FA-76000 / Chat ordering and read receipts / Member archive
Place the "new messages" divider in a loaded window: older unread boundary · case 05
The "older unread messages" banner appears when the window begins right after the marker, or misses a single unloaded unread.
Case contract
messages are ascending [seq, author] pairs of the loaded window that starts at window_start. Sending a message implies having read everything before it, so the effective marker is the larger of marker and the highest seq authored by me. The divider goes before the first loaded message after the effective marker not written by me (index, or None). older_unread is true when some unread seq lies before the loaded window (effective marker < window_start - 1).
Why this case matters
The divider tells users where to resume reading; misplacing it hides or repeats unread history.
One recorded failure
Sample boundary fixtureThis sample comes from the broken implementation of a controlled reproducer.
| Boundary fixture | Actual | Expected | Outcome |
|---|---|---|---|
| window starts right after marker | {"index": 0, "older_unread": true} | {"index": 0, "older_unread": false} | Failed |
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