Thread reply missing from thread in the mix of sending my own thread reply

[MadLittleMods]

Steps to reproduce

1. Someone else threads off their own message and sends some thread replies
2. In the mix of them sending more thread replies, you send a thread reply of your own
3. Notice a missing thread reply from the other person. Probably some race condition between your message being sent and the other thread replies.

(details in rageshake issue https://github.com/matrix-org/element-web-rageshakes/issues/19979)

Also discussed in an internal room.

Outcome

What did you expect?

All thread replies visible especially since it seems like it reached my client.

What happened instead?

There is a missing thread reply from the other person.

I did see an unread message indicator in the room preview. This phenomena is very hard to notice since I only caught the missing message from a notification on my other computer.

Operating system

Windows 10

Browser information

Chrome Version 109.0.5414.120

URL for webapp

develop.element.io

Application version

Element version: 27f1d99-react-39fe72e53ad6-js-8a3d7d5671e5 Olm version: 3.2.12

Homeserver

matrix.org

Will you send logs?

Yes, https://github.com/matrix-org/element-web-rageshakes/issues/19979

[Johennes]

@MadLittleMods how often have you experienced this since filing the issue?

[MadLittleMods]

@Johennes I think 3 times. It seems pretty easy to reproduce in a situation as described above. Was pretty common with two of us going back and forth in a thread (answering someone's Gitter questions).

[Johennes]

I think I've run into another occurrence of this. Details in https://github.com/matrix-org/element-web-rageshakes/issues/22198.

[daniellekirkwood]

I think we've had several more reports of this (i've had a few people in my DMs this past week)

@Johennes does that mean there's a new, different issue, or this one is getting worse (and needs to be changed from occasional)?

[Johennes]

It's hard to say since we don't know the actual root cause. It might as well be something new or just the old issue increased in frequency by a tangential change.

[Johennes]

We chatted this through in the WAT weekly today and we'll not escalate fixing this. We have other urgent work that is aligned with company-wide strategy this week and are generally not confident in making changes around the threads model without the refactoring and test suite mentioned in #24392. So this is, sadly, gated behind that work unless somebody would like to step in.