Permalinks to threads are difficult for clients to render

[clokep]

The /context API gets used to render the room timeline when clicking on a permalink to a message (since the client may not have all the surrounding events). Due to the /context API returning non-sense for events in a thread (see below) it is difficult to properly render a portion of a thread when clicking on a permalink to a thread.

The workaround is to fetch the entire thread using the /relations API, which could be expensive for a large thread and involve many round-trips, which is a poor user experience on mobile.

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Technical bits

Calling /context on a threaded event returns other events around it in the topological order of the room, which is not very useful.

For example if you have the following events in a room where D is a threaded reply to A:

flowchart RL;
	F-->E;
	E-->D;
	D-->C;
	C-->B;
	B-->A;
	D-->|`m.thread` relation|A;
	F-->|`m.thread` relation|A;	

Loading

Calling /rooms/!roomId/context/D?limit=1 would return something like (note that this is simplified since we just care about the events):

{
	"event": "D",
	"events_before": ["C"],
    "events_after": ["E"]
}

This is nonsensical since D has nothing to do with C or E, it would likely make more sense to include A as the "event before" (as the root event) and perhaps F as "events after" as another event relating to A.

In the contrived example above you can get around it by increasing the limit, but this is not a reasonable workaround in a busy room.

I think this probably applies to any relation, not just threads, but is "worse" for threads since you likely wouldn't call /context on e.g. an annotation.

As a related piece of work, the above also applies to any location where we fetch "context" for an event. Other locations include:

• Search results can include surrounding events and suffer from the exact same problem described above.
• Email notifications sent from Synapse include surrounding messages, but I believe this could be changed without any spec changes. (See Improve email notification context for threads matrix-org/synapse#11976.)

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Potential work breakdown:

1. Write an MSC to describe the change in behavior.
2. Implement the MSC in Synapse.
3. Implement the MSC in clients.

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Background

It was briefly discussed in MSC2675 whether the /context API should be changed for relations. See the screenshot below because that PR seems to break GitHub:

[clokep]

Note that the workaround for this isn't as dire as mentioned in the above, essentially if the event returned includes a relation, then a second request to /relations can be made on the root event. Hopefully the thread is short and pagination is then not needed.