Fix PaintPropertyTreeBuilder for root layer scrolling.

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:
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@@ skipping 1118 matching lines @@
             bottomLayer = layer;
         }
     };
 
     updateBottomLayer(m_childTransformLayer.get());
     updateBottomLayer(m_childContainmentLayer.get());
     updateBottomLayer(m_scrollingLayer.get());
 
     // Now constructing the subtree for the overflow controls.
     bottomLayer = m_graphicsLayer.get();
-    if (m_isMainFrameLayoutViewLayer)
+    if (m_isMainFrameLayoutViewLayer && !RuntimeEnabledFeatures::slimmingPaintV2Enabled())

[skobes, 2016/08/17 01:38:24]

This still makes me nervous but if the paint team is ok with it then *shrug*.

[pdr., 2016/08/17 04:30:41]

I agree but we just aren't far enough along to block you on this :(

I've filed https://crbug.com/638473 for this work.

[trchen, 2016/08/17 05:57:47]

I think this line is totally fine. The long term goal is to bypass
core/layout/compositing. Today we are still using PaintLayerCompositor just
because it is a pain to duplicate some basic setup. If possible I don't even
want LayoutView to create CLM... Because it pops main frame scrollbars to a
graphics layer.

         bottomLayer = layoutObject()->frame()->page()->frameHost().visualViewport().containerLayer();
     updateBottomLayer(m_overflowControlsAncestorClippingLayer.get());
     updateBottomLayer(m_overflowControlsHostLayer.get());
     if (m_layerForHorizontalScrollbar)
         m_overflowControlsHostLayer->addChild(m_layerForHorizontalScrollbar.get());
     if (m_layerForVerticalScrollbar)
         m_overflowControlsHostLayer->addChild(m_layerForVerticalScrollbar.get());
     if (m_layerForScrollCorner)
         m_overflowControlsHostLayer->addChild(m_layerForScrollCorner.get());
 
@@ skipping 1541 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink