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 49 matching lines @@
             host_impl_.active_tree(), kId, HORIZONTAL, kThumbThickness,
             kTrackStart, kIsLeftSideVerticalScrollbar, kIsOverlayScrollbar);
     scrollbar_layer_ = scrollbar.get();
 
     scroll_layer->test_properties()->AddChild(std::move(scrollbar));
     clip_layer_->test_properties()->AddChild(std::move(scroll_layer));
     host_impl_.active_tree()->SetRootLayerForTesting(std::move(clip));
 
     scrollbar_layer_->SetScrollLayerId(scroll_layer_ptr->id());
     scrollbar_layer_->test_properties()->opacity_can_animate = true;
+

[bokan, 2016/09/23 20:15:17]

Nit: spurious new line

     clip_layer_->SetBounds(gfx::Size(100, 100));
     scroll_layer_ptr->SetBounds(gfx::Size(200, 200));
     host_impl_.active_tree()->BuildLayerListAndPropertyTreesForTesting();
 
     scrollbar_controller_ = ScrollbarAnimationControllerThinning::Create(
         scroll_layer_ptr->id(), this, base::TimeDelta::FromSeconds(2),
         base::TimeDelta::FromSeconds(5), base::TimeDelta::FromSeconds(3));
   }
 
   FakeImplTaskRunnerProvider task_runner_provider_;
@@ skipping 10 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);

[bokan, 2016/09/23 20:15:18]

DidScrollUpdate gets called when we scroll though, right? Currently, the
scrollbars appear after the user scrolls. Does this test fail without your fix?

[sahel, 2016/09/23 20:25:11]

It get's called when we are scrolling, AND during pushPropertiesTo,
and the latter call causes the bug on load, because it's getting called without
scrolling and during sets the opacity_change_ to be DECREASE.

Yes, it fails without this fix.

[bokan, 2016/09/23 20:26:03]

Got it. Thanks!

+  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