Viewing 2 of the 3 threads in a specific room causes an unread dot on the room

[turt2live]

Steps to reproduce

Just in case it's relevant:

1. Be on a version of Synapse without stable threads support
2. Be on a version of Element with stable threads support (and threads enabled)
3. Have 3 threads in the room, using stable identifiers
4. Upgrade Synapse
5. Refresh Element

Then for the issue:

1. View the threads one by one, noting the unread dot between them

Outcome

What did you expect?

For the room to stay marked as read, as I had the room read.

What happened instead?

The unread dot appears.

Some potentially relevant information:

• The room is set to "Mentions Only", so there might be an unread count I'm not seeing
• The thread which "fixes" the issue is the thread with my event as the root event
• The thread which "fixes" the issue is the middle thread in the panel
• The thread with my event as a root event was not started by me

Operating system

Windows 10

Application version

Nightly (2021-03-17)

How did you install the app?

The Internet

Homeserver

t2l.io

Will you send logs?

Yes

[germain-gg]

Duplicate of #21114