Relation event insertion breaks during backwards pagination if parent is on another page (if supportsThreads: true)

[davidisaaclee]

Assume that we have a PARENT event a CHILD event which is an m.replace of PARENT, sent a long time after PARENT was sent (so there are some pages between the two events).

If I have supportsThreads: true, and I'm backfilling the room via paginateEventTimeline (going in backwards direction), I hit CHILD first, going through addEventsToTimeline -> eventShouldLiveIn, leading me to...

matrix-js-sdk/src/models/room.ts

Line 2171 in b515cdb

parentEvent = this.findEventById(parentEventId) ?? events?.find((e) => e.getId() === parentEventId);

Since I didn't hit PARENT yet (and it is not in the page that is currently being processed, i.e. events), parentEvent is not found, causing the following case to fail (I think this case is the one that should be hit, although we don't have parent so we can't recur):

matrix-js-sdk/src/models/room.ts

Lines 2174 to 2177 in b515cdb

	// Treat non-thread-relations, redactions, and replies as extensions of their parents so evaluate parentEvent instead
	if (parentEvent && !isThreadRelation) {
	return this.eventShouldLiveIn(parentEvent, events, roots);
	}

All other branches fail, causing the function to return:

matrix-js-sdk/src/models/room.ts

Lines 2198 to 2204 in b515cdb

	// We've exhausted all scenarios,
	// we cannot assume that it lives in the main timeline as this may be a relation for an unknown thread
	// adding the event in the wrong timeline causes stuck notifications and can break ability to send read receipts
	return {
	shouldLiveInRoom: false,
	shouldLiveInThread: false,
	};

This omits CHILD from the timeline (and I think shows PARENT instead?).

(I'm probably going to just disable supportsThreads to workaround this for myself.)