Very old events appear out of order in the live timeline after federation backfill

[matrixbot]

Created by @ richvdh:sw1v.org.

when synapse decides to backfill get_missing_events events over federation, these end up being sent to clients over /sync, which leads to very confusing timelines with old messages

[ara4n]

element-hq/element-web#897 might be related?

[richvdh]

yes, they sound like duplicates

[richvdh]

... so let's close this one.

[richvdh]

Hrm, I'm not convinced they are dups

[ara4n]

This is getting really nasty with federation glitches recently. It'd almost be better to drop events with ancient timestamps than append them to the live timeline.

[jfrederickson]

I'd say drop events with ancient timestamps if the surrounding events aren't in the current timeline view, otherwise insert them in the correct place in the timeline.

Possible gotcha: a malicious homeserver could forge an event that appears to have been sent in the past. (But maybe that's already possible and they look like older events after a refresh?)