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

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

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. 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 2606 matching lines @@
     default:
       return (obj != layoutObject() &&
               compositedLayerMapping()->scrollingContentsLayer())
                  ? compositedLayerMapping()->scrollingContentsLayer()
                  : compositedLayerMapping()->mainGraphicsLayer();
   }
 }
 
 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.

   return location;
 }
 
 void PaintLayer::ensureCompositedLayerMapping() {
   if (m_rareData && m_rareData->compositedLayerMapping)
     return;
 
   ensureRareData().compositedLayerMapping =
       WTF::wrapUnique(new CompositedLayerMapping(*this));
   m_rareData->compositedLayerMapping->setNeedsGraphicsLayerUpdate(
@@ skipping 591 matching lines @@
 }
 
 void showLayerTree(const blink::LayoutObject* layoutObject) {
   if (!layoutObject) {
     LOG(INFO) << "Cannot showLayerTree. Root is (nil)";
     return;
   }
   showLayerTree(layoutObject->enclosingLayer());
 }
 #endif