The other option that I can see, though I wouldn't like it as much, is to prevent the code tags from being wider than the screen size and make users scroll horizontally on long lines.
That's the way it's meant to be (as it shows on other browsers). Why wouldn't you like to see it? If the code tags are wider than the page you'll have to scroll horizontally anyway, so why not make it happen within the code box and preserve the readability of the rest of the page. With the old forum's behaviour, if one person posted long code it made every post in the thread frustrating to read.
