[janekm]

Nothing so complicated. All that's needed is a local cache so that when you type a new message in the chat window, you immediately see it appear when you hit submit (optionally with an indication of when the message was received by the peer). But there's quite a bit of tooling required to reliably update the local cache, run the code both in the client and on the server.

[yencabulator]

Firebase does this brilliantly with Firestore queries. Any data mutation by the client shows up in persistent searches immediately, flagged as tentative until server acknowledges.