Double-clicking on it switches to the mini-player. I'd like to either disable it or instead switch to the theater-mode.
Disabling would be simple: just don't double-click on it?
On a more serious note, I think this is some undocumented feature. Possibly unintentional even.
I would expect the albumart to get displayed full-size in a new panel when its double-clicked.
That would be consistent with how that works elsewhere in MusicBee.