As of right now, the sidebar is appearing below the content, with vertical and horizontal scrollbars around the #content div.
Imagine there is a red freehand circle around the misplaced sidebar...
This appears on Physics and Physics Meta, but I haven't seen it on any other sites.
I'm using Google Chrome 4545.0.2454.101 46.0.24542490.10171 provided by Google's Yum repository, on Fedora 22 with KDE. I don't see it in other browsers, so I think it may be something specific to Chrome.
Interestingly, when I reduce zoom from 100% to 90%, then the scrollbars disappear and the sidebar returns to its proper place, but then everything's too small...