Show Overlay Scrollbar when GestureScrollUpdate

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 9363993b1b235d12e33e6e6a4e73e4fe502fc9df..3510d0cbe500fb7483349979ede7847539f20c18 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -2737,6 +2737,8 @@ class LayerTreeHostImplTestScrollbarAnimation : public LayerTreeHostImplTest {
     settings.scrollbar_animator = animator;
     settings.scrollbar_fade_in_delay = base::TimeDelta::FromMilliseconds(20);
     settings.scrollbar_fade_out_delay = base::TimeDelta::FromMilliseconds(20);
+    settings.scrollbar_fade_out_resize_delay =
+        base::TimeDelta::FromMilliseconds(20);
     settings.scrollbar_fade_out_duration =
         base::TimeDelta::FromMilliseconds(20);
 
@@ -2761,14 +2763,24 @@ class LayerTreeHostImplTestScrollbarAnimation : public LayerTreeHostImplTest {
       EXPECT_EQ(base::TimeDelta(), requested_animation_delay_);
     }
 
-    // If no scroll happened during a scroll gesture, it should have no effect.
+    // If no scroll happened during a scroll gesture, it appears scrollbars and

[bokan, 2017/02/27 23:34:17]

I would leave this case as is and make sure no GSU means no effect. Add a test
below that checks that an empty ScrollBy queues an animation. Also, please add a
check for !ScrollbarsHidden

+    // schedules a fade out delay.
     host_impl_->ScrollBegin(BeginState(gfx::Point()).get(),
                             InputHandler::WHEEL);
+    host_impl_->ScrollBy(UpdateState(gfx::Point(), gfx::Vector2dF(0, 0)).get());
     host_impl_->ScrollEnd(EndState().get());
     EXPECT_FALSE(did_request_next_frame_);
     EXPECT_FALSE(did_request_redraw_);
-    EXPECT_EQ(base::TimeDelta(), requested_animation_delay_);
-    EXPECT_TRUE(animation_task_.Equals(base::Closure()));
+    if (expecting_animations) {
+      EXPECT_EQ(base::TimeDelta::FromMilliseconds(20),
+                requested_animation_delay_);
+      EXPECT_FALSE(animation_task_.Equals(base::Closure()));
+      requested_animation_delay_ = base::TimeDelta();
+      animation_task_ = base::Closure();
+    } else {
+      EXPECT_EQ(base::TimeDelta(), requested_animation_delay_);
+      EXPECT_TRUE(animation_task_.Equals(base::Closure()));
+    }
 
     // Before the scrollbar animation exists, we should not get redraws.
     BeginFrameArgs begin_frame_args =
@@ -2890,8 +2902,17 @@ class LayerTreeHostImplTestScrollbarAnimation : public LayerTreeHostImplTest {
     host_impl_->ScrollEnd(EndState().get());
     EXPECT_FALSE(did_request_next_frame_);
     EXPECT_FALSE(did_request_redraw_);
-    EXPECT_EQ(base::TimeDelta(), requested_animation_delay_);
-    EXPECT_TRUE(animation_task_.Equals(base::Closure()));
+
+    if (expecting_animations) {
+      EXPECT_EQ(base::TimeDelta::FromMilliseconds(20),

[bokan, 2017/02/27 23:34:16]

Is this because the layer is already scrolled to the maximum offset so
previously it wouldn't have queued an animation?

[chaopeng, 2017/02/28 01:51:12]

Yes,
https://cs.chromium.org/chromium/src/cc/trees/layer_tree_host_impl_unittest.c...

+                requested_animation_delay_);
+      EXPECT_FALSE(animation_task_.Equals(base::Closure()));
+      requested_animation_delay_ = base::TimeDelta();
+      animation_task_ = base::Closure();
+    } else {
+      EXPECT_EQ(base::TimeDelta(), requested_animation_delay_);
+      EXPECT_TRUE(animation_task_.Equals(base::Closure()));
+    }
 
     // Changing page scale triggers scrollbar animation.
     host_impl_->active_tree()->PushPageScaleFromMainThread(1.f, 1.f, 4.f);