Thanks for the suggestion. I skimmed their docs before briefly; I'm giving it a second look now, and realizing I passed by way too quickly last time.

It looks like they did tackle acausality head-on; I like what I see about previous message IDs being included in https://matrix.org/docs/spec/#pdus

Nervous about ever christening JSON "today's technology". JSON has the same problem XML in XMPP does: can't tunnel binary e.g. video without extreme backbending (jingle for XMPP is pretty much a textbook example of "too much friction -> not fit for purpose and near zero adoption"). But it looks like matrix is pluggable on transports, too, so that's potentially a sweet spot for lazy REST clients and powerful performance situations to coexist.

... And I popped into their self-hosted chat room. Observation 1: It works (!). Observation 2: the devs are dogfooding and seem both knowledgeable and friendly :) I haven't taken the leap to self-host yet (will later!); but from preliminaries, matrix is looking remarkably solid and well thought out. I'm going to try to build something with this! Thanks again for the link!