type ActionChannel = (UnboundedSender<UserAction>, Mutex<Option<UnboundedReceiver<UserAction>>>);Expand description
The user-action channel exists from first use, not from start: the tray
(and its Pause/Resume/Open menu) comes up before srv does, and a pick made
in that window must queue until the presenter connects rather than vanish
(ReAgent P2 on #3654). start takes the receiver.