Squashed fixed position layers shouldn't flicker while scrolling

Source/core/rendering/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 956 matching lines @@
     if (!scrollingCoordinator)
         return;
 
     scrollingCoordinator->updateLayerPositionConstraint(&m_owningLayer);
 
     // Page scale is applied as a transform on the root render view layer. Because the scroll
     // layer is further up in the hierarchy, we need to avoid marking the root render view
     // layer as a container.
     bool isContainer = m_owningLayer.hasTransform() && !m_owningLayer.isRootLayer();
     // FIXME: we should make certain that childForSuperLayers will never be the m_squashingContainmentLayer here
-    scrollingCoordinator->setLayerIsContainerForFixedPositionLayers(localRootForOwningLayer(), isContainer);
+    scrollingCoordinator->setLayerIsContainerForFixedPositionLayers(childForSuperlayers(), isContainer);
 }
 
 void CompositedLayerMapping::updateInternalHierarchy()
 {
     // m_foregroundLayer has to be inserted in the correct order with child layers,
     // so it's not inserted here.
     if (m_ancestorClippingLayer)
         m_ancestorClippingLayer->removeAllChildren();
 
     m_graphicsLayer->removeFromParent();
@@ skipping 863 matching lines @@
 
     if (m_childContainmentLayer)
         return m_childContainmentLayer.get();
 
     if (m_childTransformLayer)
         return m_childTransformLayer.get();
 
     return m_graphicsLayer.get();
 }
 
-GraphicsLayer* CompositedLayerMapping::localRootForOwningLayer() const
-{
-    if (m_ancestorClippingLayer)
-        return m_ancestorClippingLayer.get();
-
-    return m_graphicsLayer.get();
-}
-
 GraphicsLayer* CompositedLayerMapping::childForSuperlayers() const
 {
     if (m_squashingContainmentLayer)
         return m_squashingContainmentLayer.get();
 
-    return localRootForOwningLayer();
+    if (m_ancestorClippingLayer)
+        return m_ancestorClippingLayer.get();
+
+    return m_graphicsLayer.get();
 }
 
 GraphicsLayer* CompositedLayerMapping::layerForChildrenTransform() const
 {
     if (GraphicsLayer* clipLayer = clippingLayer())
         return clipLayer;
     if (m_scrollingLayer)
         return m_scrollingLayer.get();
     return m_childTransformLayer.get();
 }
@@ skipping 24 matching lines @@
         || m_owningLayer.transform()
         || m_owningLayer.clipsCompositingDescendantsWithBorderRadius() // FIXME: Revisit this if the paintsIntoCompositedAncestor state is removed.
         || renderer->isTransparent()
         || renderer->hasMask()
         || renderer->hasReflection()
         || renderer->hasFilter();
 
     if (paintsIntoCompositedAncestor() != previousPaintsIntoCompositedAncestor)
         compositor()->repaintOnCompositingChange(&m_owningLayer);
 
-
     return m_requiresOwnBackingStoreForIntrinsicReasons != previousRequiresOwnBackingStoreForIntrinsicReasons;
 }
 
 void CompositedLayerMapping::setBlendMode(blink::WebBlendMode blendMode)
 {
     if (m_ancestorClippingLayer) {
         m_ancestorClippingLayer->setBlendMode(blendMode);
         m_graphicsLayer->setBlendMode(blink::WebBlendModeNormal);
     } else {
         m_graphicsLayer->setBlendMode(blendMode);
@@ skipping 357 matching lines @@
     } else if (graphicsLayer == m_scrollingBlockSelectionLayer.get()) {
         name = "Scrolling Block Selection Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink