MSC1763 local retention is bypassed for events paginated into filtered timeline sets

[itsmorninghao]

Description

When unstableMSC1763Retention is enabled and a room has an effective
m.room.retention policy with max_lifetime, expired events can still
become visible after backward pagination.

This can be reproduced in Element Web with feature_retention enabled:
messages older than max_lifetime disappear initially, but scrolling
far enough backwards can load and display them again.

Suspected cause

There appear to be two related paths:

1. Room.addEventsToTimeline() checks
   this.retention?.shouldEventBeRetained(event), but pagination code
   calls EventTimelineSet.addEventsToTimeline() directly.

2. RoomRetentionPolicy.processTimeline() only scans
   room.getLiveTimeline(). Events inserted into filtered or detached
   timeline sets are therefore not found by the cleanup pass.

Minimal regression test

Tested against matrix-js-sdk develop commit:

2a625eb8f611a701867ac47f3e30345cfbb13f00

it("does not expose expired events paginated into a filtered timeline", async () => {
    getCachedMock.mockReturnValue({
        policies: {
            [ROOM_ID]: { max_lifetime: ONE_DAY_MS },
        },
    });
    await applyPolicy();

    const filter = new Filter(USER_ID, "test_filter");
    filter.setDefinition({});

    const timelineSet = room.getOrCreateFilteredTimelineSet(filter, {
        prepopulateTimeline: false,
        useSyncEvents: false,
    });

    const expiredEvent = makeMessageEvent(
        Date.now() - 2 * ONE_DAY_MS,
    );

    timelineSet.addEventsToTimeline(
        [expiredEvent],
        true,
        false,
        timelineSet.getLiveTimeline(),
        null,
    );

    await vi.advanceTimersByTimeAsync(201);

    expect(
        timelineSet.getLiveTimeline().getEvents(),
    ).toHaveLength(0);
});

Result:

Expected: 0 events
Received: 1 event

All 31 existing room-retention tests passed; only this new pagination
regression test failed.

Runtime logging also reported:

Running processTimeline
Found no expired events

Expected behavior

Expired events should not be inserted into any visible timeline set, or
they should be synthetically redacted/removed immediately after
pagination.

Actual behavior

Expired events inserted through historical pagination remain in a
filtered timeline set and can be displayed.

Affected consumer

Reproduced in Element Web 1.12.25 with feature_retention enabled and a
room policy using max_lifetime.

Possible direction

Either retention should be enforced at the EventTimelineSet insertion
boundary, pagination should route through the retention-aware Room API,
or the cleanup process should cover all relevant room timeline sets.

[shyamsharmas124-commits]

Opened a fix for this — root cause matches your analysis exactly: EventTimelineSet.addEventsToTimeline() has no retention check, so both pagination (client.ts) and thread timelines (thread.ts) bypass the filtering Room.addEventsToTimeline() does.

Went with your first suggested direction — enforcing at the EventTimelineSet insertion boundary rather than routing pagination through Room's API or extending the sweep in processTimeline(). EventTimelineSet already holds a reference to its parent Room, so this covers every insertion path (live, filtered, thread) in one place without new plumbing, and Room.addEventsToTimeline() now just delegates instead of duplicating the check.

processTimeline()'s periodic sweep still matters independently of this — it catches events that were valid on insertion but have since expired while the timeline stayed open, which this fix doesn't touch.

Added your regression test as-is to room-retention.spec.ts, confirmed it fails on unpatched develop and passes with the fix. Full existing retention suite plus every spec touching addEventsToTimeline still green.

PR: https://github.com/shyamsharmas124-commits/matrix-js-sdk/pull/new/fix-msc1763-timelineset-retention