When both the store and the cache held a marker for one session, the merge
chose between them by completed_at while the clear comparison used unread_order.
Two completions in the same millisecond therefore kept the earlier marker and
its message count and cron profile metadata.
Compare unread_order on both sides, matching the ordering the rest of the unread
state uses, and record in the RFC that a deletion record's expiry can let a very
long-lived tab re-add a count for a session deleted more than seven days ago —
bounded and invisible, since that session is no longer listed.
Michael Snoyman
d61d7212fb
