Fairly certain this is the right category for this thread, as it relates to the forum and the software itself, more than anything else. I've never used Discourse in particular, though I've done a lot of research on it (and its similarities to early Vanilla), so I think this should be possible.
In the images below, you can see what the navigation looks like in Google Chrome, and I've posted a small suggestion for the navigation underneath that. The new navigation box wouldn't have to scroll with the rest of the forum, so it would be fairly simple to implement, honestly. Just thought I would throw it out there, as it would make the forum a bit more dynamic and clean up just a small nitpick.