Hi Daniel,
I can confirm that existing configs shouldn't break unless the user has behavior that operates on TabbarLocation (which seems weird, that kind of stuff really belongs in XMonad.Layout.Tabbed). If they *do* have that kind of behavior, they'll need to update their pattern matching to handle the LeftS/RightS cases.

The naming is weird, but it's to prevent conflicts with Either. I was going for a Leftside/Rightside mnemonic. I actually like Adam's idea of just using Direction2D. That should also not break user configs since the interface functions don't ask for Top/Bottom/etc. and instead have separate functions for each task.

Thanks,
Arjun