I've been updating album cover art, and found this bug.
If I update anything in the extended tag window, and click >> before the save completes, an exception occurs, and the save dialog will not go away.
Can continue working, so it's not too big a deal, but the ideal solution is to disable the controls in the extended tag window until the save is complete.
There no longer seems to be an issue with the progress dialog.
Personally, I would have just disabled the controls on the tag window, but looks like you opted for some other way as I never saw the controls go inactive.