Use full mapLocalToAncestor machinery in ScrollingCoordinator.

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 2573 matching lines @@
   LayoutViewItem viewItem = layoutViewItem();
   if (!viewItem.isNull())
     viewItem.clearHitTestCache();
 }
 
 IntRect FrameView::scrollableAreaBoundingBox() const {
   LayoutPartItem ownerLayoutItem = frame().ownerLayoutItem();
   if (ownerLayoutItem.isNull())
     return frameRect();
 
-  return ownerLayoutItem.absoluteContentQuad().enclosingBoundingBox();
+  return ownerLayoutItem.absoluteContentQuad(TraverseDocumentBoundaries)

[chrishtr, 2017/01/27 20:33:30]

It seems you need to update the definition in ScrollableArea, and also update
the other
overrides such as:

https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/frame/Vis...

[szager1, 2017/01/30 18:25:41]

Done.

+      .enclosingBoundingBox();
 }
 
 bool FrameView::isScrollable() {
   return getScrollingReasons() == Scrollable;
 }
 
 bool FrameView::isProgrammaticallyScrollable() {
   return !m_inUpdateScrollbars;
 }
 
@@ skipping 2456 matching lines @@
     std::unique_ptr<CompositorAnimationTimeline> timeline) {
   m_animationTimeline = std::move(timeline);
 }
 
 void FrameView::setAnimationHost(
     std::unique_ptr<CompositorAnimationHost> host) {
   m_animationHost = std::move(host);
 }
 
 }  // namespace blink