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 168383e4077bc5da12b3a5d8b84841d30bb4efcc..4a6fc138a15b867a7b26faed424d4fbc1141562f 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayer.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayer.cpp
@@ -2624,10 +2624,13 @@ GraphicsLayer* PaintLayer::graphicsLayerBacking(const LayoutObject* obj) const {
 
 BackgroundPaintLocation PaintLayer::backgroundPaintLocation(
     uint32_t* reasons) const {
-  BackgroundPaintLocation location =
-      isRootLayer() || !scrollsOverflow()
-          ? BackgroundPaintInGraphicsLayer
-          : layoutObject()->backgroundPaintLocation(reasons);
+  BackgroundPaintLocation location = BackgroundPaintInGraphicsLayer;
+  if (scrollsOverflow()) {
+    if (RuntimeEnabledFeatures::rootLayerScrollingEnabled() || !isRootLayer()) {

[flackr, 2017/01/26 00:03:16]

I wonder if this check is really necessary? The scrollsOverflow() should be
false on the root layer if we don't have root layer scrolling right?

+      location = layoutObject()->backgroundPaintLocation(reasons);
+    }
+  }
+
   m_stackingNode->updateLayerListsIfNeeded();
   if (m_stackingNode->hasNegativeZOrderList())
     location = BackgroundPaintInGraphicsLayer;

[flackr, 2017/01/26 00:03:16]

This seems wrong, do we actually create a layer behind the scrolling contents
layer for negative z index layers on the root scroller?

If we do, and we paint a scrolling / local background into a non-scrolling layer
then we'd have to repaint on scroll.