Overlay scrollbars are painted onload.

cc/input/scrollbar_animation_controller_thinning_unittest.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 "cc/layers/solid_color_scrollbar_layer_impl.h"
 #include "cc/test/fake_impl_task_runner_provider.h"
 #include "cc/test/fake_layer_tree_host_impl.h"
 #include "cc/test/geometry_test_utils.h"
@@ skipping 79 matching lines @@
   bool did_request_animate_;
 };
 
 // Check initialization of scrollbar.
 TEST_F(ScrollbarAnimationControllerThinningTest, Idle) {
   scrollbar_controller_->Animate(base::TimeTicks());
   EXPECT_FLOAT_EQ(0.7f, scrollbar_layer_->Opacity());
   EXPECT_FLOAT_EQ(0.4f, scrollbar_layer_->thumb_thickness_scale_factor());
 }
 
+// Check that scrollbar appears again, when the layer becomes scrollable.
+TEST_F(ScrollbarAnimationControllerThinningTest, AppearOnResize) {
+  scrollbar_controller_->DidScrollUpdate(false);
+  // Make the Layer non-scrollable, scrollbar disappears.
+  clip_layer_->SetBounds(gfx::Size(200, 200));
+  scrollbar_controller_->DidScrollUpdate(false);
+  EXPECT_FLOAT_EQ(0.0f, scrollbar_layer_->Opacity());
+  // Make the layer scrollable, scrollbar appears again.
+  clip_layer_->SetBounds(gfx::Size(100, 100));
+  scrollbar_controller_->DidScrollUpdate(false);
+  EXPECT_FLOAT_EQ(0.7f, scrollbar_layer_->Opacity());
+}
+
 // Check that scrollbar disappears when the layer becomes non-scrollable.
 TEST_F(ScrollbarAnimationControllerThinningTest, HideOnResize) {
   LayerImpl* scroll_layer = host_impl_.active_tree()->LayerById(1);
   ASSERT_TRUE(scroll_layer);
   EXPECT_EQ(gfx::Size(200, 200), scroll_layer->bounds());
 
   EXPECT_EQ(HORIZONTAL, scrollbar_layer_->orientation());
 
   // Shrink along X axis, horizontal scrollbar should appear.
   clip_layer_->SetBounds(gfx::Size(100, 200));
@@ skipping 271 matching lines @@
 
   time += base::TimeDelta::FromSeconds(1);
   scrollbar_controller_->Animate(time);
   EXPECT_FLOAT_EQ(0.7f, scrollbar_layer_->Opacity());
   // The thickness now gets big again.
   EXPECT_FLOAT_EQ(1.0f, scrollbar_layer_->thumb_thickness_scale_factor());
 }
 
 }  // namespace
 }  // namespace cc