I don't think it's just on Mac Mini - my MacBook Pro late 13" 2016 retina will show similar symptoms if I try and resize the window to full screen (not the maximise button, but try and drag the bottom corner of the world to the last 5-10 pixels at the bottom of the screen).
I think Ted noticed similar in that it looks frozen but it's actually the screen not redrawing as you can fiddle to get it back (can't recall what I did - maybe drag a window on top?)
Tim
Sent from my iPhone
I don't know if it matters, but on my Retina 5K, 27 inch, Late 2015 iMac, AMD Radeon R9 M395 wirh 2048 MB running MacOS 10.12.6 I don't have a problem resizing or maximizing Pharo 6.1.