control.actor.HELD_WHILE_AWAITING

Reading continues while something is waiting for a reply, because the reply arrives on the connection that would otherwise pause. That is bounded by how long a reply takes, and tmux answers most commands at once -- but not all. run-shell without -b answers its own block immediately and then parks the queue for as long as its shell command runs, so the next command sent is outstanding for that long and this end would hold events for the duration. A ceiling turns that into a pause rather than a memory leak.