Prevent overlay scrollbars expand or hover together

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index e27073c6351fc2341e27cb411ae7621e9c8730d3..28e153fccc1dcb838900dd7eea64bcc77fce6abb 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1658,7 +1658,6 @@ bool LayerTreeHostImpl::DrawLayers(FrameData* frame) {
     }
   }
 
-
   CompositorFrame compositor_frame;
   compositor_frame.metadata = std::move(metadata);
   resource_provider_->PrepareSendToParent(resources,
@@ -3228,7 +3227,6 @@ void LayerTreeHostImpl::MouseUp() {
 }
 
 void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
-  float distance_to_scrollbar = std::numeric_limits<float>::max();
   gfx::PointF device_viewport_point = gfx::ScalePoint(
       gfx::PointF(viewport_point), active_tree_->device_scale_factor());
   LayerImpl* layer_impl =
@@ -3241,10 +3239,7 @@ void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
   int new_id = Layer::INVALID_ID;
   if (layer_impl && layer_impl->ToScrollbarLayer())
     new_id = layer_impl->ToScrollbarLayer()->ScrollLayerId();
-  if (new_id != Layer::INVALID_ID) {
-    // Mouse over a scrollbar.
-    distance_to_scrollbar = 0;
-  } else {
+  if (new_id == Layer::INVALID_ID) {
     bool scroll_on_main_thread = false;
     uint32_t main_thread_scrolling_reasons;
     LayerImpl* scroll_layer_impl = FindScrollLayerForDeviceViewportPoint(
@@ -3273,12 +3268,13 @@ void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
   if (!new_animation_controller)
     return;
 
-  for (ScrollbarLayerImplBase* scrollbar : ScrollbarsFor(new_id))
-    distance_to_scrollbar =
-        std::min(distance_to_scrollbar,
-                 DeviceSpaceDistanceToLayer(device_viewport_point, scrollbar));
-  new_animation_controller->DidMouseMoveNear(
-      distance_to_scrollbar / active_tree_->device_scale_factor());
+  for (ScrollbarLayerImplBase* scrollbar : ScrollbarsFor(new_id)) {
+    new_animation_controller->DidMouseMoveNear(
+        scrollbar->orientation(),
+        DeviceSpaceDistanceToLayer(device_viewport_point, scrollbar) /
+            active_tree_->device_scale_factor());
+  }
+  new_animation_controller->EnsureScrollbarFadeIn();

[bokan, 2016/12/16 14:57:42]

Make this private and move it into DidMouseMoveNear.
Also, shouldn't this only be called if the mouse is actually over the scrollbar?

[chaopeng, 2016/12/16 21:02:25]

No, we need to keep scrollbar fade in while the mouse is near. 

 }
 
 void LayerTreeHostImpl::MouseLeave() {