This SE site has a lot of long code postings. Usually these are just copy pasted from actual source code.
The maximum line width before a horizontal scrollbar appears, seems to be only 80 characters. Times when 80 was considered an optimal maximum line width are long gone.
For other SE sites, I feel it's the posters duty to make sure the code is readable, and only the necessary code is presented. For Code Review I do feel you should be able to just post code without having to edit it for better viewing.
Several proposals have already been done:
Perhaps a simple or intelligent wordwrap is also suitable. Consider the revision of this question I posted as something I would like to see automated.
I forgot to mention, couldn't a different column layout work as well? Where the code/question part resizes freely to the remaining space, and only the right column is fixed?