Prevent overlay scrollbars expand or hover together

cc/input/scrollbar_animation_controller_thinning.h

Index: cc/input/scrollbar_animation_controller_thinning.h
diff --git a/cc/input/scrollbar_animation_controller_thinning.h b/cc/input/scrollbar_animation_controller_thinning.h
index 236e595ea263ec5d9f090d913005b34bfbbcd967..1c53a977a25500ec8962a729151052275e43bc01 100644
--- a/cc/input/scrollbar_animation_controller_thinning.h
+++ b/cc/input/scrollbar_animation_controller_thinning.h
@@ -10,11 +10,17 @@
 #include "base/macros.h"
 #include "cc/base/cc_export.h"
 #include "cc/input/scrollbar_animation_controller.h"
+#include "cc/input/single_scrollbar_animation_controller_thinning.h"
 
 namespace cc {
 
-// Scrollbar animation that partially fades and thins after an idle delay,
-// and reacts to mouse movements.
+// This class fade in scrollbars when scroll and fade out after an idle delay.
+// The fade animations works on both scrollbars and is controlled in this class.
+
+// TODO(chaopeng) clean up the inheritance hierarchy after
+// ScrollbarAnimationControllerLinearFade merge into
+// ScrollbarAnimationControllerThinning so we can remove the empty overrides and
+// NOTREACHED() checks.
 class CC_EXPORT ScrollbarAnimationControllerThinning
     : public ScrollbarAnimationController {
  public:
@@ -28,22 +34,14 @@ class CC_EXPORT ScrollbarAnimationControllerThinning
 
   ~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 DidScrollEnd() override;
-
-  void DidMouseDown() override;
-  void DidMouseUp() override;
-  void DidMouseLeave() override;
-  void DidMouseMoveNear(float distance) override;
   bool ScrollbarsHidden() const override;
 
  protected:
+  bool NeedThinningAnimation() const override;
+
+ private:
   ScrollbarAnimationControllerThinning(
       int scroll_layer_id,
       ScrollbarAnimationControllerClient* client,
@@ -52,38 +50,12 @@ class CC_EXPORT ScrollbarAnimationControllerThinning
       base::TimeDelta fade_duration,
       base::TimeDelta thinning_duration);
 
+  void ApplyOpacityToScrollbars(float opacity) override;
   void RunAnimationFrame(float progress) override;
   const base::TimeDelta& Duration() override;
 
- private:
-  // Describes whether the current animation should INCREASE (thicken)
-  // a bar or DECREASE it (thin).
-  enum AnimationChange { NONE, INCREASE, DECREASE };
-  enum AnimatingProperty { OPACITY, THICKNESS };
-  float ThumbThicknessScaleAt(float progress);
-  float AdjustScale(float new_value,
-                    float current_value,
-                    AnimationChange animation_change,
-                    float min_value,
-                    float max_value);
-  void ApplyOpacity(float opacity);
-  void ApplyThumbThicknessScale(float thumb_thickness_scale);
-
-  void SetCurrentAnimatingProperty(AnimatingProperty property);
-
   float opacity_;
-  bool captured_;
-  bool mouse_is_over_scrollbar_;
-  bool mouse_is_near_scrollbar_;
-  // Are we narrowing or thickening the bars.
-  AnimationChange thickness_change_;
-  // How close should the mouse be to the scrollbar before we thicken it.
-  float mouse_move_distance_to_trigger_animation_;
-
   base::TimeDelta fade_duration_;
-  base::TimeDelta thinning_duration_;
-
-  AnimatingProperty current_animating_property_;
 
   DISALLOW_COPY_AND_ASSIGN(ScrollbarAnimationControllerThinning);
 };