Ok, so the freezing UI thing is a bug in Qt:
https://bugreports.qt.io/browse/QTBUG-39220
https://stackoverflow.com/questions/14554903/widgets-freezing-after-minimise-window

Shouldn't be hard to solve it, I'll see if I can fix it and a few other things and have a new build tonight or tomorrow.

Thanks everyone for reporting it and narrowing it down!

- Michael