Kevin Kofler (./2) :
Without your complete, compilable testcase, there's no way people can debug your issue. The only code you posted is the one which doesn't reproduce it!

I suspect your issue is that the complete program is not using the event loop properly. Any refreshes are done in the event loop.

Yes, I know, that's what's frustrating me. That code is pretty much exactly what I have in the whole program. I just copy-pasted it together. (It's the exact same event callback). I was half hoping someone would be able to say "Aha! you're not setting attribute X on your window, which is needed for TextEdit to work reliably." Anyway, I'll see if I can narrow down where things are going wrong, and hopefully create a properly broken testcase.

Unfortunately, I can't share the actual source because the copyright belongs to my university.