Draw main frame custom scrollbars in root layer scrolling mode.

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 2621 matching lines @@
     if (GraphicsLayer* layerForHorizontalScrollbar = view.compositor()->layerForHorizontalScrollbar()) {
         synchronizedPaintRecursively(layerForHorizontalScrollbar);
     }
     if (GraphicsLayer* layerForVerticalScrollbar = view.compositor()->layerForVerticalScrollbar()) {
         synchronizedPaintRecursively(layerForVerticalScrollbar);
     }
     if (GraphicsLayer* layerForScrollCorner = view.compositor()->layerForScrollCorner()) {
         synchronizedPaintRecursively(layerForScrollCorner);
     }
 
+    if (!layoutViewItem().isNull() && layoutViewItem().layer()->hasCompositedLayerMapping()) {

[bokan, 2016/08/11 14:34:29]

IMO, instead of doing this, we should fix FrameView::layerForHozizontalScrollbar
and friends to return the appropriate layer and use that above.

+skobes@: Is it true that if LayoutView has a CompositedLayerMapping then we in
root-layer-scrolls? If so, then layerForHorizontalScrollbar can check that and
return the CLM scrollbar layer, otherwise the PLC scrollbar layer. Otherwise, I
guess you could just key off settings()->rootLayerScrolls(). Does that sound ok?

[MuVen, 2016/08/11 14:45:57]

That's a great idea to fix in layerForHorizontalScrollbar. I shall add tests,
and modified patch, if skobes is ok with this.

+        CompositedLayerMapping* compositedLayerMapping = layoutViewItem().layer()->compositedLayerMapping();
+        if (GraphicsLayer* layerForHorizontalScrollbar = compositedLayerMapping->layerForHorizontalScrollbar()) {
+            synchronizedPaintRecursively(layerForHorizontalScrollbar);
+        }
+        if (GraphicsLayer* layerForVerticalScrollbar = compositedLayerMapping->layerForVerticalScrollbar()) {
+            synchronizedPaintRecursively(layerForVerticalScrollbar);
+        }
+        if (GraphicsLayer* layerForScrollCorner = compositedLayerMapping->layerForScrollCorner()) {
+            synchronizedPaintRecursively(layerForScrollCorner);
+        }
+    }
+
     forAllNonThrottledFrameViews([](FrameView& frameView) {
         frameView.lifecycle().advanceTo(DocumentLifecycle::PaintClean);
         LayoutViewItem layoutViewItem = frameView.layoutViewItem();
         if (!layoutViewItem.isNull())
             layoutViewItem.layer()->clearNeedsRepaintRecursively();
     });
 }
 
 void FrameView::synchronizedPaintRecursively(GraphicsLayer* graphicsLayer)
 {
@@ skipping 1632 matching lines @@
 }
 
 bool FrameView::canThrottleRendering() const
 {
     if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
         return false;
     return m_subtreeThrottled || (m_hiddenForThrottling && m_crossOriginForThrottling);
 }
 
 } // namespace blink