Bug 976370. Move overlay scrollbar display items above scroll layer items for the same scroll frame. r=roc

This has two desirable side effects. The first and more important is that it prevents a scrollbar item from sitting between scroll layer items and preventing them from merging into one and thus preventing the creation of an async scrollable layer. The second is that scrollbars should be on top of the content they are scrolling in general, and this will make that happen in more cases.

layout/base/nsDisplayList.cpp

@@ -1004,6 +1004,31 @@ TreatAsOpaque(nsDisplayItem* aItem, nsDisplayListBuilder* aBuilder)
   return opaqueClipped;
 }
 
+/* Checks if aPotentialScrollItem is a scroll layer item and aPotentialScrollbarItem
+ * is an overlay scrollbar item for the same scroll frame.
+ */
+static bool
+IsScrollLayerItemAndOverlayScrollbarForScrollFrame(
+  nsDisplayItem* aPotentialScrollItem, nsDisplayItem* aPotentialScrollbarItem)
+{
+  if (aPotentialScrollItem->GetType() == nsDisplayItem::TYPE_SCROLL_LAYER &&
+      aPotentialScrollbarItem &&
+      aPotentialScrollbarItem->GetType() == nsDisplayItem::TYPE_OWN_LAYER &&
+      LookAndFeel::GetInt(LookAndFeel::eIntID_UseOverlayScrollbars)) {
+    nsDisplayScrollLayer* scrollItem =
+      static_cast<nsDisplayScrollLayer*>(aPotentialScrollItem);
+    nsDisplayOwnLayer* layerItem =
+      static_cast<nsDisplayOwnLayer*>(aPotentialScrollbarItem);
+    if ((layerItem->GetFlags() &
+         (nsDisplayOwnLayer::VERTICAL_SCROLLBAR |
+          nsDisplayOwnLayer::HORIZONTAL_SCROLLBAR)) &&
+        layerItem->Frame()->GetParent() == scrollItem->GetScrollFrame()) {
+      return true;
+    }
+  }
+  return false;
+}
+
 bool
 nsDisplayList::ComputeVisibilityForSublist(nsDisplayListBuilder* aBuilder,
                                            nsRegion* aVisibleRegion,
@@ -1036,6 +1061,17 @@ nsDisplayList::ComputeVisibilityForSublist(nsDisplayListBuilder* aBuilder,
         continue;
       }
 
+      // If an overlay scrollbar item is between a scroll layer item and the
+      // other scroll layer items that we need to merge with just move the
+      // scrollbar item up, that way it will be on top of the scrolled content
+      // and we can try to merge all the scroll layer items.
+      if (IsScrollLayerItemAndOverlayScrollbarForScrollFrame(item, belowItem)) {
+        elements[i] = belowItem;
+        elements[i-1] = item;
+        i++;
+        continue;
+      }
+
       if (list && item->ShouldFlattenAway(aBuilder)) {
         // The elements on the list >= i no longer serve any use.
         elements.SetLength(i);

layout/base/nsDisplayList.h

@@ -2688,6 +2688,7 @@ public:
     // Don't allow merging, each sublist must have its own layer
     return false;
   }
+  uint32_t GetFlags() { return mFlags; }
   NS_DISPLAY_DECL_NAME("OwnLayer", TYPE_OWN_LAYER)
 private:
   uint32_t mFlags;