This is a tracker bug of the issue that an ARC++ notification is not shown in the right z-ordering.

- ARC++ notifications use NativeViewHost, which attaches the notification widget directly to the tray bubble widget regardless of views hierarchy.
- So an ARC++ notification layer is a direct child of the unified bubble's toplevel layer
- ARC++ notifications are shown above the scroll bar and SystemTray part of UnifiedSystemTray.

I had long offline discussion with yoshiki@ about this, and although it's not immediately possible, we have number of issues that cannot be fixed cleanly without this (Issue 873818, Issue 858940), so in the long term it'd be nice if it can be fixed.