Layer squashing causes incorrect rendering while scrolling in Gmail

Source/core/rendering/RenderLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 3410 matching lines @@
         result.unite(m_reflectionInfo->reflectionLayer()->boundingBoxForCompositing(this));
 
     ASSERT(m_stackingNode->isStackingContainer() || !m_stackingNode->hasPositiveZOrderList());
 
 #if !ASSERT_DISABLED
     LayerListMutationDetector mutationChecker(const_cast<RenderLayer*>(this)->stackingNode());
 #endif
 
     RenderLayerStackingNodeIterator iterator(*m_stackingNode.get(), AllChildren);
     while (RenderLayerStackingNode* node = iterator.next()) {
-        if (node->layer()->hasCompositedLayerMapping() && options != ApplyBoundsChickenEggHacks)
+        // Here we exclude both directly composted layers and squashing layers
+        // because those RenderLayers don't paint into the graphics layer
+        // for this RenderLayer. For example, the bounds of squashed RenderLayers
+        // will be included in the computation of the appropriate squashing
+        // GraphicsLayer.
+        if (options != ApplyBoundsChickenEggHacks && node->layer()->compositingState() != NotComposited)
             continue;
         result.unite(node->layer()->boundingBoxForCompositing(this, options));
     }
 
     // FIXME: We can optimize the size of the composited layers, by not enlarging
     // filtered areas with the outsets if we know that the filter is going to render in hardware.
     // https://bugs.webkit.org/show_bug.cgi?id=81239
     m_renderer->style()->filterOutsets().expandRect(result);
 
     if (shouldIncludeTransform)
@@ skipping 573 matching lines @@
     }
 }
 
 void showLayerTree(const WebCore::RenderObject* renderer)
 {
     if (!renderer)
         return;
     showLayerTree(renderer->enclosingLayer());
 }
 #endif