Only paint the background onto the scrolling contents layer if we have a scrolling contents layer.

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:
  * 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 281 matching lines @@
     bool isolate = m_owningLayer.shouldIsolateCompositedDescendants();
 
     // non stacking context layers should never isolate
     ASSERT(m_owningLayer.stackingNode()->isStackingContext() || !isolate);
 
     m_graphicsLayer->setIsRootForIsolatedGroup(isolate);
 }
 
 void CompositedLayerMapping::updateBackgroundPaintsOntoScrollingContentsLayer()
 {
-    bool shouldPaintOntoScrollingContentsLayer = m_owningLayer.shouldPaintBackgroundOntoScrollingContentsLayer();
+    bool shouldPaintOntoScrollingContentsLayer =
+        m_owningLayer.canPaintBackgroundOntoScrollingContentsLayer()
+        && m_owningLayer.needsCompositedScrolling();

[Stephen Chennney, 2016/09/15 20:08:37]

Indent.

And add a comment here about why composited scrolling is needed. Basically what
you have in the review comment below.

[flackr, 2016/09/15 20:28:37]

I'm confused, isn't that the correct indent for two lines that are part of the
same expression? Should I not wrap the first line?
Done.

     if (shouldPaintOntoScrollingContentsLayer != backgroundPaintsOntoScrollingContentsLayer()) {
         m_backgroundPaintsOntoScrollingContentsLayer = shouldPaintOntoScrollingContentsLayer;
         // If the background is no longer painted onto the scrolling contents
         // layer the scrolling contents layer needs to be updated. If it is
         // going to be painted onto the scrolling contents layer this update
         // will be triggered by LayoutBoxModelObject::setBackingNeedsPaintInvalidationInRect
         if (hasScrollingLayer() && !shouldPaintOntoScrollingContentsLayer)
             m_scrollingContentsLayer->setNeedsDisplay();
     }
 }
@@ skipping 2384 matching lines @@
     } else if (graphicsLayer == m_scrollingContentsLayer.get()) {
         name = "Scrolling Contents Layer";
     } else {
         ASSERT_NOT_REACHED();
     }
 
     return name;
 }
 
 } // namespace blink