Paint local background colors onto foreground layer.

third_party/WebKit/Source/core/paint/PaintLayer.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
@@ skipping 229 matching lines @@
 
     // FIXME: Many people call this function while it has out-of-date information.
     bool isSelfPaintingLayer() const { return m_isSelfPaintingLayer; }
 
     bool isTransparent() const { return layoutObject()->isTransparent() || layoutObject()->style()->hasBlendMode() || layoutObject()->hasMask(); }
 
     bool isReflection() const { return layoutObject()->isReplica(); }
     PaintLayerReflectionInfo* reflectionInfo() { return m_rareData ? m_rareData->reflectionInfo.get() : nullptr; }
     const PaintLayerReflectionInfo* reflectionInfo() const { return const_cast<PaintLayer*>(this)->reflectionInfo(); }
 
+    // Returns true for layers with scrollable overflow which have a background
+    // that can be painted into the composited scrolling contents layer (i.e.
+    // the background can scroll with the content). When the background is also
+    // opaque this allows us to composite the scroller even on low DPI as we can
+    // draw with subpixel anti-aliasing.
+    bool shouldPaintBackgroundOntoScrollingContentsLayer() const;

[chrishtr, 2016/08/05 00:40:08]

Move this method to CompositedLayerMapping.

[flackr, 2016/08/08 18:01:30]

Done.

+
     const PaintLayer* root() const
     {
         const PaintLayer* curr = this;
         while (curr->parent())
             curr = curr->parent();
         return curr;
     }
 
     const LayoutPoint& location() const { ASSERT(!m_needsPositionUpdate); return m_location; }
     // FIXME: size() should ASSERT(!m_needsPositionUpdate) as well, but that fails in some tests,
@@ skipping 632 matching lines @@
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 void showLayerTree(const blink::PaintLayer*);
 void showLayerTree(const blink::LayoutObject*);
 #endif
 
 #endif // Layer_h