Aura Overlay Scrollbars appear when mouse hovers over scroller edge

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 25 matching lines @@
 // independent between vertical/horizontal and are managed by the
 // SingleScrollbarAnimationControllerThinnings.
 class CC_EXPORT ScrollbarAnimationController {
  public:
   // ScrollbarAnimationController for Android. It only has fade in/out
   // animation.
   static std::unique_ptr<ScrollbarAnimationController>
   CreateScrollbarAnimationControllerAndroid(
       int scroll_layer_id,
       ScrollbarAnimationControllerClient* client,
-      base::TimeDelta delay_before_starting,
-      base::TimeDelta resize_delay_before_starting,
-      base::TimeDelta fade_duration);
+      base::TimeDelta fade_out_delay,
+      base::TimeDelta fade_out_resize_delay,
+      base::TimeDelta fade_out_duration);
 
   // ScrollbarAnimationController for Desktop Overlay Scrollbar. It has fade in/
   // out animation and thinning animation.
   static std::unique_ptr<ScrollbarAnimationController>
   CreateScrollbarAnimationControllerAuraOverlay(
       int scroll_layer_id,
       ScrollbarAnimationControllerClient* client,
-      base::TimeDelta delay_before_starting,
-      base::TimeDelta resize_delay_before_starting,
-      base::TimeDelta fade_duration,
+      base::TimeDelta fade_in_delay,
+      base::TimeDelta fade_out_delay,
+      base::TimeDelta fade_out_resize_delay,
+      base::TimeDelta fade_out_duration,
       base::TimeDelta thinning_duration);
 
   ~ScrollbarAnimationController();
 
   bool ScrollbarsHidden() const;
 
   bool Animate(base::TimeTicks now);
 
   void DidScrollBegin();
   void DidScrollUpdate(bool on_resize);
   void DidScrollEnd();
 
   void DidMouseDown();
   void DidMouseUp();
   void DidMouseLeave();
   void DidMouseMoveNear(ScrollbarOrientation, float);
 
   bool mouse_is_over_scrollbar(ScrollbarOrientation orientation) const;
   bool mouse_is_near_scrollbar(ScrollbarOrientation orientation) const;
   bool mouse_is_near_any_scrollbar() const;
 
+  static constexpr float kMouseMoveDistanceToTriggerFadeInAnimation = 50.0f;

[bokan, 2017/02/28 17:12:31]

Sebastian actually corrected this to 30 so please change.

+
  private:
   ScrollbarAnimationController(int scroll_layer_id,
                                ScrollbarAnimationControllerClient* client,
-                               base::TimeDelta delay_before_starting,
-                               base::TimeDelta resize_delay_before_starting,
-                               base::TimeDelta fade_duration);
+                               base::TimeDelta fade_out_delay,
+                               base::TimeDelta fade_out_resize_delay,
+                               base::TimeDelta fade_out_duration);
 
   ScrollbarAnimationController(int scroll_layer_id,
                                ScrollbarAnimationControllerClient* client,
-                               base::TimeDelta delay_before_starting,
-                               base::TimeDelta resize_delay_before_starting,
-                               base::TimeDelta fade_duration,
+                               base::TimeDelta fade_in_delay,
+                               base::TimeDelta fade_out_delay,
+                               base::TimeDelta fade_out_resize_delay,
+                               base::TimeDelta fade_out_duration,
                                base::TimeDelta thinning_duration);
 
   ScrollbarSet Scrollbars() const;
   SingleScrollbarAnimationControllerThinning& GetScrollbarAnimationController(
       ScrollbarOrientation) const;
 
   // Returns how far through the animation we are as a progress value from
   // 0 to 1.
   float AnimationProgressAtTime(base::TimeTicks now);
   void RunAnimationFrame(float progress);
 
   void StartAnimation();
   void StopAnimation();
 
-  ScrollbarAnimationControllerClient* client_;
+  void FadeIn();
 
-  void PostDelayedAnimationTask(bool on_resize);
+  void PostDelayedFadeIn();
+  void PostDelayedFadeOut(bool on_resize);
 
   bool Captured() const;
 
+  bool CalcNeedTriggerScrollbarFadeIn(ScrollbarOrientation orientation,
+                                      float distance) const;
+
   void ApplyOpacityToScrollbars(float opacity);
 
+  ScrollbarAnimationControllerClient* client_;
+
   base::TimeTicks last_awaken_time_;
-  base::TimeDelta delay_before_starting_;
-  base::TimeDelta resize_delay_before_starting_;
+
+  // fade_in_delay_ is only for the case where the mouse hovers near the screen
+  // edge.
+  base::TimeDelta fade_in_delay_;
+  base::TimeDelta fade_out_delay_;
+  base::TimeDelta fade_out_resize_delay_;
+
+  bool need_trigger_scrollbar_fade_in_;
 
   bool is_animating_;
 
   const int scroll_layer_id_;
   bool currently_scrolling_;
   bool scroll_gesture_has_scrolled_;
 
-  base::CancelableClosure delayed_scrollbar_fade_;
+  base::CancelableClosure delayed_scrollbar_fade_in_;
+  base::CancelableClosure delayed_scrollbar_fade_out_;
+
   float opacity_;
-  base::TimeDelta fade_duration_;
+  base::TimeDelta fade_out_duration_;
 
   const bool need_thinning_animation_;
   std::unique_ptr<SingleScrollbarAnimationControllerThinning>
       vertical_controller_;
   std::unique_ptr<SingleScrollbarAnimationControllerThinning>
       horizontal_controller_;
 
   base::WeakPtrFactory<ScrollbarAnimationController> weak_factory_;
 };
 
 }  // namespace cc
 
 #endif  // CC_INPUT_SCROLLBAR_ANIMATION_CONTROLLER_H_