Keep expanded if mouse moves off of scrollbar while dragging

cc/input/scrollbar_animation_controller.h

 // Copyright 2012 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_SCROLLBAR_ANIMATION_CONTROLLER_H_
 #define CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_H_
 
 #include "base/cancelable_callback.h"
 #include "base/memory/weak_ptr.h"
 #include "base/time/time.h"
@@ skipping 23 matching lines @@
 // specialized implementation.
 class CC_EXPORT ScrollbarAnimationController {
  public:
   virtual ~ScrollbarAnimationController();
 
   bool Animate(base::TimeTicks now);
 
   virtual void DidScrollBegin();
   virtual void DidScrollUpdate(bool on_resize);
   virtual void DidScrollEnd();
+  virtual void DidCaptureScrollbarBegin() {}
+  virtual void DidCaptureScrollbarEnd() {}
   virtual void DidMouseMoveOffScrollbar() {}
   virtual void DidMouseMoveNear(float distance) {}
 
  protected:
   ScrollbarAnimationController(int scroll_layer_id,
                                ScrollbarAnimationControllerClient* client,
                                base::TimeDelta delay_before_starting,
                                base::TimeDelta resize_delay_before_starting,
                                base::TimeDelta duration);
 
@@ skipping 23 matching lines @@
   bool currently_scrolling_;
   bool scroll_gesture_has_scrolled_;
   base::CancelableClosure delayed_scrollbar_fade_;
 
   base::WeakPtrFactory<ScrollbarAnimationController> weak_factory_;
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_H_