Initial viewport is not the same as FrameView rect.

third_party/WebKit/Source/core/frame/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights
    reserved.
 
    This library is free software; you can redistribute it and/or
@@ skipping 147 matching lines @@
   IntSize layoutSize(IncludeScrollbarsInRect = ExcludeScrollbars) const;
   void setLayoutSize(const IntSize&);
 
   // If this is set to false, the layout size will need to be explicitly set by
   // the owner.  E.g. WebViewImpl sets its mainFrame's layout size manually
   void setLayoutSizeFixedToFrameSize(bool isFixed) {
     m_layoutSizeFixedToFrameSize = isFixed;
   }
   bool layoutSizeFixedToFrameSize() { return m_layoutSizeFixedToFrameSize; }
 
+  void setInitialViewportSize(const IntSize&);
+  int initialViewportWidth() const;
+  int initialViewportHeight() const;
+
   void updateAcceleratedCompositingSettings();
 
   void recalcOverflowAfterStyleChange();
 
   bool isEnclosedInCompositingLayer() const;
 
   void dispose() override;
   void detachScrollbars();
   void recalculateCustomScrollbarStyle();
   void invalidateAllCustomScrollbarsOnActiveChanged();
@@ skipping 792 matching lines @@
   std::unique_ptr<ResizerAreaSet> m_resizerAreas;
   std::unique_ptr<ViewportConstrainedObjectSet> m_viewportConstrainedObjects;
   unsigned m_stickyPositionObjectCount;
   ViewportConstrainedObjectSet m_backgroundAttachmentFixedObjects;
   Member<FrameViewAutoSizeInfo> m_autoSizeInfo;
 
   IntSize m_inputEventsOffsetForEmulation;
   float m_inputEventsScaleFactorForEmulation;
 
   IntSize m_layoutSize;
+  IntSize m_initialViewportSize;
   bool m_layoutSizeFixedToFrameSize;
 
   Timer<FrameView> m_didScrollTimer;
 
   Vector<IntRect> m_tickmarks;
 
   float m_topControlsViewportAdjustment;
 
   bool m_needsUpdateWidgetGeometries;
   bool m_needsUpdateViewportIntersection;
@@ skipping 117 matching lines @@
                   widget.isFrameView());
 DEFINE_TYPE_CASTS(FrameView,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->isFrameView(),
                   scrollableArea.isFrameView());
 
 }  // namespace blink
 
 #endif  // FrameView_h