History is swallowed before point of joining room

[bwindels]

Caused by #160 mitigation.

I guess Hydrogen could be a bit more forgiving in how it handles duplicate events in /messages

It assumes that the first duplicate event is on a different fragment and that your pagination is colliding into that fragment and hence they need to be linked up and the rest of the events will also overlap with the fragment. If that fragment is the same as the fragment who's gap your filling, it screams #160 and prevents it linking up with itself.

Perhaps I could check overlap for every event in the /messages response and filter out the ones on the same fragment.

I also wonder if federation makes this more likely to happen on the server side. IIRC federation can cause events through /sync and /messages to be in different order. I've filed this before as matrix-org/synapse#7164 but it's expected (and somewhat underspecced) behaviour.