Messages can appear out of order if /sync is delayed but message PUTs are 200ing

[ara4n]

If you send a bunch of messages on a slow connection, they stack up in grey. It's possible they then go black, as you receive 200s for them. However, if you then receive /sync response the syncs can end up before the queued messages, making them appear out of order. At least, i think that's what's going on.

It's closely related to element-hq/element-web#639.

I've seen it a bunch and we just got a report at https://matrix.to/#/!DgvjtOljKujDBrxyHk:matrix.org/%2414864078871816754pzfgA:matrix.org of it.

[axelsimon]

Can expand a bit. The experience is that message end up appearing earlier in the conversation (which I think is ok) but without any visual cue this has happened (bad!) which can lead to real world/real human misunderstandings and frustration ;)

Also, it's somewhat inconsistent in terms of experience with the fact that when messages have failed to send, riot offers to resend them, showing it's clearly aware of intermittent connectivity issues :)

[richvdh]

also related to #1900?