Love this extension! Couple of things I've noticed...
1) The title bar seems to deactivate when I click inside the game itself. The frame only shows active when I click on the frame itself. This doesn't break anything, but does seem like an odd user experience all the same. This may just be an unavoidable consequence of the fact that there are technically two windows. It'd be lovely if there's a way to fix it, but not end of the world if not.
2) A bit more problematic is that the resize arrows seem to stop showing up after the first time I resize the frame. It's still resizable, the cursors just don't show correctly. I'm working around this by setting the cursor myself, but I thought I'd let you know this is happening in case you weren't aware of it.
I'm not super familiar with WinAPI so I don't really know how easy either of these is to fix, but if you don't get to it in the next... oh, few months while I'm working on my game, I'll probably look into it eventually and see if I can't make you a pull request for it, since you've helpfully provided source. :)