Overlay scrollbars expand only when mouse is near thumb

cc/input/single_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_SINGLE_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_
 #define CC_INPUT_SINGLE_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 15 matching lines @@
   static constexpr float kDefaultMouseMoveDistanceToTriggerAnimation = 25.f;
 
   static std::unique_ptr<SingleScrollbarAnimationControllerThinning> Create(
       ElementId scroll_element_id,
       ScrollbarOrientation orientation,
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta thinning_duration);
 
   ~SingleScrollbarAnimationControllerThinning() {}
 
-  bool mouse_is_over_scrollbar() const { return mouse_is_over_scrollbar_; }
+  bool mouse_is_over_scrollbar_thumb() const {
+    return mouse_is_over_scrollbar_thumb_;
+  }
+  bool mouse_is_near_scrollbar_thumb() const {
+    return mouse_is_near_scrollbar_thumb_;
+  }
   bool mouse_is_near_scrollbar() const { return mouse_is_near_scrollbar_; }
   bool captured() const { return captured_; }
 
   bool Animate(base::TimeTicks now);
   void StartAnimation();
   void StopAnimation();
 
   void UpdateThumbThicknessScale();
 
   void DidMouseDown();
   void DidMouseUp();
   void DidMouseLeave();
-  void DidMouseMoveNear(float distance);
+  void DidMouseMoveNear(float distance_to_scrollbar, float distance_to_thumb);
 
  private:
   SingleScrollbarAnimationControllerThinning(
       ElementId scroll_element_id,
       ScrollbarOrientation orientation,
       ScrollbarAnimationControllerClient* client,
       base::TimeDelta thinning_duration);
 
   float AnimationProgressAtTime(base::TimeTicks now);
   void RunAnimationFrame(float progress);
@@ skipping 13 matching lines @@
 
   ScrollbarAnimationControllerClient* client_;
 
   base::TimeTicks last_awaken_time_;
   bool is_animating_;
 
   ElementId scroll_element_id_;
 
   ScrollbarOrientation orientation_;
   bool captured_;
-  bool mouse_is_over_scrollbar_;
+  bool mouse_is_over_scrollbar_thumb_;
+  bool mouse_is_near_scrollbar_thumb_;
   bool mouse_is_near_scrollbar_;
   // Are we narrowing or thickening the bars.
   AnimationChange thickness_change_;
 
   base::TimeDelta thinning_duration_;
 
   DISALLOW_COPY_AND_ASSIGN(SingleScrollbarAnimationControllerThinning);
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SINGLE_SCROLLBAR_ANIMATION_CONTROLLER_THINNING_H_