Keep expanded if mouse moves off of scrollbar while dragging

cc/input/input_handler.h

 // Copyright 2011 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 CC_INPUT_INPUT_HANDLER_H_
 #define CC_INPUT_INPUT_HANDLER_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 135 matching lines @@
   // accumulated within this ScrollBegin() scope is reported in the return
   // value's |accumulated_overscroll| field. Should only be called if
   // ScrollBegin() returned SCROLL_STARTED.
   virtual InputHandlerScrollResult ScrollBy(ScrollState* scroll_state) = 0;
 
   // Returns SCROLL_STARTED if a layer was actively being scrolled,
   // SCROLL_IGNORED if not.
   virtual ScrollStatus FlingScrollBegin() = 0;
 
   virtual void MouseMoveAt(const gfx::Point& mouse_position) = 0;
+  virtual void MouseDownAt(const gfx::Point& mouse_position) = 0;

[weiliangc, 2016/09/29 21:37:33]

I don't see the mouse_position being used. Is it possible to rename to MouseDown
and not have mouse_position passed in?

+  virtual void MouseUp() = 0;
 
   // Stop scrolling the selected layer. Should only be called if ScrollBegin()
   // returned SCROLL_STARTED.
   virtual void ScrollEnd(ScrollState* scroll_state) = 0;
 
   // Requests a callback to UpdateRootLayerStateForSynchronousInputHandler()
   // giving the current root scroll and page scale information.
   virtual void RequestUpdateForSynchronousInputHandler() = 0;
 
   // Called when the root scroll offset has been changed in the synchronous
@@ skipping 43 matching lines @@
   InputHandler() {}
   virtual ~InputHandler() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(InputHandler);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_INPUT_HANDLER_H_