Don't know how I managed to do it, but MC13 is trapped by a modal dialog box. I think I searched for some cover art but clicked back on the main window. This left the dialog in the background, for quite a while, as I was doing a lot of other edits. I dislike the lack of defined edges to one of the skins so I switched to ThunderOS. Lo and behold that Get Cover dialog box came back to the foreground. And in the process it made it impossible to select the main dialog. Nor does it let me use Cancel or any of the other options on the dialog box. It has effectively stranded my otherwise working MC13 session behind this orphaned dialog box.
I'm struggling with the Cover Art handling. The UI for it is just way too unwieldy. Yeah, it does a lot of great things but it's really a pain in the ass to try and use it productively. This is not an area where MC is alone in being a pain in the ass, lots of programs suck at this. But they all suck differently enough to really make it infuriating to come across yet another bad example. Yes, that was a rant and I feel no better for it. Please, someone buckle down and rethink the UI for managing Cover Art.
Meanwhile, check whether an orphaned dialog box would cause problems after a skin change. I think that's how this problem developed. Either make sure it's impossible to orphan the dialog (probably easier) or do it during the skin change (probably impossible, or at least impractical).