Composited fixed-pos element should still cause repaints if it paintsIntoCompositedAncestor

Source/core/page/scrolling/ScrollingCoordinator.cpp

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 789 matching lines @@
         return false;
 
     for (FrameView::ViewportConstrainedObjectSet::const_iterator it = viewportConstrainedObjects->begin(), end = viewportConstrainedObjects->end(); it != end; ++it) {
         RenderObject* viewportConstrainedObject = *it;
         if (!viewportConstrainedObject->isBoxModelObject() || !viewportConstrainedObject->hasLayer())
             return true;
         RenderLayer* layer = toRenderBoxModelObject(viewportConstrainedObject)->layer();
         // Any explicit reason that a fixed position element is not composited shouldn't cause slow scrolling.
         if (!layer->isComposited() && layer->viewportConstrainedNotCompositedReason() == RenderLayer::NoNotCompositedReason)
             return true;
+
+        // Composited layers that actually paint into their enclosing ancestor
+        // must also force main thread scrolling.
+        if (layer->isComposited() && layer->backing()->paintsIntoCompositedAncestor())
+            return true;
     }
     return false;
 }
 
 MainThreadScrollingReasons ScrollingCoordinator::mainThreadScrollingReasons() const
 {
     // The main thread scrolling reasons are applicable to scrolls of the main
     // frame. If it does not exist or if it is not scrollable, there is no
     // reason to force main thread scrolling.
     FrameView* frameView = m_page->mainFrame()->view();
@@ skipping 30 matching lines @@
         stringBuilder.resize(stringBuilder.length() - 2);
     return stringBuilder.toString();
 }
 
 String ScrollingCoordinator::mainThreadScrollingReasonsAsText() const
 {
     return mainThreadScrollingReasonsAsText(mainThreadScrollingReasons());
 }
 
 } // namespace WebCore