Overlay scrollbars must respect ancestor clip

Source/core/rendering/compositing/GraphicsLayerUpdater.cpp

 /*
  * Copyright (C) 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Google 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
@@ skipping 44 matching lines @@
             if (layer.stackingNode()->isStackingContext())
                 m_compositingStackingContext = &layer;
         }
     }
 
     const RenderLayer* compositingContainer(const RenderLayer& layer) const
     {
         return layer.stackingNode()->isNormalFlowOnly() ? m_compositingAncestor : m_compositingStackingContext;
     }
 
+    const RenderLayer* compositingStackingContext() const
+    {
+        return m_compositingStackingContext;
+    }
+
 private:
     const RenderLayer* m_compositingStackingContext;
     const RenderLayer* m_compositingAncestor;
 };
 
 GraphicsLayerUpdater::GraphicsLayerUpdater()
     : m_needsRebuildTree(false)
 {
 }
 
@@ skipping 27 matching lines @@
             mapping->updateCompositedBounds();
 
             if (RenderLayerReflectionInfo* reflection = layer.reflectionInfo()) {
                 if (reflection->reflectionLayer()->hasCompositedLayerMapping())
                     reflection->reflectionLayer()->compositedLayerMapping()->updateCompositedBounds();
             }
 
             if (mapping->updateGraphicsLayerConfiguration())
                 m_needsRebuildTree = true;
 
-            mapping->updateGraphicsLayerGeometry(compositingContainer, layersNeedingPaintInvalidation);
+            mapping->updateGraphicsLayerGeometry(compositingContainer, context.compositingStackingContext(), layersNeedingPaintInvalidation);
 
             if (mapping->hasUnpositionedOverflowControlsLayers())
                 layer.scrollableArea()->positionOverflowControls(IntSize());
 
             updateType = mapping->updateTypeForChildren(updateType);
             mapping->clearNeedsGraphicsLayerUpdate();
         }
     }
 
     UpdateContext childContext(context, layer);
     for (RenderLayer* child = layer.firstChild(); child; child = child->nextSibling())
         updateRecursive(*child, updateType, childContext, layersNeedingPaintInvalidation);
 }
 
 #if ENABLE(ASSERT)
 
 void GraphicsLayerUpdater::assertNeedsToUpdateGraphicsLayerBitsCleared(RenderLayer& layer)
 {
     if (layer.hasCompositedLayerMapping())
         layer.compositedLayerMapping()->assertNeedsToUpdateGraphicsLayerBitsCleared();
 
     for (RenderLayer* child = layer.firstChild(); child; child = child->nextSibling())
         assertNeedsToUpdateGraphicsLayerBitsCleared(*child);
 }
 
 #endif
 
 } // namespace blink