Iframe/div MD scrollbars get idle when mouse leaves the nested elements.

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 d47f504871741c890f5daf7280c13c7412cf7e2e..28eba769b00d0bdb3fbd922dcfd5ea7677f221ad 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -209,6 +209,7 @@ LayerTreeHostImpl::LayerTreeHostImpl(
       wheel_scrolling_(false),
       scroll_affects_scroll_handler_(false),
       scroll_layer_id_when_mouse_over_scrollbar_(Layer::INVALID_ID),
+      scroll_layer_id_when_mouse_near_scrollbar_(Layer::INVALID_ID),
       captured_scrollbar_layer_id_(Layer::INVALID_ID),
       tile_priorities_dirty_(false),
       settings_(settings),
@@ -3256,9 +3257,17 @@ void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
   if (scroll_on_main_thread || !scroll_layer_impl)
     return;
 
-  ScrollbarAnimationController* animation_controller =
+  if (scroll_layer_impl->id() != scroll_layer_id_when_mouse_near_scrollbar_) {
+    ScrollbarAnimationController* old_animation_controller =
+        ScrollbarAnimationControllerForId(

[bokan, 2016/10/18 15:44:35]

Hmm...too much of this logic is starting to creep into LayerTreeHostImpl.
scroll_layer_id_when_mouse_near_scrollbar_ should always be the same layer as
scroll_layer_id_when_mouse_over_scrollbar_ (if the latter is non-null), right? I
think we could keep just one variable, scroll_layer_id_mouse_currently_over and
send the distance from scrollbar to the controller on each mouse move. The
controller could then keep all this state and make the transitions internally.

[sahel, 2016/10/19 21:29:17]

Done.

+            scroll_layer_id_when_mouse_near_scrollbar_);
+    if (old_animation_controller)
+      old_animation_controller->DidMouseMoveOffScrollbar();
+  }
+
+  ScrollbarAnimationController* new_animation_controller =
       ScrollbarAnimationControllerForId(scroll_layer_impl->id());
-  if (!animation_controller)
+  if (!new_animation_controller)
     return;
 
   float distance_to_scrollbar = std::numeric_limits<float>::max();
@@ -3268,8 +3277,10 @@ void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
         std::min(distance_to_scrollbar,
                  DeviceSpaceDistanceToLayer(device_viewport_point, scrollbar));
 
-  animation_controller->DidMouseMoveNear(distance_to_scrollbar /
-                                         active_tree_->device_scale_factor());
+  new_animation_controller->DidMouseMoveNear(
+      distance_to_scrollbar / active_tree_->device_scale_factor());
+  if (new_animation_controller->MouseIsNearScrollbar())
+    scroll_layer_id_when_mouse_near_scrollbar_ = scroll_layer_impl->id();
 }
 
 void LayerTreeHostImpl::HandleMouseOverScrollbar(LayerImpl* layer_impl) {