Overlay scrollbars are painted onload.

cc/input/scrollbar_animation_controller_thinning_unittest.cc

Index: cc/input/scrollbar_animation_controller_thinning_unittest.cc
diff --git a/cc/input/scrollbar_animation_controller_thinning_unittest.cc b/cc/input/scrollbar_animation_controller_thinning_unittest.cc
index e1a1aa3cd53a5b79aafd9cd8c544d86f539232cf..df1adbf666e05bfb0259189a1ef72cf7fb12d36a 100644
--- a/cc/input/scrollbar_animation_controller_thinning_unittest.cc
+++ b/cc/input/scrollbar_animation_controller_thinning_unittest.cc
@@ -67,6 +67,7 @@ class ScrollbarAnimationControllerThinningTest
 
     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();
@@ -97,6 +98,19 @@ TEST_F(ScrollbarAnimationControllerThinningTest, Idle) {
   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);