Hang on... I've already done this one, haven't I?
(In all seriousness, this problem has already been fixed, but it fell off of theymos' radar and has just been collecting dust. I recently reminded theymos about it, and I'm pretty certain he'll accept/merge the patch once I'm finished with one last testing/polishing pass. It's good/fortunate timing that logfiles posted a new topic about this, though, because the approach that PX-Z came up with a few posts up is smarter/neater than the approach I used originally, so I'll probably end up revising the patch as a result. @PX-Z: Nice job, man!)
But, if you look at the stylesheet, it's pretty obvious that the SMF devs didn't want [code] to be presented that way (with "wrapped" contents), and I agree with them. So, while the simple CSS tweak is tempting, I don't think that's the right solution here: changing how code renders in response to a pretty specific layout bug seems clumsy/ill-considered to me.
(BTW, last time this came up, you mentioned something similar happening when replying to wide-code-containing PMs: I've got a patch for that, too, and I'll include it when I update that topic.)