I am not an expert in CSS, but it seems that the issue is in
#footer. I think that since Mi Yodeya (unlike all the other SE sites I checked1) is using a repeated image as a background, the
repeat-x property is repeating enough to fill only its div's width. In this case, it seems that the two offending widths are
100% and a default
auto, respectively. Adding a
min-width property to both seems to fix it.
1Upon further investigation, it seems that the current SE beta theme has the same problem.