Viewport resize anchoring should use the current layout viewport.

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
    modify it under the terms of the GNU Library General Public
@@ skipping 556 matching lines @@
     // be the RootFrameViewport, which adds pinch-zoom semantics to scrolling.
     // For non-root frames, this will be the the ScrollableArea used by the
     // FrameView, depending on whether root-layer-scrolls is enabled.
     ScrollableArea* getScrollableArea();
 
     // Used to get at the underlying layoutViewport in the rare instances where
     // we actually want to scroll *just* the layout viewport (e.g. when sending
     // deltas from CC). For typical scrolling cases, use getScrollableArea().
     ScrollableArea* layoutViewportScrollableArea();
 
+    // If this is the main frame, this will return the RootFrameViewport used
+    // to scroll the main frame. Otherwise returns nullptr. Unless you need a
+    // unique method on RootFrameViewport, you should probably use
+    // getScrollableArea.
+    RootFrameViewport* getRootFrameViewport();
+
     int viewportWidth() const;
 
     LayoutAnalyzer* layoutAnalyzer() { return m_analyzer.get(); }
 
     // Returns true if this frame should not render or schedule visual updates.
     bool shouldThrottleRendering() const;
 
     // Returns true if this frame could potentially skip rendering and avoid
     // scheduling visual updates.
     bool canThrottleRendering() const;
@@ skipping 299 matching lines @@
 
     std::unique_ptr<LayoutAnalyzer> m_analyzer;
 
     // Mark if something has changed in the mapping from Frame to GraphicsLayer
     // and the Frame Timing regions should be recalculated.
     bool m_frameTimingRequestsDirty;
 
     // Exists only on root frame.
     // TODO(bokan): crbug.com/484188. We should specialize FrameView for the
     // main frame.
-    Member<ScrollableArea> m_viewportScrollableArea;
+    Member<RootFrameViewport> m_viewportScrollableArea;
 
     // This frame's bounds in the root frame's content coordinates, clipped
     // recursively through every ancestor view.
     IntRect m_viewportIntersection;
     bool m_viewportIntersectionValid;
 
     // The following members control rendering pipeline throttling for this
     // frame. They are only updated in response to intersection observer
     // notifications, i.e., not in the middle of the lifecycle.
     bool m_hiddenForThrottling;
@@ skipping 56 matching lines @@
     if (m_visuallyNonEmptyPixelCount > visualPixelThreshold)
         setIsVisuallyNonEmpty();
 }
 
 DEFINE_TYPE_CASTS(FrameView, Widget, widget, widget->isFrameView(), widget.isFrameView());
 DEFINE_TYPE_CASTS(FrameView, ScrollableArea, scrollableArea, scrollableArea->isFrameView(), scrollableArea.isFrameView());
 
 } // namespace blink
 
 #endif // FrameView_h