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();

[chrishtr, 2016/09/15 20:15:47]

It's PaintLayerScrollableArea::usesCompositedScrolling that is the most direct
and foolproof. It's also used in some other places. FYI
I made the same mistake elsewhere as you are fixing in this CL, see:

https://codereview.chromium.org/2343963002

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

Done. I can add it, but I shouldn't need to check if we have a scrollable area
given canPaintBackgroundOntoScrollingContentsLayer must have returned true
before we evaluate this.

     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