Re-target wheel events only when a new scroll sequence has started.

third_party/WebKit/Source/core/input/MouseWheelEventManager.h

 // Copyright 2017 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 MouseWheelEventManager_h
 #define MouseWheelEventManager_h
 
 #include "platform/heap/Handle.h"
 #include "platform/heap/Visitor.h"
 #include "public/platform/WebInputEventResult.h"
 
 namespace blink {
 
+class Document;
 class LocalFrame;
+class LocalFrameView;
+class Node;
 class ScrollManager;
 class WebMouseWheelEvent;
 
 class MouseWheelEventManager final
     : public GarbageCollectedFinalized<MouseWheelEventManager> {
   WTF_MAKE_NONCOPYABLE(MouseWheelEventManager);
 
  public:
   explicit MouseWheelEventManager(LocalFrame&, ScrollManager&);
   DECLARE_TRACE();
 
+  void Clear();
+
   WebInputEventResult HandleWheelEvent(const WebMouseWheelEvent&);
 
  private:
+  Node* FindTargetNode(const WebMouseWheelEvent&,
+                       const Document*,
+                       const LocalFrameView*);
+
   const Member<LocalFrame> frame_;
+  Member<Node> wheel_target_;
   Member<ScrollManager> scroll_manager_;
 };
 
 }  // namespace blink
 
 #endif  // MouseWheelEventManager_h