Draw main frame custom scrollbars in root layer scrolling mode.

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

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 5c308d51c0a1411b380ee76631b51c9c74d3781e..838870f022b6cef919212e9c6d2293d47eaaf744 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -2639,6 +2639,19 @@ void FrameView::synchronizedPaint()
         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();