Bug 786672 - Use the scrolled frame for nsDisplayScrollLayer when finding the active scrolled root. r=roc

The underlying frame of an nsDisplayScrollLayer can change and end up returning
different values when finding the active scrolled root. Instead of relying on
display-item ordering/merging, get the scrolled frame from the item (which was
already storing it).

layout/base/FrameLayerBuilder.cpp

@@ -1807,6 +1807,12 @@ ContainerState::ProcessDisplayItems(const nsDisplayList& aList,
         nsLayoutUtils::GetActiveScrolledRootFor(mContainerFrame,
                                                 mBuilder->ReferenceFrame());
       forceInactive = true;
+    } else if (item->GetType() == nsDisplayItem::TYPE_SCROLL_LAYER) {
+      nsDisplayScrollLayer* scrollLayerItem =
+        static_cast<nsDisplayScrollLayer*>(item);
+      activeScrolledRoot =
+        nsLayoutUtils::GetActiveScrolledRootFor(scrollLayerItem->GetScrolledFrame(),
+                                                mBuilder->ReferenceFrame());
     } else {
       activeScrolledRoot = nsLayoutUtils::GetActiveScrolledRootFor(item, mBuilder);
     }

layout/base/nsDisplayList.h

@@ -2038,6 +2038,8 @@ public:
   intptr_t GetScrollLayerCount();
   intptr_t RemoveScrollLayerCount();
 
+  virtual nsIFrame* GetScrolledFrame() { return mScrolledFrame; }
+
 private:
   nsIFrame* mScrollFrame;
   nsIFrame* mScrolledFrame;