Oh! I know what you are talking about and remember the issue. You had/have to add `setWMName "LG3D"' to the configuration since versions older than Java 7 don't include Xmonad on their hardcoded list of suppported window managers. Correct me if I'm wrong, but it doesn't seem like that one was Xmonad's fault.

Interesting, to be honest I don't really use any extensions. I do use Java apps and openoffice these days and they work fine.

Thanks for your response! I'll have to try adding some extensions and see what happens.