A single "last viewed" timestamp probably solves 99% of the problems. Lists should have their one.
Sounds infeasible
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It's also very simple, lightweight and easy to implement. Pro and cons.
but it won't scale
You just want to sync some clients at the same temporal point, where do you see scaling issues?
The only downside I can see is if a relay is down and so the first client doesn't fetch some post, you lose them.
