Building from source is very difficult. Just read the very next conversation topic here with Reuben Martin for more information. I do not offer much support for people making their own builds.
OS X 10.6 might be a part of the problem. Both Qt and Shotcut team have a problem to support that due to infrequent testing even though it should work "in theory." I do have a really old Mac Mini on which Shotcut is being built that I can test it on, but it may be insufficient to even run. I will have to try again.
As for Debian, from the description (menu outline only and ATI fglrx vs. NVIDIA nouveau) and based on the fact that the menu is basically declarative QML, which uses GL for rendering, it sounds like a Qt GL bug. However, the way Shotcut is embedding QML Quick views into traditional widget "dock" windows could be a factor. That explains why I was asking about your window arrangement.
I am curious to know if the problem also occurs when you click the menu button in the timeline toolbar instead of right-click.