The CRDT I was referencing was Shelf by Greg Little. He's given a few talks about it at the braid meetups. When he first showed it off, Kevin Jahns (the Yjs author) was also there and was as impressed as I was:

https://braid.org/meeting-8 (the shelf part with Greg starts at about 43 minutes in to the recording.)

The code is all here. Its tiny:

https://github.com/dglittle/shelf