Initial viewport is not the same as FrameView rect.

third_party/WebKit/Source/web/WebViewImpl.cpp

 /*
  * Copyright (C) 2011, 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 1888 matching lines @@
 
   pageScaleConstraintsSet().didChangeInitialContainingBlockSize(ICBSize);
 
   updatePageDefinedViewportConstraints(
       mainFrameImpl()->frame()->document()->viewportDescription());
   updateMainFrameLayoutSize();
 
   page()->frameHost().visualViewport().setSize(m_size);
 
   if (mainFrameImpl()->frameView()) {
+    mainFrameImpl()->frameView()->setInitialViewportSize(ICBSize);
     if (!mainFrameImpl()->frameView()->needsLayout())
       postLayoutResize(mainFrameImpl());
   }
 }
 
 void WebViewImpl::updateTopControlsState(WebTopControlsState constraint,
                                          WebTopControlsState current,
                                          bool animate) {
   topControls().updateConstraintsAndState(constraint, current, animate);
 
@@ skipping 2083 matching lines @@
   if (!m_client || !frame->isLocalRoot())
     return;
 
   if (m_shouldAutoResize) {
     WebSize frameSize = frame->view()->frameRect().size();
     if (frameSize != m_size) {
       m_size = frameSize;
 
       page()->frameHost().visualViewport().setSize(m_size);
       pageScaleConstraintsSet().didChangeInitialContainingBlockSize(m_size);
+      frame->view()->setInitialViewportSize(m_size);
 
       m_client->didAutoResize(m_size);
       sendResizeEventAndRepaint();
     }
   }
 
   if (pageScaleConstraintsSet().constraintsDirty())
     refreshPageScaleFactorAfterLayout();
 
   FrameView* view = webframe->frame()->view();
@@ skipping 526 matching lines @@
   if (focusedFrame->localFrameRoot() != mainFrameImpl()->frame())
     return nullptr;
   return focusedFrame;
 }
 
 LocalFrame* WebViewImpl::focusedLocalFrameAvailableForIme() const {
   return m_imeAcceptEvents ? focusedLocalFrameInWidget() : nullptr;
 }
 
 }  // namespace blink