Disable overlay scrollbars in Blink when hidden by the compositor.

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 146 matching lines @@
 
   if (mouse_is_near_scrollbar_ != mouse_is_near_scrollbar) {
     mouse_is_near_scrollbar_ = mouse_is_near_scrollbar;
     thickness_change_ = mouse_is_near_scrollbar_ ? INCREASE : DECREASE;
   }
 
   SetCurrentAnimatingProperty(THICKNESS);
   StartAnimation();
 }
 
+bool ScrollbarAnimationControllerThinning::ScrollbarsHidden() const {
+  return opacity_ == 0.0f;
+}
+
 float ScrollbarAnimationControllerThinning::ThumbThicknessScaleAt(
     float progress) {
   if (thickness_change_ == NONE)
     return mouse_is_near_scrollbar_ ? 1.f : kIdleThicknessScale;
   float factor = thickness_change_ == INCREASE ? progress : (1.f - progress);
   return ((1.f - kIdleThicknessScale) * factor) + kIdleThicknessScale;
 }
 
 float ScrollbarAnimationControllerThinning::AdjustScale(
     float new_value,
@@ skipping 29 matching lines @@
     // be overwritten anyway when property trees are pushed.
     if (property_trees->IsInIdToIndexMap(PropertyTrees::TreeType::EFFECT,
                                          scrollbar->id())) {
       property_trees->effect_tree.OnOpacityAnimated(
           effective_opacity,
           property_trees->effect_id_to_index_map[scrollbar->id()],
           scrollbar->layer_tree_impl());
     }
   }
 
+  bool previouslyVisible = opacity_ > 0.0f;
+  bool currentlyVisible = opacity > 0.0f;
+
   opacity_ = opacity;
+
+  if (previouslyVisible != currentlyVisible)
+    client_->DidChangeScrollbarVisibility();
 }
 
 void ScrollbarAnimationControllerThinning::ApplyThumbThicknessScale(
     float thumb_thickness_scale) {
   for (ScrollbarLayerImplBase* scrollbar : Scrollbars()) {
     if (!scrollbar->is_overlay_scrollbar())
       continue;
 
     scrollbar->SetThumbThicknessScaleFactor(AdjustScale(
         thumb_thickness_scale, scrollbar->thumb_thickness_scale_factor(),
         thickness_change_, kIdleThicknessScale, 1));
   }
 }
 
 void ScrollbarAnimationControllerThinning::SetCurrentAnimatingProperty(
     AnimatingProperty property) {
   if (current_animating_property_ == property)
     return;
 
   StopAnimation();
   current_animating_property_ = property;
   if (current_animating_property_ == THICKNESS)
     ApplyOpacity(1.f);
 }
 
 }  // namespace cc