Hi Rob and Cleon,
Yes, I've seen this problem a few times. In my Mac this only happens seldomly and only with the 64-bit version of PixInsight for Mac OS X. I've never seen this problem with the 32-bit version ?and it should never happen in the 32-bit application.
Are you using the 64-bit version? If so, remember that the 64-bit version of PixInsight for Mac OS X is still experimental. It has some GUI glitches and problems and this is one of them. I'll try to fix this particularly nasty problem as soon as possible.