Determine the layoutobject of middleClickAutoscroll by the scroll direction.

third_party/WebKit/Source/core/page/AutoscrollController.h

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 #ifndef AutoscrollController_h
 #define AutoscrollController_h
 
 #include "core/CoreExport.h"
+#include "platform/geometry/FloatSize.h"
 #include "platform/geometry/IntPoint.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class LocalFrame;
 class FrameView;
 class Node;
 class Page;
 class PlatformMouseEvent;
@@ skipping 33 matching lines @@
                                                   const PlatformMouseEvent&);
   void startMiddleClickAutoscroll(LayoutBox*, const IntPoint&);
 
  private:
   explicit AutoscrollController(Page&);
 
   void startAutoscroll();
 
   void updateMiddleClickAutoscrollState(FrameView*,
                                         const IntPoint& lastKnownMousePosition);
+  FloatSize calculateAutoscrollDelta();
 
   Member<Page> m_page;
   LayoutBox* m_autoscrollLayoutObject;
   LayoutBox* m_pressedLayoutObject;
   AutoscrollType m_autoscrollType;
   IntPoint m_dragAndDropAutoscrollReferencePosition;
   double m_dragAndDropAutoscrollStartTime;
   IntPoint m_middleClickAutoscrollStartPos;
+  bool m_didLatchForMiddleClickAutoscroll;
 };
 
 }  // namespace blink
 
 #endif  // AutoscrollController_h