Improve scroll anchoring's interactions with the visual viewport

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

Index: third_party/WebKit/Source/core/frame/RootFrameViewport.h
diff --git a/third_party/WebKit/Source/core/frame/RootFrameViewport.h b/third_party/WebKit/Source/core/frame/RootFrameViewport.h
index 1342700f07b33f4c227f23ce62015ebce79d5059..75cfe52370771438b1e9b3329edaf536239b958f 100644
--- a/third_party/WebKit/Source/core/frame/RootFrameViewport.h
+++ b/third_party/WebKit/Source/core/frame/RootFrameViewport.h
@@ -34,6 +34,7 @@ public:
     void setLayoutViewport(ScrollableArea&);
 
     // ScrollableArea Implementation
+    bool isRootFrameViewport() const override { return true; }
     void setScrollPosition(const DoublePoint&, ScrollType, ScrollBehavior = ScrollBehaviorInstant) override;
     LayoutRect scrollIntoView(
         const LayoutRect& rectInContent,
@@ -73,13 +74,19 @@ public:
     ScrollBehavior scrollBehaviorStyle() const override;
     Widget* getWidget() override;
     void clearScrollAnimators() override;
+    LayoutBox* layoutBox() const override;
 
 private:
     RootFrameViewport(ScrollableArea& visualViewport, ScrollableArea& layoutViewport);
 
+    enum ViewportToScrollFirst {
+        VisualViewport,
+        LayoutViewport
+    };
+
     DoublePoint scrollOffsetFromScrollAnimators() const;
 
-    void distributeScrollBetweenViewports(const DoublePoint&, ScrollType, ScrollBehavior);
+    void distributeScrollBetweenViewports(const DoublePoint&, ScrollType, ScrollBehavior, ViewportToScrollFirst);
 
     // If either of the layout or visual viewports are scrolled explicitly (i.e. not
     // through this class), their updated offset will not be reflected in this class'
@@ -93,6 +100,8 @@ private:
     Member<ScrollableArea> m_layoutViewport;
 };
 
+DEFINE_TYPE_CASTS(RootFrameViewport, ScrollableArea, scrollableArea, scrollableArea->isRootFrameViewport(), scrollableArea.isRootFrameViewport());
+
 } // namespace blink
 
 #endif // RootFrameViewport_h