Right now all I know is that toggling fullscreen mode disables the accent preview that appears before hitting the letter. I have no clue what makes it output nothing after that.
Initially:

After toggling full-screen (this is then persistent even when changing screen modes):

After ???? happens, and until the app is closed and restarted:

I've experienced it in builds 2088-2089 at least, on OS X Snow Leopard.