Fix scroll anchoring during viewport resize.

third_party/WebKit/Source/web/ResizeViewportAnchor.h

Index: third_party/WebKit/Source/web/ResizeViewportAnchor.h
diff --git a/third_party/WebKit/Source/web/ResizeViewportAnchor.h b/third_party/WebKit/Source/web/ResizeViewportAnchor.h
index 49ec20eaeb1e5acf0ee31f0f57632b8a83eefb26..e3138cc1cd38163e9e6fd08162910e6c851b8b3e 100644
--- a/third_party/WebKit/Source/web/ResizeViewportAnchor.h
+++ b/third_party/WebKit/Source/web/ResizeViewportAnchor.h
@@ -5,28 +5,62 @@
 #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();
+
+    // The amount of resize-induced clamping drift accumulated during the
+    // ResizeScope.  Note that this should NOT include other kinds of scrolling
+    // that may occur during layout, such as from ScrollAnchor.
+    DoubleSize m_drift;
+    Member<Page> m_page;
+    int m_scopeCount;
 };
 
 } // namespace blink