Background attachment behavior update for rootlayer with root-layer-scrolls enabled

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

Index: third_party/WebKit/Source/core/paint/PaintLayer.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayer.cpp b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
index bf4faf4eff447320bad99429f6ec2a72e5b5e13d..f4c22197ed9086f4d67f121f7811b8159e7f2dd7 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -2603,10 +2603,14 @@ GraphicsLayer* PaintLayer::graphicsLayerBacking(const LayoutObject* obj) const {
 }
 
 BackgroundPaintLocation PaintLayer::backgroundPaintLocation() const {
-  BackgroundPaintLocation location =
-      isRootLayer() || !scrollsOverflow()
-          ? BackgroundPaintInGraphicsLayer
-          : layoutObject()->backgroundPaintLocation();
+  BackgroundPaintLocation location;
+  if (RuntimeEnabledFeatures::rootLayerScrollingEnabled()) {
+    location = layoutObject()->backgroundPaintLocation();

[flackr, 2017/02/15 05:52:19]

Don't we still need to check scrollsOverflow? Otherwise we could return
BackgroundPaintInScrollingContentsLayer for non scrolling paint layers.

[yigu, 2017/02/15 16:00:38]

Done.

+  } else {
+    location = isRootLayer() || !scrollsOverflow()
+                   ? BackgroundPaintInGraphicsLayer
+                   : layoutObject()->backgroundPaintLocation();
+  }
   m_stackingNode->updateLayerListsIfNeeded();
   if (m_stackingNode->hasNegativeZOrderList())
     location = BackgroundPaintInGraphicsLayer;