Viewport resize anchoring should use the current layout viewport.

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 801 matching lines @@
     document->updateStyleAndLayoutTree();
     lifecycle().advanceTo(DocumentLifecycle::StyleClean);
 
     if (m_frame->isMainFrame() && !m_viewportScrollableArea) {
         ScrollableArea& visualViewport = m_frame->host()->visualViewport();
         ScrollableArea* layoutViewport = layoutViewportScrollableArea();
         DCHECK(layoutViewport);
         m_viewportScrollableArea = RootFrameViewport::create(visualViewport, *layoutViewport);
     }
 
-    if (!m_scrollAnchor.hasScroller())
-        m_scrollAnchor.setScroller(m_viewportScrollableArea ? m_viewportScrollableArea : this);
+    if (!m_scrollAnchor.hasScroller()) {
+        ScrollableArea* scroller = m_viewportScrollableArea;
+        if (!scroller)
+            scroller = this;
+
+        m_scrollAnchor.setScroller(scroller);
+    }
 
     if (shouldPerformScrollAnchoring())
         m_scrollAnchor.save();
 }
 
 bool FrameView::shouldPerformScrollAnchoring() const
 {
     return RuntimeEnabledFeatures::scrollAnchoringEnabled()
         && m_frame->settings() && !m_frame->settings()->rootLayerScrolls()
         && m_scrollAnchor.hasScroller()
@@ skipping 3290 matching lines @@
 ScrollableArea* FrameView::layoutViewportScrollableArea()
 {
     Settings* settings = frame().settings();
     if (!settings || !settings->rootLayerScrolls())
         return this;
 
     LayoutViewItem layoutViewItem = this->layoutViewItem();
     return layoutViewItem.isNull() ? nullptr : layoutViewItem.getScrollableArea();
 }
 
+RootFrameViewport* FrameView::getRootFrameViewport()
+{
+    return m_viewportScrollableArea.get();
+}
+
 LayoutObject* FrameView::viewportLayoutObject() const
 {
     if (Document* document = frame().document()) {
         if (Element* element = document->viewportDefiningElement())
             return element->layoutObject();
     }
     return nullptr;
 }
 
 void FrameView::collectAnnotatedRegions(LayoutObject& layoutObject, Vector<AnnotatedRegionValue>& regions) const
@@ skipping 164 matching lines @@
 }
 
 bool FrameView::canThrottleRendering() const
 {
     if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
         return false;
     return m_subtreeThrottled || (m_hiddenForThrottling && m_crossOriginForThrottling);
 }
 
 } // namespace blink