Fix Compositing of Opaque Scrolling Layers and Add Tests

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.h
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.h b/third_party/WebKit/Source/core/paint/PaintLayer.h
index d003945cf4f18d84301a29ab3fa3d2d1ac88a7ab..b7e75297fa1ce0f52cea514e3262d19b347ad72e 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.h
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.h
@@ -433,6 +433,12 @@ public:
     CompositedLayerMapping* compositedLayerMapping() const;
     GraphicsLayer* graphicsLayerBacking() const;
     GraphicsLayer* graphicsLayerBackingForScrolling() const;
+    // 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

[chrishtr, 2016/09/02 01:19:38]

"can be painted into the composited scrolling contents layer when it exists"
(last three words are my suggested addition)

[Stephen Chennney, 2016/09/06 21:06:21]

Will do.

+    // opaque this allows us to composite the scroller even on low DPI as we can
+    // draw with subpixel anti-aliasing.
+    bool shouldPaintBackgroundOntoScrollingContentsLayer() const;
     // NOTE: If you are using hasCompositedLayerMapping to determine the state of compositing for this layer,
     // (and not just to do bookkeeping related to the mapping like, say, allocating or deallocating a mapping),
     // then you may have incorrect logic. Use compositingState() instead.