IFrame Scrollbar needs reconstruction.

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 2747 matching lines @@
     // layout and make sure they are up to date.
     // Mac actually tests for intersection with the dirty region and tries not to
     // update layout for frames that are outside the dirty region.  Not only does this seem
     // pointless (since those frames will have set a zero timer to layout anyway), but
     // it is also incorrect, since if two frames overlap, the first could be excluded from the dirty
     // region but then become included later by the second frame adding rects to the dirty region
     // when it lays out.
 
     m_frame->document()->updateStyleAndLayoutTree();
 
+    updateScrollbarsIfNeeded();

[skobes, 2016/09/06 17:39:19]

FrameView::layout calls updateScrollbarsIfNeeded.  Why do we need to call it
here as well?

[MuVen, 2016/09/07 14:50:17]

as this is a recursively calls for all the frameviews present in the mainframe;
so for each frameview we need to call updatescrollbarsifneeded which checks for
needsreconstruction, based on that it will update the scrollbars.

[skobes, 2016/09/07 19:04:38]

But layout is also recursive.  Scrollbars changing between native and custom
should invalidate layout.

+
     CHECK(!shouldThrottleRendering());
     CHECK(m_frame->document()->isActive());
     CHECK(!m_nestedLayoutCount);
 
     if (needsLayout())
         layout();
 
     checkDoesNotNeedLayout();
 
     // WebView plugins need to update regardless of whether the LayoutEmbeddedObject
@@ skipping 1544 matching lines @@
 }
 
 bool FrameView::canThrottleRendering() const
 {
     if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
         return false;
     return m_subtreeThrottled || (m_hiddenForThrottling && m_crossOriginForThrottling);
 }
 
 } // namespace blink