Iframe/div MD scrollbars get idle when mouse leaves the nested elements.

cc/input/scrollbar_animation_controller_thinning.cc

 // 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.
 
 #include "cc/input/scrollbar_animation_controller_thinning.h"
 
 #include "base/memory/ptr_util.h"
 #include "base/time/time.h"
 #include "cc/layers/layer_impl.h"
 #include "cc/layers/scrollbar_layer_impl_base.h"
@@ skipping 48 matching lines @@
       ThumbThicknessScaleAtAnimationProgress(progress);
   ApplyOpacityAndThumbThicknessScale(opacity, thumb_thickness_scale);
   client_->SetNeedsRedrawForScrollbarAnimation();
   if (progress == 1.f) {
     opacity_change_ = NONE;
     thickness_change_ = NONE;
     StopAnimation();
   }
 }
 
-void ScrollbarAnimationControllerThinning::DidCaptureScrollbarBegin() {
+void ScrollbarAnimationControllerThinning::DidMouseDown() {
+  if (!mouse_is_over_scrollbar_)
+    return;
+
   captured_ = true;
   ApplyOpacityAndThumbThicknessScale(1, 1.f);
 }
 
-void ScrollbarAnimationControllerThinning::DidCaptureScrollbarEnd() {
+void ScrollbarAnimationControllerThinning::DidMouseUp() {
+  if (!captured_)
+    return;
+
   captured_ = false;
-
   if (!mouse_is_over_scrollbar_)
     opacity_change_ = DECREASE;
   if (!mouse_is_near_scrollbar_)
     thickness_change_ = DECREASE;
   StartAnimation();
 }
 
 void ScrollbarAnimationControllerThinning::DidMouseMoveOffScrollbar() {
   if (!mouse_is_over_scrollbar_ && !mouse_is_near_scrollbar_)
     return;
@@ skipping 112 matching lines @@
           property_trees->effect_id_to_index_map[scrollbar->id()],
           scrollbar->layer_tree_impl());
     }
     scrollbar->SetThumbThicknessScaleFactor(AdjustScale(
         thumb_thickness_scale, scrollbar->thumb_thickness_scale_factor(),
         thickness_change_, kIdleThicknessScale, 1));
   }
 }
 
 }  // namespace cc