Live updates may omit newly rejoined rooms

[kegsay]

Thanks to ganfra for finding this.

I have seen proxyman logs for the following scenario:

• Left the room #ghidra-main:matrix.org from EW
• The room disappear from my room list in EX
• I try to join the room #ghidra-main:matrix.org again from EX
  the room appears back in EW but not in EX
• I send an invite to a private room for the account
• I accept the invite and it appears in the room list, but still no trace of #ghidra-main:matrix.org
• I kill and restart the app : I finally got #ghidra-main:matrix.org

I see the room disappearing, and it not returning until a /sync without a ?pos= is performed. Server-side logs show no errors. The room has ~14k users, which do appear in the changed device lists in the encryption connection, so it's clearly processing the join in a timely manner. 15:54:49 INF Poller: accumulated data device [events,changed,left,account]=[0,14269,51,0] device_id=XYBRWRTVYZ rooms [timeline,state,typing,receipts,invites]=[1,1,0,1,0] user_id=@ganfra0804:matrix.org

Ganfra asserts that this is only apparent on large rooms, and it cannot be reproduced on smaller private rooms.

Timings wise:

• The leave event at 1712764431388 (15:53:51)
• The rejoin event at 1712764447579 (15:54:07)
• Gave up on the sync connection at 15:55:38 (pos=22)
• The initial sync which returns it correctly at 15:55:49

On v0.99.15.

---

Initial thoughts:

• is it really only for large rooms? What about small public rooms?
• If a cache still thought that the user was joined this could explain why it doesn't tell the connection, and why it works on refreshing the connection.
• write an e2e test to repro this.

[kegsay]

This happens due to bad interactions with left rooms remaining in the room list. I think it'll be easiest to just remove them completely when they leave (which matches Element X anyways which looks at the timeline).