Close buttons at the bottom of Combinators.
Since they can get quite tall when the devices are open. There's plenty of blank space at the bottom for this. Could be a nice shortcut when you're not using the Navigators. Obviously it should automatically scroll you up to the collapsed combi so you're not suddenly staring at completely unrelated devices should there be many of them after your combi.
(P.S. I mean collapsed as in "hide devices")