Overlay scrollbars expand only when mouse is near thumb

cc/input/scrollbar_animation_controller.h

Index: cc/input/scrollbar_animation_controller.h
diff --git a/cc/input/scrollbar_animation_controller.h b/cc/input/scrollbar_animation_controller.h
index c34273e57ee835c1cd082014b65aae4544008ae6..1bb84362e723086f476e1cf4e505a4d34a48a8ec 100644
--- a/cc/input/scrollbar_animation_controller.h
+++ b/cc/input/scrollbar_animation_controller.h
@@ -82,13 +82,14 @@ class CC_EXPORT ScrollbarAnimationController {
   void DidMouseDown();
   void DidMouseUp();
   void DidMouseLeave();
-  void DidMouseMoveNear(ScrollbarOrientation, float);
+  void DidMouseMove(const gfx::PointF& device_viewport_point);
 
   // Called when Blink wants to show the scrollbars (via
   // ScrollableArea::showOverlayScrollbars).
   void DidRequestShowFromMainThread();
 
-  bool MouseIsOverScrollbar(ScrollbarOrientation orientation) const;
+  bool MouseIsOverScrollbarThumb(ScrollbarOrientation orientation) const;
+  bool MouseIsNearScrollbarThumb(ScrollbarOrientation orientation) const;
   bool MouseIsNearScrollbar(ScrollbarOrientation orientation) const;
   bool MouseIsNearAnyScrollbar() const;
 
@@ -112,6 +113,7 @@ class CC_EXPORT ScrollbarAnimationController {
                                base::TimeDelta thinning_duration);
 
   ScrollbarSet Scrollbars() const;
+  ScrollbarLayerImplBase* GetScrollbar(ScrollbarOrientation orientation) const;
   SingleScrollbarAnimationControllerThinning& GetScrollbarAnimationController(
       ScrollbarOrientation) const;
 
@@ -129,9 +131,6 @@ class CC_EXPORT ScrollbarAnimationController {
 
   bool Captured() const;
 
-  bool CalcNeedTriggerScrollbarShow(ScrollbarOrientation orientation,
-                                    float distance) const;
-
   void ApplyOpacityToScrollbars(float opacity);
 
   ScrollbarAnimationControllerClient* client_;
@@ -143,7 +142,7 @@ class CC_EXPORT ScrollbarAnimationController {
 
   base::TimeDelta fade_duration_;
 
-  bool need_trigger_scrollbar_show_;
+  bool need_trigger_scrollbar_fade_in_;
 
   bool is_animating_;
   AnimationChange animation_change_;
@@ -158,10 +157,11 @@ class CC_EXPORT ScrollbarAnimationController {
 
   const bool show_scrollbars_on_scroll_gesture_;
   const bool need_thinning_animation_;
+
   std::unique_ptr<SingleScrollbarAnimationControllerThinning>
-      vertical_controller_;
-  std::unique_ptr<SingleScrollbarAnimationControllerThinning>
-      horizontal_controller_;
+      scrollbar_controllers_[2];

[bokan, 2017/04/28 14:15:17]

Please keep these as vertical_controller_ and horizontal_controller_. IMO,
having `for` loops to do something for both controllers ends up being less
readable and, in most cases, doesn't save us from any code copying. For cases
where you want to do something based on ScrollbarOrientation, you can keep the
helper return the correct controller.

+
+  bool mouse_is_near_scrollbar_[2];

[bokan, 2017/04/28 14:15:17]

Lets move this into the thinning controller, pass the actual point in
DidMoveMouse, and calculate "is near/over track/thumb" inside the individual
controller. Then this class can just ask the Thinning controllers for the
current mouse<->scrollbar state.

The "Thinning" controller should really be an "IndividualScrollbar" controller,
but we can rename later. The point being that ThinningController manages state
for each individual scrollbar. That way the code becomes more natural as you
don't have to duplicate the Vertical/Horizontal split in this class more than
once.

 
   base::WeakPtrFactory<ScrollbarAnimationController> weak_factory_;
 };