we have posted a new update, 3.1.148 http://meshmixer.com/beta.html
. This version does not have any significant feature additions, the main change is that we switched from Qt 4 (which is unsupported on all major OSs) to Qt5. I know it doesn't sound very exciting but it was a major change behind-the-scenes, that will improve stability going forward.
In particular this should resolve many OSX platform bugs, however it may introduce other issues. We have not done extensive testing, we are hoping you can help with that.
Unfortunately as part of this we had to drop support for 32-bit Windows, at least temporarily. Hoping to bring that back once we work out all the kinks.
Also, you might notice that the download on OSX has gotten enormous (250mb!!). We are currently including /all/ of Qt, not just what we are actually using - this will get sorted out in time.