Unread count goes from zero through the roof after restarting the app

[Johennes]

Steps to reproduce

1. Go to inbox zero in https://matrix.to/#/#ps-features-team:matrix.org
2. Restart and relaunch Element Desktop
3. Open the room
4. Notice how the unread count slowly jumps up to ~500

Outcome

What did you expect?

Lots of messages are marked unread

What happened instead?

No unread count in the room

Operating system

macOS

Application version

Element Desktop nightly

How did you install the app?

No response

Homeserver

element.io

Will you send logs?

Yes

[turt2live]

I've noticed this primarily on DMs, ftr

[Johennes]

Can't reproduce the rapidly rising unread count anymore as of this morning. Travis had posted to the main timeline in the room to clear his stuck notification which might be related.

[weeman1337]

I can also see this problem from time to time. That happens rarely.

[germain-gg]

This issue is a duplicate of #25450

Digging through the logs I can find the following exception

2023-05-30T14:48:53.690Z E Error sending receipt {"room":"!pDXDyxlzCKaaGYESIR:matrix.org","error":{"httpStatus":400,"url":"https://element.ems.host/_matrix/client/r0/rooms/!pDXDyxlzCKaaGYESIR%3Amatrix.org/receipt/m.read/%24JgIrwa6haKY-PoeAGUzyzR3Ceii9ywNLl_057T39dPg","errcode":"M_INVALID_PARAM","data":{"errcode":"M_INVALID_PARAM","error":"event_id $JgIrwa6haKY-PoeAGUzyzR3Ceii9ywNLl_057T39dPg is not related to thread main"},"name":"M_INVALID_PARAM"}}

The last event in the main live timeline is a reaction that belongs to a thread. Your accumulated read receipt refers to an older event.
What happens is that the client tries to send a read receipt agains the latest event in the live timeline, but it should not have ended up there.
The client eagerly clears the notification count, but the API call fails, and this failure mode is not taken into account... which means that upon a restart, your accumulated read receipt and the server read receipt has not changed because the API call has failed, you're therefore replaying the notification.