Keep expanded if mouse moves off of scrollbar while dragging

cc/input/scrollbar_animation_controller_thinning.h

 // Copyright 2013 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_THINNING_H_
 #define CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 17 matching lines @@
 
   ~ScrollbarAnimationControllerThinning() override;
 
   void set_mouse_move_distance_for_test(float distance) {
     mouse_move_distance_to_trigger_animation_ = distance;
   }
   bool mouse_is_over_scrollbar() const { return mouse_is_over_scrollbar_; }
   bool mouse_is_near_scrollbar() const { return mouse_is_near_scrollbar_; }
 
   void DidScrollUpdate(bool on_resize) override;
+
+  void DidCaptureScrollbarBegin() override;
+  void DidCaptureScrollbarEnd() override;
   void DidMouseMoveOffScrollbar() override;
   void DidMouseMoveNear(float distance) override;
 
  protected:
   ScrollbarAnimationControllerThinning(
       int scroll_layer_id,
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta delay_before_starting,
       base::TimeDelta resize_delay_before_starting,
       base::TimeDelta duration);
 
   void RunAnimationFrame(float progress) override;
 
  private:
   // Describes whether the current animation should INCREASE (darken / thicken)
   // a bar or DECREASE it (lighten / thin).
   enum AnimationChange { NONE, INCREASE, DECREASE };
   float OpacityAtAnimationProgress(float progress);
   float ThumbThicknessScaleAtAnimationProgress(float progress);
   float AdjustScale(float new_value,
                     float current_value,
                     AnimationChange animation_change);
   void ApplyOpacityAndThumbThicknessScale(float opacity,
                                           float thumb_thickness_scale);
 
+  bool captured_;
   bool mouse_is_over_scrollbar_;
   bool mouse_is_near_scrollbar_;
   // Are we narrowing or thickening the bars.
   AnimationChange thickness_change_;
   // Are we darkening or lightening the bars.
   AnimationChange opacity_change_;
   // How close should the mouse be to the scrollbar before we thicken it.
   float mouse_move_distance_to_trigger_animation_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarAnimationControllerThinning);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_