Matthew Weidner was a student of Martin Kleppmann. He made collabs, a typescript CRDT library similar to Yjs or Automerge.
As a transport protocol he uses a matrix group chat, which I find really interesting. The app itself then is displayed as an element custom widget.
Fun quote:
In the single user case, you could just make it in the weekend, put it up and forget about it.
[…]
Basically at this point, you are thinking of quitting your job and starting a startup, all just to add collaboration to this app.