Overlay scrollbars expand only when mouse is near thumb

cc/input/single_scrollbar_animation_controller_thinning.cc

Index: cc/input/single_scrollbar_animation_controller_thinning.cc
diff --git a/cc/input/single_scrollbar_animation_controller_thinning.cc b/cc/input/single_scrollbar_animation_controller_thinning.cc
index f09756ee5e636571e659691d1c146f6424b18bc3..86b1a2bce2f9650ca7821e213ba96902dd86fded 100644
--- a/cc/input/single_scrollbar_animation_controller_thinning.cc
+++ b/cc/input/single_scrollbar_animation_controller_thinning.cc
@@ -36,8 +36,8 @@ SingleScrollbarAnimationControllerThinning::
       scroll_element_id_(scroll_element_id),
       orientation_(orientation),
       captured_(false),
-      mouse_is_over_scrollbar_(false),
-      mouse_is_near_scrollbar_(false),
+      mouse_is_over_scrollbar_thumb_(false),
+      mouse_is_near_scrollbar_thumb_(false),
       thickness_change_(NONE),
       thinning_duration_(thinning_duration) {
   ApplyThumbThicknessScale(kIdleThicknessScale);
@@ -92,7 +92,7 @@ void SingleScrollbarAnimationControllerThinning::StopAnimation() {
 }
 
 void SingleScrollbarAnimationControllerThinning::DidMouseDown() {
-  if (!mouse_is_over_scrollbar_)
+  if (!mouse_is_over_scrollbar_thumb_)
     return;
 
   StopAnimation();
@@ -107,7 +107,7 @@ void SingleScrollbarAnimationControllerThinning::DidMouseUp() {
   captured_ = false;
   StopAnimation();
 
-  if (!mouse_is_near_scrollbar_) {
+  if (!mouse_is_near_scrollbar_thumb_) {
     thickness_change_ = DECREASE;
     StartAnimation();
   } else {
@@ -116,11 +116,11 @@ void SingleScrollbarAnimationControllerThinning::DidMouseUp() {
 }
 
 void SingleScrollbarAnimationControllerThinning::DidMouseLeave() {
-  if (!mouse_is_over_scrollbar_ && !mouse_is_near_scrollbar_)
+  if (!mouse_is_over_scrollbar_thumb_ && !mouse_is_near_scrollbar_thumb_)
     return;
 
-  mouse_is_over_scrollbar_ = false;
-  mouse_is_near_scrollbar_ = false;
+  mouse_is_over_scrollbar_thumb_ = false;
+  mouse_is_near_scrollbar_thumb_ = false;
 
   if (captured_)
     return;
@@ -129,24 +129,25 @@ void SingleScrollbarAnimationControllerThinning::DidMouseLeave() {
   StartAnimation();
 }
 
-void SingleScrollbarAnimationControllerThinning::DidMouseMoveNear(
-    float distance) {
-  bool mouse_is_over_scrollbar = distance == 0.0f;
-  bool mouse_is_near_scrollbar =
-      distance < kDefaultMouseMoveDistanceToTriggerAnimation;
+void SingleScrollbarAnimationControllerThinning::DidMouseMove(
+    float distance_to_thumb) {
+  bool mouse_is_over_scrollbar_thumb = distance_to_thumb == 0.0f;
+  bool mouse_is_near_scrollbar_thumb =
+      distance_to_thumb < kMouseMoveDistanceToTriggerExpand;
 
-  if (!captured_ && mouse_is_near_scrollbar != mouse_is_near_scrollbar_) {
-    thickness_change_ = mouse_is_near_scrollbar ? INCREASE : DECREASE;
+  if (!captured_ &&
+      mouse_is_near_scrollbar_thumb != mouse_is_near_scrollbar_thumb_) {
+    thickness_change_ = mouse_is_near_scrollbar_thumb ? INCREASE : DECREASE;
     StartAnimation();
   }
-  mouse_is_near_scrollbar_ = mouse_is_near_scrollbar;
-  mouse_is_over_scrollbar_ = mouse_is_over_scrollbar;
+  mouse_is_near_scrollbar_thumb_ = mouse_is_near_scrollbar_thumb;
+  mouse_is_over_scrollbar_thumb_ = mouse_is_over_scrollbar_thumb;
 }
 
 float SingleScrollbarAnimationControllerThinning::ThumbThicknessScaleAt(
     float progress) {
   if (thickness_change_ == NONE)
-    return mouse_is_near_scrollbar_ ? 1.f : kIdleThicknessScale;
+    return mouse_is_near_scrollbar_thumb_ ? 1.f : kIdleThicknessScale;
   float factor = thickness_change_ == INCREASE ? progress : (1.f - progress);
   return ((1.f - kIdleThicknessScale) * factor) + kIdleThicknessScale;
 }
@@ -173,8 +174,8 @@ float SingleScrollbarAnimationControllerThinning::AdjustScale(
 
 void SingleScrollbarAnimationControllerThinning::UpdateThumbThicknessScale() {
   StopAnimation();
-  ApplyThumbThicknessScale(mouse_is_near_scrollbar_ ? 1.f
-                                                    : kIdleThicknessScale);
+  ApplyThumbThicknessScale(
+      mouse_is_near_scrollbar_thumb_ ? 1.f : kIdleThicknessScale);
 }
 
 void SingleScrollbarAnimationControllerThinning::ApplyThumbThicknessScale(