Hmm, no, it seems that the problem is not that it's not basing it off of the width of the desktop rather than the browser window: I just maximized my browser, and the result was just that the right-hand column got even wider. Still have to horizontally scroll to see the right of it.
I guess maybe it's more like the widths are being specified as percentages, but those percentages add up to more than 100%?
When playing a game or otherwise looking at a page with a board on it, the right-hand column (which typically has, for example, the move list of the game) is now much too wide, to the point that I have to horizontally scroll to look at the right-hand side of it. Was the site's CSS recently changed?
I'm just guessing, but perhaps it's basing the displayed width off of the width of my desktop, rather than the width of my browser window? Off the top of my head, I don't know if that's even possible with CSS and such, but at the very least, what's happening looks like what I imagine would happen if it were doing that.