I have implemented communication between users using Yjs. And guess what? It works both with webxdc and online (with WebRTC).
To check out WebRTC sync, head to Vikunja , go to “Settings” and check “Enable sharing” and enter room name and password, then open another instance (say, in a different browser) and enter the same credentials. If the signaling server works, your instances will sync (gotta reload the page for now).
.xdc can be found here, as usual.
Source code of the sync part: src/localBackend/sync.ts
I’ll pause the development of Vikunja for now. I’ll be happy to assist whoever finds this relatable.