Handle nested position:sticky elements correctly (compositor)

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index f7e90ea7b47277c8df23da1df91cfd84ecc80de1..929cacf42b940b4df7ef90823f4ffb55a9f27048 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -113,7 +113,24 @@ struct StickyPositionNodeData {
   // by computing the additional offset necessary to keep the element stuck.
   gfx::Vector2dF main_thread_offset;
 
-  StickyPositionNodeData() : scroll_ancestor(-1) {}
+  // In order to properly compute the sticky offset, we need to know if we have
+  // any sticky ancestors both between ourselves and our containing block and
+  // between our containing block and the viewport. These ancestors are then
+  // used to correct the constraining rect locations.
+  int nearest_node_shifting_sticky_box;
+  int nearest_node_shifting_containing_block;
+
+  // For performance we cache our accumulated sticky offset to allow descendant
+  // sticky elements to offset their constraint rects. Because we can either
+  // affect the sticky box constraint rect or the containing block constraint
+  // rect, we need to accumulate both.
+  gfx::Vector2dF total_sticky_box_sticky_offset;
+  gfx::Vector2dF total_containing_block_sticky_offset;
+
+  StickyPositionNodeData()
+      : scroll_ancestor(-1),

[flackr, 2017/03/17 18:04:08]

nit: Can we change these ids to kInvalidNodeId?

[smcgruer, 2017/03/17 18:42:42]

Done.

+        nearest_node_shifting_sticky_box(-1),
+        nearest_node_shifting_containing_block(-1) {}
 };
 
 class CC_EXPORT TransformTree final : public PropertyTree<TransformNode> {