No threads, and it happens while the game is running.   Also, I'm using the latest ruby gem.

Strangely, it only happens in my level editor, and not in the game proper, even though I'm pretty sure the rendering logic is the same.  I'm getting other issues as well, like things not loading correctly, so it might just be me.  However, I don't think I got this issue with the other versions of gosu, and I heard that recently there was a modification which dealt with window damage in X.

...just tested it again and I can't reproduce the error =/
Sorry about that.  Just ignore this problem for now, and I'll post again if I can isolate the issue.