The recent Steam2 leak apparently leaked the entire codebase of F-Stop (I think they said it was in an early L4D2 build, but I'm not certain). Still no maps, but it's pretty cool. I'd compile it myself, but I already spent 7 hours getting nowhere with it, so hopefully someone here can do it.
Download:
https://drive.google.com/file/d/1W9Xm_QM5x3VRuBWUigKCCtSXoKznTcCa/view?usp=sharingCombine this with the F-Stop assets found in the 2009 Corehub build, and we're
that much closer to actually having F-Stop.
(PS, it also contains code for other extremely early Portal 2 entities, like an actually-hovering Hover Turret.)