servo: Merge #14475 - Don't scroll background and borders of overflow:scroll blocks (from mrobinson:backrounds-borders-scrolling-div); r=pcwalton

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---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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We want them to be children of the parent scrolling root instead of the
scrolling root of the particular block they belong to. Also, we adjust
the scroll root to only include the content region.

Source-Repo: https://github.com/servo/servo
Source-Revision: 900cb3b142ed6158c74f8a6d835a59dedb965f10

servo/components/layout/display_list_builder.rs

@@ -15,7 +15,6 @@ use block::{BlockFlow, BlockStackingContextType};
 use canvas_traits::{CanvasData, CanvasMsg, FromLayoutMsg};
 use context::SharedLayoutContext;
 use euclid::{Matrix4D, Point2D, Radians, Rect, SideOffsets2D, Size2D};
-use euclid::point::TypedPoint2D;
 use flex::FlexFlow;
 use flow::{BaseFlow, Flow, IS_ABSOLUTELY_POSITIONED};
 use flow_ref::FlowRef;
@@ -98,6 +97,7 @@ pub struct DisplayListBuildState<'a> {
     pub scroll_roots: HashMap<ScrollRootId, ScrollRoot>,
     pub stacking_context_id_stack: Vec<StackingContextId>,
     pub scroll_root_id_stack: Vec<ScrollRootId>,
+    pub processing_scroll_root_element: bool,
 }
 
 impl<'a> DisplayListBuildState<'a> {
@@ -111,6 +111,7 @@ impl<'a> DisplayListBuildState<'a> {
             scroll_roots: HashMap::new(),
             stacking_context_id_stack: vec!(stacking_context_id),
             scroll_root_id_stack: vec!(ScrollRootId::root()),
+            processing_scroll_root_element: false,
         }
     }
 
@@ -162,6 +163,14 @@ impl<'a> DisplayListBuildState<'a> {
                                 cursor: Option<Cursor>,
                                 section: DisplayListSection)
                                 -> BaseDisplayItem {
+        let scroll_root_id = if (section == DisplayListSection::BackgroundAndBorders ||
+                                 section == DisplayListSection::BlockBackgroundsAndBorders) &&
+                                 self.processing_scroll_root_element {
+            self.parent_scroll_root_id()
+        } else {
+            self.scroll_root_id()
+        };
+
         BaseDisplayItem::new(&bounds,
                              DisplayItemMetadata {
                                  node: node,
@@ -170,7 +179,7 @@ impl<'a> DisplayListBuildState<'a> {
                              &clip,
                              section,
                              self.stacking_context_id(),
-                             self.scroll_root_id())
+                             scroll_root_id)
     }
 
     pub fn to_display_list(mut self) -> DisplayList {
@@ -1977,21 +1986,25 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
 
         if self.has_scrolling_overflow() {
             let size = self.base.overflow.scroll.size;
-            let mut clip = self.fragment.stacking_relative_border_box(
+            let coordinate_system = if establishes_stacking_context {
+                CoordinateSystem::Own
+            } else {
+                CoordinateSystem::Parent
+            };
+
+            let border_box = self.fragment.stacking_relative_border_box(
                 &self.base.stacking_relative_position,
                 &self.base.early_absolute_position_info.relative_containing_block_size,
                 self.base.early_absolute_position_info.relative_containing_block_mode,
-                CoordinateSystem::Parent);
-            if establishes_stacking_context {
-                clip = Rect::new(TypedPoint2D::zero(), clip.size);
-            }
+                coordinate_system);
 
             let parent_id = state.parent_scroll_root_id();
+            state.processing_scroll_root_element = true;
             state.add_scroll_root(
                 ScrollRoot {
                     id: self.base.scroll_root_id,
                     parent_id: parent_id,
-                    clip: clip,
+                    clip: self.fragment.stacking_relative_content_box(&border_box),
                     size: size,
                 }
             );
@@ -2012,6 +2025,8 @@ impl BlockFlowDisplayListBuilding for BlockFlow {
                                 &self.base.clip);
 
         self.base.build_display_items_for_debugging_tint(state, self.fragment.node);
+
+        state.processing_scroll_root_element = false;
     }
 
     fn switch_coordinate_system_if_necessary(&mut self) {