Waaaaa! Don't make me reply again, I'm going cross eyed.
*ahem*
I have a suspicion that it might be the "Quick Reply" that causes the problem.
Prior to today, I replied to a thread using quick reply and noticed that the width
had gone awry on the reply screen.
Today I replied to a thread - it defaulted to "Quick Reply" and I noticed the same
issue and switched to "Go Advanced" and the problem disappeared, but once I'd
submitted my reply the width was all over the place again.