Return main thread scroll layer in FindScrollLayerForDeviceViewportPoint

cc/trees/layer_tree_host_impl_unittest.cc

Index: cc/trees/layer_tree_host_impl_unittest.cc
diff --git a/cc/trees/layer_tree_host_impl_unittest.cc b/cc/trees/layer_tree_host_impl_unittest.cc
index 761b4a6bd9e7dd4c561511e8a9e2d9102fbdd620..ed8d79e1f7d6db8893a07a4cc7d1f8e2d235df68 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -464,6 +464,8 @@ class LayerTreeHostImplTest : public testing::Test,
 
   void SetupMouseMoveAtWithDeviceScale(float device_scale_factor);
 
+  void SetupMouseMoveAtTestScrollbarStates(bool main_thread_scrolling);
+
   scoped_refptr<AnimationTimeline> timeline() { return timeline_; }
 
  protected:
@@ -11621,7 +11623,8 @@ TEST_F(LayerTreeHostImplTest, RecomputeGpuRasterOnCompositorFrameSinkChange) {
   EXPECT_FALSE(host_impl_->use_gpu_rasterization());
 }
 
-TEST_F(LayerTreeHostImplTest, LayerTreeHostImplTestScrollbarStates) {
+void LayerTreeHostImplTest::SetupMouseMoveAtTestScrollbarStates(

[bokan, 2017/01/13 19:23:38]

IMO, TestScrollbarStates would be a better name

+    bool main_thread_scrolling) {
   LayerTreeSettings settings = DefaultSettings();
   settings.scrollbar_fade_delay = base::TimeDelta::FromMilliseconds(500);
   settings.scrollbar_fade_duration = base::TimeDelta::FromMilliseconds(300);
@@ -11647,6 +11650,11 @@ TEST_F(LayerTreeHostImplTest, LayerTreeHostImplTestScrollbarStates) {
   LayerImpl* root_scroll =
       host_impl_->active_tree()->OuterViewportScrollLayer();
 
+  if (main_thread_scrolling) {
+    root_scroll->set_main_thread_scrolling_reasons(

[chaopeng, 2017/01/13 19:15:28]

Can child layer be not main thread scroll if the parent has
main_thread_scrolling_reasons?

[bokan, 2017/01/13 19:23:38]

Yes, they're independent so a child that scrolls on impl can have a parent that
scrolls on main. A child that scrolls on main can have a parent that scrolls on
impl.

That said, in this case I'd also make the child main thread scroll.

+        MainThreadScrollingReason::kHasBackgroundAttachmentFixedObjects);
+  }
+
   // scrollbar_1 on root scroll.
   std::unique_ptr<SolidColorScrollbarLayerImpl> scrollbar_1 =
       SolidColorScrollbarLayerImpl::Create(host_impl_->active_tree(),
@@ -11746,5 +11754,15 @@ TEST_F(LayerTreeHostImplTest, LayerTreeHostImplTestScrollbarStates) {
   EXPECT_FALSE(scrollbar_2_animation_controller->mouse_is_over_scrollbar());
 }
 
+TEST_F(LayerTreeHostImplTest,
+       LayerTreeHostImplTestScrollbarStatesInMainThreadScorlling) {
+  SetupMouseMoveAtTestScrollbarStates(true);
+}
+
+TEST_F(LayerTreeHostImplTest,
+       LayerTreeHostImplTestScrollbarStatesInNotMainThreadScorlling) {
+  SetupMouseMoveAtTestScrollbarStates(false);
+}
+
 }  // namespace
 }  // namespace cc