They are exchanged between the two devices.
Have you tried using Signal on desktop? It doesn’t offer history syncing. Cross device for whatsapp for example is also a terrible experience. Unusable for something like Discord.
For a seamless experience Discord would probably have to store the keys themselves, defeating the whole point.
Yep, guess that’d do it.
You’d still have to handle transferring keys across your devices, every time you login on a new one.
Also, searching would probably not work, at least as well as it does right now, since all messages are indexed on the discord side, which they wouldn’t be able to do without seeing them. Everything would have to happen on device, meaning the devide would have to store all messages.