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 2661 matching lines @@
     LayoutViewItem view = layoutViewItem();
     ASSERT(!view.isNull());
     forAllNonThrottledFrameViews([](FrameView& frameView) { frameView.lifecycle().advanceTo(DocumentLifecycle::InPaint); });
 
     // A null graphics layer can occur for painting of SVG images that are not parented into the main frame tree,
     // or when the FrameView is the main frame view of a page overlay. The page overlay is in the layer tree of
     // the host page and will be painted during synchronized painting of the host page.
     if (GraphicsLayer* rootGraphicsLayer = view.compositor()->rootGraphicsLayer()) {
         synchronizedPaintRecursively(rootGraphicsLayer);
     }
-    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 (m_viewportScrollableArea) {

[bokan, 2016/09/02 16:01:54]

This can be a DCHECK since we're the main frame.

[MuVen, 2016/09/06 10:07:29]

Need if(m_viewportScrollableArea) check as without which webkit_unit_tests will
crash ex: FrameThrottlingTest.cpp.

[bokan, 2016/09/07 15:00:44]

Interesting, those tests need to be fixed. Please file a bug (cc me) and list
all the tests that break. The problem is that "Main frame doesn't create
RootFrameViewport in some webkit_unit_tests". Please add a TODO here linked to
that bug.

+        if (GraphicsLayer* layerForHorizontalScrollbar = m_viewportScrollableArea->layerForHorizontalScrollbar()) {
+            synchronizedPaintRecursively(layerForHorizontalScrollbar);
+        }
+        if (GraphicsLayer* layerForVerticalScrollbar = m_viewportScrollableArea->layerForVerticalScrollbar()) {
+            synchronizedPaintRecursively(layerForVerticalScrollbar);
+        }
+        if (GraphicsLayer* layerForScrollCorner = m_viewportScrollableArea->layerForScrollCorner()) {
+            synchronizedPaintRecursively(layerForScrollCorner);
+        }
     }
 
     forAllNonThrottledFrameViews([](FrameView& frameView) {
         frameView.lifecycle().advanceTo(DocumentLifecycle::PaintClean);
         LayoutViewItem layoutViewItem = frameView.layoutViewItem();
         if (!layoutViewItem.isNull())
             layoutViewItem.layer()->clearNeedsRepaintRecursively();
     });
 }
 
@@ skipping 1622 matching lines @@
 }
 
 bool FrameView::canThrottleRendering() const
 {
     if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
         return false;
     return m_subtreeThrottled || (m_hiddenForThrottling && m_crossOriginForThrottling);
 }
 
 } // namespace blink