IFrame Scrollbar needs reconstruction.

third_party/WebKit/Source/core/layout/LayoutPart.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2000 Simon Hausmann <hausmann@kde.org>
  *           (C) 2000 Stefan Schimanski (1Stein@gmx.de)
  * Copyright (C) 2004, 2005, 2006, 2009 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 291 matching lines @@
         return;
 
     LayoutRect newFrame = replacedContentRect();
     DCHECK(newFrame.size() == roundedIntSize(newFrame.size()));
     bool boundsWillChange = LayoutSize(widget->frameRect().size()) != newFrame.size();
 
     FrameView* frameView = widget->isFrameView() ? toFrameView(widget) : nullptr;
 
     // If frame bounds are changing mark the view for layout. Also check the
     // frame's page to make sure that the frame isn't in the process of being
-    // destroyed.
-    if (frameView && boundsWillChange && frameView->frame().page())
+    // destroyed. If iframe scrollbars needs reconstruction from native to custom
+    // scrollbar, then also we need to layout the frameview.
+    if (frameView && frameView->frame().page() && (boundsWillChange || frameView->needsScrollbarReconstruction()))
         frameView->setNeedsLayout();
 
     updateWidgetGeometryInternal();
 
     // If view needs layout, either because bounds have changed or possibly
     // indicating content size is wrong, we have to do a layout to set the right
     // widget size.
     if (frameView && frameView->needsLayout() && frameView->frame().page())
         frameView->layout();
 
@@ skipping 46 matching lines @@
 
 bool LayoutPart::isThrottledFrameView() const
 {
     if (!widget() || !widget()->isFrameView())
         return false;
     const FrameView* frameView = toFrameView(widget());
     return frameView->shouldThrottleRendering();
 }
 
 } // namespace blink