Fix flattening for scrollable areas.

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

 /*
  * Copyright (C) 2009, 2010, 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 1493 matching lines @@
     ApplyToGraphicsLayers(this, functor, mode);
 
     // Note, if we apply perspective, we have to set should flatten differently
     // so that the transform propagates to child layers correctly.
     if (hasChildTransformLayer()) {
         ApplyToGraphicsLayers(this, [](GraphicsLayer* layer) {
             layer->setShouldFlattenTransform(false);
         }, ApplyToChildContainingLayers);
     }
 
-    // Regardless, mark the scrolling contents layer and scrolling block
+    // Regardless, mark the graphics layer, scrolling layer and scrolling block
     // selection layer (if they exist) as not flattening. Having them flatten
     // causes unclipped render surfaces which cause bugs.
     // http://crbug.com/521768
     if (hasScrollingLayer()) {
-        m_scrollingContentsLayer->setShouldFlattenTransform(false);
+        m_graphicsLayer->setShouldFlattenTransform(false);
+        m_scrollingLayer->setShouldFlattenTransform(false);
         if (m_scrollingBlockSelectionLayer)
             m_scrollingBlockSelectionLayer->setShouldFlattenTransform(false);
     }
 }
 
 void CompositedLayerMapping::updateElementIdAndCompositorMutableProperties()
 {
     if (!RuntimeEnabledFeatures::compositorWorkerEnabled())
         return;
 
@@ skipping 1071 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink