Overlay scrollbars are painted onload.

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 106 matching lines @@
     ThumbThicknessScaleAtAnimationProgress(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,
     float current_value,
-    AnimationChange animation_change) {
+    AnimationChange animation_change,
+    float min_value,
+    float max_value) {
+  float result;
   if (animation_change == INCREASE && current_value > new_value)
-    return current_value;
-  if (animation_change == DECREASE && current_value < new_value)
-    return current_value;
-  return new_value;
+    result = current_value;
+  else if (animation_change == DECREASE && current_value < new_value)
+    result = current_value;
+  else
+    result = new_value;
+  if (result > max_value)
+    return max_value;
+  if (result < min_value)
+    return min_value;
+  return result;
 }
 
 void ScrollbarAnimationControllerThinning::ApplyOpacityAndThumbThicknessScale(
     float opacity,
     float thumb_thickness_scale) {
   for (ScrollbarLayerImplBase* scrollbar : Scrollbars()) {
     if (!scrollbar->is_overlay_scrollbar())
       continue;
     float effective_opacity =
         scrollbar->CanScrollOrientation()
-            ? AdjustScale(opacity, scrollbar->Opacity(), opacity_change_)
+            ? AdjustScale(opacity, scrollbar->Opacity(), opacity_change_,
+                          kIdleOpacity, 1)
             : 0;
-
     PropertyTrees* property_trees =
         scrollbar->layer_tree_impl()->property_trees();
     // If this method is called during LayerImpl::PushPropertiesTo, we may not
     // yet have valid effect_id_to_index_map entries as property trees are
     // pushed after layers during activation. We can skip updating opacity in
     // that case as we are only registering a scrollbar and because opacity will
     // 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());
     }
     scrollbar->SetThumbThicknessScaleFactor(AdjustScale(
         thumb_thickness_scale, scrollbar->thumb_thickness_scale_factor(),
-        thickness_change_));
+        thickness_change_, kIdleThicknessScale, 1));
   }
 }
 
 }  // namespace cc