I've spent about an hour now trying to figure out why the width goes nuts when you post anything. I installed a couple other themes to see if they did it too. They do, but some are in a less annoying way. The default SMF theme doesn't seem to do it.
I turned off all the forum mods, did a fresh install (just to the files, not to the MySQL database) and put all the mods back. None of that did any good. I looked in the code of this theme and the CSS, but didn't notice anything that would cause this. I don't think the problem is in any particular theme, though, since this seems to be happening on all of the themes. I'll start looking through all the settings next. Maybe murd0c got drunk and made the posting width 9,000,000.
In other news...all of the shoutbox mods suck. The one we used before hasn't been updated for the new versions of SMF. Same for SMF Arcade - they don't seem to be updating their software anymore. Maybe we should move to a more popular forum software.