matrix-doc/proposals/3816-clarify-thread-participation.md

MSC3816: Clarify Thread Participation

MSC3440 defines the m.thread relation type, and the format of the serverside aggregation for them. The definition of the aggregation includes a current_user_participated flag, which is not fully defined:

A flag set to true if the current logged in user has participated in the thread

In particular, it is unclear whether sending the initial event (i.e., the event which is the target of the m.thread relation) counts as participating in the thread.

Known implementations do not count the initial event in this way, and instead implement this as: "has the current user sent an event with an m.thread relation targeting the event", but this has found to give poor user experience in practice.

For example, consider A as the root event in a thread from @alice:example.com, and B as a threaded reply from @bob:example.com. The bundled aggregations for A would include:

Requester	current_user_participated
@alice:example.com	false
@bob:example.com	true

If @alice:example.com sends reply C, this would change:

Requester	current_user_participated
@alice:example.com	true
@bob:example.com	true

The proposed clarification is that @alice:example.com should have always have participated in the thread (i.e. both tables would be true in the example above).

Proposal

The definition of the current_user_participated flag from the bundled aggregations for m.thread relations is updated:

A boolean flag, which is set to true if the current logged in user has participated in the thread. The user has participated if:

• They created the current event.
• They created an event with a m.thread relation targeting the current event.

This better matches the intention of this flag, which is that a client is able to visually separate threads which might be of interest.

Potential issues

The current implementations will need to be updated to take into account the sender of the current event when generating bundled aggregations. This should be trivial since all of the needed information is directly available.

MSC3440 proposes using new filter parameters in order to list threads in a room that a user has participated in. There would now be an inconsistency that threads where the current user sent the root event but has not replied to the thread could not easily be fetched. A future MSC may solve this problem.

Alternatives

Do not clarify MSC3440 and leave it up to implementations to define the behavior of the current_user_participated flag.

Security considerations

None

Unstable prefix

None, the changes above shouldn't dramatically change behavior for clients.