Don't promote all scrollable regions to stacking contexts

Instead annotate all flows with their owning ScrollRoots. When
processing the display list items into a flattened display list, we add
PushScrollRoot and PopScrollRoot to signal when scrolling regions start
and end. It is possible for content from different scrolling regions to
intersect and when they do, the stack of scrolling regions is
duplicated.  When these duplicated scrolling regions stacks reach
WebRender, it will scroll them in tandem.

The PushScrollRoot and PopScrollRoot items are currently represented as
StackingContexts in WebRender, but eventually these will be replaced
with special WebRender display items.

Fixes #13529.
Fixed #13298.

1 files changed, 19 insertions, 19 deletions

diff --git a/components/layout/traversal.rs b/components/layout/traversal.rs
index c897838875a..1792f1134ff 100644
--- a/components/layout/traversal.rs
+++ b/components/layout/traversal.rs
@@ -270,34 +270,34 @@ pub struct BuildDisplayList<'a> {
 impl<'a> BuildDisplayList<'a> {
     #[inline]
     pub fn traverse(&mut self, flow: &mut Flow) {
-        if self.should_process() {
-            let new_stacking_context =
-                flow::base(flow).stacking_context_id != self.state.stacking_context_id();
-            if new_stacking_context {
-                self.state.push_stacking_context_id(flow::base(flow).stacking_context_id);
-            }
+        let new_stacking_context =
+            flow::base(flow).stacking_context_id != self.state.stacking_context_id();
+        if new_stacking_context {
+            self.state.push_stacking_context_id(flow::base(flow).stacking_context_id);
+        }
 
-            let new_scroll_root =
-                flow::base(flow).scroll_root_id != self.state.scroll_root_id();
-            if new_scroll_root {
-                self.state.push_scroll_root_id(flow::base(flow).scroll_root_id);
-            }
+        let new_scroll_root =
+            flow::base(flow).scroll_root_id != self.state.scroll_root_id();
+        if new_scroll_root {
+            self.state.push_scroll_root_id(flow::base(flow).scroll_root_id);
+        }
 
+        if self.should_process() {
             flow.build_display_list(&mut self.state);
             flow::mut_base(flow).restyle_damage.remove(REPAINT);
-
-            if new_stacking_context {
-                self.state.pop_stacking_context_id();
-            }
-
-            if new_scroll_root {
-                self.state.pop_scroll_root_id();
-            }
         }
 
         for kid in flow::child_iter_mut(flow) {
             self.traverse(kid);
         }
+
+        if new_stacking_context {
+            self.state.pop_stacking_context_id();
+        }
+
+        if new_scroll_root {
+            self.state.pop_scroll_root_id();
+        }
     }
 
     #[inline]