IFrame Scrollbar needs reconstruction.

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 3b1b7cc80d33f2edf14595b69711e9ab47901362..c941412ac02a8a7a89a76000ec255dec0cae99bf 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -2765,6 +2765,8 @@ void FrameView::updateStyleAndLayoutIfNeededRecursiveInternal()
 
     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);