When a room is left and forgotten quickly, other devices aren't told about the leave in incremental sync

[squahtx]

The spec says

Once a user leaves a room, it will no longer appear in the response to the /sync API unless it is explicitly requested via a filter with the include_leave field set to true.

Rooms which have been forgotten will never appear the response to the /sync API, until the user re-joins, is re-invited, or knocks.

which suggests that rooms that have been left and forgotten should not show up in incremental sync responses, even if the client hasn't found out about leave yet. When a user has multiple devices, this makes it impossible for other devices to know that a room has been left if an incremental sync spans the leave and forget.

Synapse appears to violate the spec and always sends leaves for forgotten rooms over incremental sync, which is arguably more useful to clients.

related: #1096

[turt2live]

Synapse appears to violate the spec and always sends leaves for forgotten rooms over incremental sync, which is arguably more useful to clients.

I wouldn't necessarily consider this as a violation, fwiw. It's certainly more sensible than breaking clients and should end up in the spec.

[Johennes]

I think this would be fixed at least eventually in Simplified Sliding Sync (MSC4186) as clients expand their window to cover the full room list in the background?

[Johennes]

Or actually, no. You would still have the same issue because Sliding Sync only communicates additions to the room list unless you do an initial sync.