Thanks very much for the examples ! I get your point but it does seem quite more involved to do all that management and broadcasting by hand rather than having a shared worker with a EventSource singleton for all, don't you think : https://stackoverflow.com/a/61636606/10469162
Even on HTTP/2, I believe it's best avoid sending 6 times the same message to the user just because they opened 6 tabs. It's less wasteful both for the server and the client.
[1] https://developer.mozilla.org/en-US/docs/Web/API/BroadcastCh...
[2] https://github.com/tracim/tracim/blob/develop/frontend_lib/s...