Fix scroll anchoring during viewport resize.

third_party/WebKit/Source/web/ResizeViewportAnchor.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ResizeViewportAnchor_h
 #define ResizeViewportAnchor_h
 
-#include "platform/geometry/DoublePoint.h"
+#include "core/page/Page.h"
 #include "platform/heap/Handle.h"
-#include "web/ViewportAnchor.h"
 
 namespace blink {
 
 class FrameView;
-class VisualViewport;
 
-// The resize anchor saves the current scroll offset of the visual viewport and
-// restores to that scroll offset so that document location appears exactly
-// unchanged to the user.
-class ResizeViewportAnchor : public ViewportAnchor {
-    STACK_ALLOCATED();
+// This class scrolls the viewports to compensate for bounds clamping caused by
+// viewport size changes.
+//
+// It is needed when the layout viewport grows (causing its own scroll position
+// to be clamped) and also when it shrinks (causing the visual viewport's scroll
+// position to be clamped).
+class ResizeViewportAnchor : public GarbageCollectedFinalized<ResizeViewportAnchor> {
+    WTF_MAKE_NONCOPYABLE(ResizeViewportAnchor);
 public:
-    ResizeViewportAnchor(FrameView& rootFrameView, VisualViewport&);
-    ~ResizeViewportAnchor();
+    ResizeViewportAnchor(Page& page)
+        : m_page(page)
+        , m_scopeCount(0)
+    {
+    }
+
+    class ResizeScope {
+        STACK_ALLOCATED();
+    public:
+        explicit ResizeScope(ResizeViewportAnchor& anchor)
+            : m_anchor(anchor)
+        {
+            m_anchor->beginScope();
+        }
+        ~ResizeScope()
+        {
+            m_anchor->endScope();
+        }
+    private:
+        Member<ResizeViewportAnchor> m_anchor;
+    };
+
+    void resizeFrameView(IntSize);
+
+    DEFINE_INLINE_TRACE()
+    {
+        visitor->trace(m_page);
+    }
 
 private:
-    // Inner viewport origin in the reference frame of the root document, in CSS
-    // pixels.
-    DoublePoint m_visualViewportInDocument;
+    void beginScope() { m_scopeCount++; }
+    void endScope();
+    FrameView* rootFrameView();
+
+    DoubleSize m_drift;

[bokan, 2016/07/28 14:55:55]

Could you add a comment describing this member? I'm assuming it's the amount
scroll was adjusted by ScrollAnchoring/layout?

[skobes, 2016/07/28 15:45:50]

The opposite actually :) Comment added.

+    Member<Page> m_page;
+    int m_scopeCount;
 };
 
 } // namespace blink
 
 #endif // ResizeViewportAnchor_h