I've been breezing through MediaWiki:Common.js on occasion now, since I am thinking of cleaning up that as well eventually (just like I did wiki's CSS already.) However, my JS skills are rusty and it's harder to figure out what the JS code is for than CSS, especially given JayAaerow somehow wasn't taught that commenting the code is one of the most important parts of a programmer's work.
I would ask him directly, but I know he hasn't been answering his message wall for a while now. You were on the wiki while he was still here, plus I saw you in the history of that file. Can you shed some light on what he or you both were trying to do?
Not sure about what he was doing tbh but the tabbers were being hella wonky at one point. I asked the wikia community for help and they told me that the code for tabbers was in there twice so i deleted the lines they said.