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Zulip web is one of the best modern chat apps for intermittently offline use, and we put a lot of effort into making it that way.

For example, if you had it open on your laptop with a window open, suspend it, and open it up on a plane, you can read the last few weeks of message history, compose replies that will send when you regain network, etc. I do this regularly on flights.

We always have ideas for how to improve this further, and the mobile app doesn't do as extensive caching as the web app does, but it's not an issue of technical feasibility. The protocol was designed for mixed online/offline use from the beginning.