Rework the way scroll roots are collected

Collect scroll roots during the collect_stacking_context phase instead
of during display list construction. This will be useful in order to
collect containing block scroll roots as well as to give scroll roots
sequential ids in the future. This change also pulls stacking context
children out of the StackingContext struct itself, which should reduce
very slightly the memory used by the finished display list. This also
simplifies the DisplayListBuilder because it no longer has to maintain
a stack of ScrollRootIds and StackingContextIds and can instead just
rely on the program stack.

1 files changed, 2 insertions, 6 deletions

diff --git a/components/layout/table_cell.rs b/components/layout/table_cell.rs
index 5e20f195078..15b3a42ed51 100644
--- a/components/layout/table_cell.rs
+++ b/components/layout/table_cell.rs
@@ -14,8 +14,6 @@ use display_list_builder::{BlockFlowDisplayListBuilding, BorderPaintingMode, Dis
 use euclid::{Point2D, Rect, SideOffsets2D, Size2D};
 use flow::{self, Flow, FlowClass, IS_ABSOLUTELY_POSITIONED, OpaqueFlow};
 use fragment::{Fragment, FragmentBorderBoxIterator, Overflow};
-use gfx::display_list::StackingContext;
-use gfx_traits::ScrollRootId;
 use gfx_traits::print_tree::PrintTree;
 use layout_debug;
 use model::MaybeAuto;
@@ -262,10 +260,8 @@ impl Flow for TableCellFlow {
         self.block_flow.build_display_list_for_block(state, border_painting_mode)
     }
 
-    fn collect_stacking_contexts(&mut self,
-                                 parent: &mut StackingContext,
-                                 parent_scroll_root_id: ScrollRootId) {
-        self.block_flow.collect_stacking_contexts(parent, parent_scroll_root_id);
+    fn collect_stacking_contexts(&mut self, state: &mut DisplayListBuildState) {
+        self.block_flow.collect_stacking_contexts(state);
     }
 
     fn repair_style(&mut self, new_style: &Arc<ServoComputedValues>) {