CSS problem with comments

When somebody goes to one of my comments, e.g.,
https://crowhill.net/blog/index.php?p=1700#comments
and starts to write in the comment box, the comment box is “under” the right nav — unlike the box I’m typing in right now which is on top of the dark gray right nav. So the comment author can’t see what he’s typing.
How do I fix this?
My style sheet is available at https://crowhill.net/data/weblog.css if you’d like to take a look.
Thanks,
Greg