[xwiki-users] Darkened background during file upload dialog problem

I'm having a bit of an odd issue. When I go into the WYSIWYG editor and click "Link" -> "Attached File" (or any of the others), when the dialog box comes up and the regular page in the background is darkened, my window width expands to a huge width. Admittedly this doesn't stop the user from doing anything, but it's annoying and shouldn't happen. I'd really appreciate it if someone could tell me why this is happening and how to fix it. Some additional info that I think may be pertinent. The skin I've created is a fixed width of 870px justified to the left. Also, this occurs in both Firefox 3.6.13 and Explorer 8, so I don't think it's a browser issue. Thanks for any help, Lenny