Disable clipping on root scroller's ancestors.

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

 /*
  * 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 320 matching lines @@
     // Return true if |m_owningLayer|'s compositing ancestor is not a descendant (inclusive) of the
     // clipping container for |m_owningLayer|.
     bool owningLayerClippedByLayerNotAboveCompositedAncestor(const PaintLayer* scrollParent);
 
     const PaintLayer* scrollParent();
 
     // Clear the groupedMapping entry on the layer at the given index, only if that layer does
     // not appear earlier in the set of layers for this object.
     bool invalidateLayerIfNoPrecedingEntry(size_t);
 
+    // Disables clipping on layers that are ancestors of the root scroller. This allows the
+    // root scroller to expand and shrink its clipping layer in response to top controls
+    // movement and have it be visible to the user even if there's other masking layers above
+    // it in the hierarchy.
+    void setClippingOnRootScrollerAncestors();

[chrishtr, 2016/09/01 23:42:56]

Name it disableClippingOnRootScrollerAncestors instead?

[bokan, 2016/09/09 00:19:02]

This method is gone now.

+
     PaintLayer& m_owningLayer;
 
     // The hierarchy of layers that is maintained by the CompositedLayerMapping looks like this:
     //
     //  + m_ancestorClippingLayer [OPTIONAL]
     //    + m_graphicsLayer
     //      + m_childTransformLayer [OPTIONAL]
     //      | + m_childContainmentLayer [OPTIONAL] <-OR-> m_scrollingLayer [OPTIONAL]
     //      |                                             + m_scrollingContentsLayer [Present iff m_scrollingLayer is present]
     //      + m_overflowControlsAncestorClippingLayer [OPTIONAL] // *The overflow controls may need to be repositioned in the
@@ skipping 116 matching lines @@
 
     // Keep track of whether the background is painted onto the scrolling contents layer for invalidations.
     unsigned m_backgroundPaintsOntoScrollingContentsLayer : 1;
 
     friend class CompositedLayerMappingTest;
 };
 
 } // namespace blink
 
 #endif // CompositedLayerMapping_h