FA-75873 / Chat ordering and read receipts / Member archive

Advance a stored read marker from a device report: monotonic advance · case 03

A delayed report from a slower device drags the read marker backwards and resurrects already-read messages.

Member previewVariant 3 · 3 implementations · 8 checks per implementation

Case contract

A report from a device session whose epoch differs from the current epoch (for example a session that predates a "mark unread") is ignored. Otherwise the report is clamped to latest_seq and the marker only moves forward. Result: {marker, broadcast (true only when the marker changed), unread = max(0, latest_seq - marker)}.

Why this case matters

Read markers drive unread badges on every device; regressions or stale sessions resurrect or hide unread messages.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
older report arrives late	`{"broadcast": true, "marker": 43, "unread": 20}`	`{"broadcast": false, "marker": 53, "unread": 10}`	Failed

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