get_border().border_left_color);
+
+        // Append the border to the display list.
+        let border_display_item = box BorderDisplayItem {
+            base: BaseDisplayItem::new(*abs_bounds, self.node, level, *clip_rect),
+            border: border.to_physical(style.writing_mode),
+            color: SideOffsets2D::new(top_color.to_gfx_color(),
+                                      right_color.to_gfx_color(),
+                                      bottom_color.to_gfx_color(),
+                                      left_color.to_gfx_color()),
+            style: SideOffsets2D::new(style.get_border().border_top_style,
+                                      style.get_border().border_right_style,
+                                      style.get_border().border_bottom_style,
+                                      style.get_border().border_left_style)
+        };
+
+        list.push(BorderDisplayItemClass(border_display_item))
+    }
+
+    fn build_debug_borders_around_text_fragments(&self,
+                                                 display_list: &mut DisplayList,
+                                                 flow_origin: Point2D<Au>,
+                                                 text_fragment: &ScannedTextFragmentInfo,
+                                                 clip_rect: &Rect<Au>) {
+        // FIXME(#2795): Get the real container size
+        let container_size = Size2D::zero();
+        // Fragment position wrt to the owning flow.
+        let fragment_bounds = self.border_box.to_physical(self.style.writing_mode, container_size);
+        let absolute_fragment_bounds = Rect(
+            fragment_bounds.origin + flow_origin,
+            fragment_bounds.size);
+
+        // Compute the text fragment bounds and draw a border surrounding them.
+        let border_display_item = box BorderDisplayItem {
+            base: BaseDisplayItem::new(absolute_fragment_bounds,
+                                       self.node,
+                                       ContentStackingLevel,
+                                       *clip_rect),
+            border: SideOffsets2D::new_all_same(Au::from_px(1)),
+            color: SideOffsets2D::new_all_same(color::rgb(0, 0, 200)),
+            style: SideOffsets2D::new_all_same(border_style::solid)
+        };
+        display_list.push(BorderDisplayItemClass(border_display_item));
+
+        // Draw a rectangle representing the baselines.
+        let ascent = text_fragment.run.ascent();
+        let mut baseline = self.border_box.clone();
+        baseline.start.b = baseline.start.b + ascent;
+        baseline.size.block = Au(0);
+        let mut baseline = baseline.to_physical(self.style.writing_mode, container_size);
+        baseline.origin = baseline.origin + flow_origin;
+
+        let line_display_item = box LineDisplayItem {
+            base: BaseDisplayItem::new(baseline, self.node, ContentStackingLevel, *clip_rect),
+            color: color::rgb(0, 200, 0),
+            style: border_style::dashed,
+        };
+        display_list.push(LineDisplayItemClass(line_display_item));
+    }
+
+    fn build_debug_borders_around_fragment(&self,
+                                           display_list: &mut DisplayList,
+                                           flow_origin: Point2D<Au>,
+                                           clip_rect: &Rect<Au>) {
+        // FIXME(#2795): Get the real container size
+        let container_size = Size2D::zero();
+        // Fragment position wrt to the owning flow.
+        let fragment_bounds = self.border_box.to_physical(self.style.writing_mode, container_size);
+        let absolute_fragment_bounds = Rect(
+            fragment_bounds.origin + flow_origin,
+            fragment_bounds.size);
+
+        // This prints a debug border around the border of this fragment.
+        let border_display_item = box BorderDisplayItem {
+            base: BaseDisplayItem::new(absolute_fragment_bounds,
+                                       self.node,
+                                       ContentStackingLevel,
+                                       *clip_rect),
+            border: SideOffsets2D::new_all_same(Au::from_px(1)),
+            color: SideOffsets2D::new_all_same(color::rgb(0, 0, 200)),
+            style: SideOffsets2D::new_all_same(border_style::solid)
+        };
+        display_list.push(BorderDisplayItemClass(border_display_item))
+    }
+
+    fn build_display_list(&mut self,
+                          display_list: &mut DisplayList,
+                          layout_context: &LayoutContext,
+                          flow_origin: Point2D<Au>,
+                          background_and_border_level: BackgroundAndBorderLevel,
+                          clip_rect: &Rect<Au>) {
+        // FIXME(#2795): Get the real container size
+        let container_size = Size2D::zero();
+        let rect_to_absolute = |writing_mode: WritingMode, logical_rect: LogicalRect<Au>| {
+            let physical_rect = logical_rect.to_physical(writing_mode, container_size);
+            Rect(physical_rect.origin + flow_origin, physical_rect.size)
+        };
+        // Fragment position wrt to the owning flow.
+        let absolute_fragment_bounds = rect_to_absolute(self.style.writing_mode, self.border_box);
+        debug!("Fragment::build_display_list at rel={}, abs={}: {}",
+               self.border_box,
+               absolute_fragment_bounds,
+               self);
+        debug!("Fragment::build_display_list: dirty={}, flow_origin={}",
+               layout_context.shared.dirty,
+               flow_origin);
+
+        if self.style().get_inheritedbox().visibility != visibility::visible {
+            return
+        }
+
+        if !absolute_fragment_bounds.intersects(&layout_context.shared.dirty) {
+            debug!("Fragment::build_display_list: Did not intersect...");
+            return
+        }
+
+        debug!("Fragment::build_display_list: intersected. Adding display item...");
+
+        if self.is_primary_fragment() {
+            let level =
+                StackingLevel::from_background_and_border_level(background_and_border_level);
+
+            // Add a pseudo-display item for content box queries. This is a very bogus thing to do.
+            let base_display_item = box BaseDisplayItem::new(absolute_fragment_bounds,
+                                                             self.node,
+                                                             level,
+                                                             *clip_rect);
+            display_list.push(PseudoDisplayItemClass(base_display_item));
+
+            // Add the background to the list, if applicable.
+            match self.inline_context {
+                Some(ref inline_context) => {
+                    for style in inline_context.styles.iter().rev() {
+                        self.build_display_list_for_background_if_applicable(
+                            &**style,
+                            display_list,
+                            layout_context,
+                            level,
+                            &absolute_fragment_bounds,
+                            clip_rect);
+                    }
+                }
+                None => {}
+            }
+            match self.specific {
+                ScannedTextFragment(_) => {},
+                _ => {
+                    self.build_display_list_for_background_if_applicable(
+                        &*self.style,
+                        display_list,
+                        layout_context,
+                        level,
+                        &absolute_fragment_bounds,
+                        clip_rect);
+                }
+            }
+
+            // Add a border, if applicable.
+            //
+            // TODO: Outlines.
+            match self.inline_context {
+                Some(ref inline_context) => {
+                    for style in inline_context.styles.iter().rev() {
+                        self.build_display_list_for_borders_if_applicable(
+                            &**style,
+                            display_list,
+                            &absolute_fragment_bounds,
+                            level,
+                            clip_rect);
+                    }
+                }
+                None => {}
+            }
+            match self.specific {
+                ScannedTextFragment(_) => {},
+                _ => {
+                    self.build_display_list_for_borders_if_applicable(
+                        &*self.style,
+                        display_list,
+                        &absolute_fragment_bounds,
+                        level,
+                        clip_rect);
+                }
+            }
+        }
+
+        let content_box = self.content_box();
+        let absolute_content_box = rect_to_absolute(self.style.writing_mode, content_box);
+
+        // Create special per-fragment-type display items.
+        match self.specific {
+            UnscannedTextFragment(_) => fail!("Shouldn't see unscanned fragments here."),
+            TableColumnFragment(_) => fail!("Shouldn't see table column fragments here."),
+            ScannedTextFragment(ref text_fragment) => {
+                // Create the text display item.
+                let orientation = if self.style.writing_mode.is_vertical() {
+                    if self.style.writing_mode.is_sideways_left() {
+                        SidewaysLeft
+                    } else {
+                        SidewaysRight
+                    }
+                } else {
+                    Upright
+                };
+
+                let metrics = &text_fragment.run.font_metrics;
+                let baseline_origin ={
+                    let mut tmp = content_box.start;
+                    tmp.b = tmp.b + metrics.ascent;
+                    tmp.to_physical(self.style.writing_mode, container_size) + flow_origin
+                };
+
+                let text_display_item = box TextDisplayItem {
+                    base: BaseDisplayItem::new(absolute_content_box,
+                                               self.node,
+                                               ContentStackingLevel,
+                                               *clip_rect),
+                    text_run: text_fragment.run.clone(),
+                    range: text_fragment.range,
+                    text_color: self.style().get_color().color.to_gfx_color(),
+                    orientation: orientation,
+                    baseline_origin: baseline_origin,
+                };
+                display_list.push(TextDisplayItemClass(text_display_item));
+
+                // Create display items for text decoration
+                {
+                    let line = |maybe_color: Option<RGBA>, rect: || -> LogicalRect<Au>| {
+                        match maybe_color {
+                            None => {},
+                            Some(color) => {
+                                display_list.push(SolidColorDisplayItemClass(
+                                                     box SolidColorDisplayItem {
+                                                        base: BaseDisplayItem::new(
+                                                                  rect_to_absolute(
+                                                                      self.style.writing_mode,
+                                                                      rect()),
+                                                               self.node,
+                                                               ContentStackingLevel,
+                                                               *clip_rect),
+                                                        color: color.to_gfx_color(),
+                                                     }));
+                            }
+                        }
+                    };
+
+                    let text_decorations =
+                        self.style().get_inheritedtext()._servo_text_decorations_in_effect;
+                    line(text_decorations.underline, || {
+                        let mut rect = content_box.clone();
+                        rect.start.b = rect.start.b + metrics.ascent - metrics.underline_offset;
+                        rect.size.block = metrics.underline_size;
+                        rect
+                    });
+
+                    line(text_decorations.overline, || {
+                        let mut rect = content_box.clone();
+                        rect.size.block = metrics.underline_size;
+                        rect
+                    });
+
+                    line(text_decorations.line_through, || {
+                        let mut rect = content_box.clone();
+                        rect.start.b = rect.start.b + metrics.ascent - metrics.strikeout_offset;
+                        rect.size.block = metrics.strikeout_size;
+                        rect
+                    });
+                }
+
+                if opts::get().show_debug_fragment_borders {
+                    self.build_debug_borders_around_text_fragments(display_list,
+                                                                   flow_origin,
+                                                                   text_fragment,
+                                                                   clip_rect);
+                }
+            }
+            GenericFragment | IframeFragment(..) | TableFragment | TableCellFragment |
+            TableRowFragment | TableWrapperFragment | InlineBlockFragment(_) | InputFragment |
+            InlineAbsoluteHypotheticalFragment(_) => {
+                if opts::get().show_debug_fragment_borders {
+                    self.build_debug_borders_around_fragment(display_list,
+                                                             flow_origin,
+                                                             clip_rect);
+                }
+            }
+            ImageFragment(ref mut image_fragment) => {
+                let image_ref = &mut image_fragment.image;
+                match image_ref.get_image(self.node.to_untrusted_node_address()) {
+                    Some(image) => {
+                        debug!("(building display list) building image fragment");
+
+                        // Place the image into the display list.
+                        let image_display_item = box ImageDisplayItem {
+                            base: BaseDisplayItem::new(absolute_content_box,
+                                                       self.node,
+                                                       ContentStackingLevel,
+                                                       *clip_rect),
+                            image: image.clone(),
+                            stretch_size: absolute_content_box.size,
+                        };
+
+                        display_list.push(ImageDisplayItemClass(image_display_item))
+                    }
+                    None => {
+                        // No image data at all? Do nothing.
+                        //
+                        // TODO: Add some kind of placeholder image.
+                        debug!("(building display list) no image :(");
+                    }
+                }
+            }
+        }
+
+        if opts::get().show_debug_fragment_borders {
+           self.build_debug_borders_around_fragment(display_list, flow_origin, clip_rect)
+        }
+
+        // If this is an iframe, then send its position and size up to the constellation.
+        //
+        // FIXME(pcwalton): Doing this during display list construction seems potentially
+        // problematic if iframes are outside the area we're computing the display list for, since
+        // they won't be able to reflow at all until the user scrolls to them. Perhaps we should
+        // separate this into two parts: first we should send the size only to the constellation
+        // once that's computed during assign-block-sizes, and second we should should send the
+        // origin to the constellation here during display list construction. This should work
+        // because layout for the iframe only needs to know size, and origin is only relevant if
+        // the iframe is actually going to be displayed.
+        match self.specific {
+            IframeFragment(ref iframe_fragment) => {
+                self.finalize_position_and_size_of_iframe(iframe_fragment,
+                                                          absolute_fragment_bounds.origin,
+                                                          layout_context)
+            }
+            _ => {}
+        }
+    }
+
+    #[inline(never)]
+    fn finalize_position_and_size_of_iframe(&self,
+                                            iframe_fragment: &IframeFragmentInfo,
+                                            offset: Point2D<Au>,
+                                            layout_context: &LayoutContext) {
+        let border_padding = (self.border_padding).to_physical(self.style.writing_mode);
+        let content_size = self.content_box().size.to_physical(self.style.writing_mode);
+        let iframe_rect = Rect(Point2D(geometry::to_frac_px(offset.x + border_padding.left) as f32,
+                                       geometry::to_frac_px(offset.y + border_padding.top) as f32),
+                               Size2D(geometry::to_frac_px(content_size.width) as f32,
+                                      geometry::to_frac_px(content_size.height) as f32));
+
+        debug!("finalizing position and size of iframe for {:?},{:?}",
+               iframe_fragment.pipeline_id,
+               iframe_fragment.subpage_id);
+        let ConstellationChan(ref chan) = layout_context.shared.constellation_chan;
+        chan.send(FrameRectMsg(iframe_fragment.pipeline_id,
+                               iframe_fragment.subpage_id,
+                               iframe_rect));
+    }
+
+    fn clip_rect_for_children(&self, current_clip_rect: Rect<Au>, flow_origin: Point2D<Au>)
+                              -> Rect<Au> {
+        // Don't clip if we're text.
+        match self.specific {
+            ScannedTextFragment(_) => return current_clip_rect,
+            _ => {}
+        }
+
+        // Only clip if `overflow` tells us to.
+        match self.style.get_box().overflow {
+            overflow::hidden | overflow::auto | overflow::scroll => {}
+            _ => return current_clip_rect,
+        }
+
+        // Create a new clip rect.
+        //
+        // FIXME(#2795): Get the real container size.
+        let physical_rect = self.border_box.to_physical(self.style.writing_mode, Size2D::zero());
+        current_clip_rect.intersection(&Rect(physical_rect.origin + flow_origin,
+                                             physical_rect.size)).unwrap_or(ZERO_RECT)
+    }
+}
+
+pub trait BlockFlowDisplayListBuilding {
+    fn build_display_list_for_block(&mut self,
+                                    layout_context: &LayoutContext,
+                                    background_border_level: BackgroundAndBorderLevel);
+    fn build_display_list_for_absolutely_positioned_block(&mut self,
+                                                          layout_context: &LayoutContext);
+    fn build_display_list_for_floating_block(&mut self, layout_context: &LayoutContext);
+}
+
+impl BlockFlowDisplayListBuilding for BlockFlow {
+    fn build_display_list_for_block(&mut self,
+                                    layout_context: &LayoutContext,
+                                    background_border_level: BackgroundAndBorderLevel) {
+        let relative_offset =
+            self.fragment.relative_position(&self.base
+                                                 .absolute_position_info
+                                                 .relative_containing_block_size);
+
+        // Add the box that starts the block context.
+        let mut display_list = DisplayList::new();
+        self.fragment.build_display_list(&mut display_list,
+                                         layout_context,
+                                         self.base.abs_position.add_size(
+                                             &relative_offset.to_physical(self.base.writing_mode)),
+                                         background_border_level,
+                                         &self.base.clip_rect);
+
+        let mut child_layers = DList::new();
+        for kid in self.base.child_iter() {
+            if kid.is_absolutely_positioned() {
+                // All absolute flows will be handled by their containing block.
+                continue
+            }
+
+            display_list.push_all_move(mem::replace(&mut flow::mut_base(kid).display_list,
+                                                    DisplayList::new()));
+            child_layers.append(mem::replace(&mut flow::mut_base(kid).layers, DList::new()))
+        }
+
+        // Process absolute descendant links.
+        for abs_descendant_link in self.base.abs_descendants.iter() {
+            // TODO(pradeep): Send in our absolute position directly.
+            display_list.push_all_move(mem::replace(
+                    &mut flow::mut_base(abs_descendant_link).display_list,
+                    DisplayList::new()));
+            child_layers.append(mem::replace(&mut flow::mut_base(abs_descendant_link).layers,
+                                             DList::new()));
+        }
+
+        self.base.display_list = display_list;
+        self.base.layers = child_layers
+    }
+
+    fn build_display_list_for_absolutely_positioned_block(&mut self,
+                                                          layout_context: &LayoutContext) {
+        self.build_display_list_for_block(layout_context, RootOfStackingContextLevel);
+
+        if !self.base.absolute_position_info.layers_needed_for_positioned_flows &&
+                !self.base.flags.needs_layer() {
+            // We didn't need a layer.
+            let z_index = self.fragment.style().get_box().z_index.number_or_zero();
+            let level = PositionedDescendantStackingLevel(z_index);
+            self.base.display_list = mem::replace(&mut self.base.display_list,
+                                                  DisplayList::new()).flatten(level);
+            return
+        }
+
+        // If we got here, then we need a new layer.
+        let layer_rect = self.base.position.union(&self.base.overflow);
+        let size = Size2D(layer_rect.size.inline.to_nearest_px() as uint,
+                          layer_rect.size.block.to_nearest_px() as uint);
+        let origin = Point2D(self.base.abs_position.x.to_nearest_px() as uint,
+                             self.base.abs_position.y.to_nearest_px() as uint);
+
+        let scroll_policy = if self.is_fixed() {
+            FixedPosition
+        } else {
+            Scrollable
+        };
+        let display_list = mem::replace(&mut self.base.display_list, DisplayList::new());
+        let new_layer = RenderLayer {
+            id: self.layer_id(0),
+            display_list: Arc::new(display_list.flatten(ContentStackingLevel)),
+            position: Rect(origin, size),
+            background_color: color::rgba(1.0, 1.0, 1.0, 0.0),
+            scroll_policy: scroll_policy,
+        };
+        self.base.layers.push(new_layer)
+    }
+
+    fn build_display_list_for_floating_block(&mut self, layout_context: &LayoutContext) {
+        self.build_display_list_for_block(layout_context, RootOfStackingContextLevel);
+        self.base.display_list = mem::replace(&mut self.base.display_list,
+                                              DisplayList::new()).flatten(FloatStackingLevel)
+    }
+}
+