Two WebUI clients on the same origin/profile share one localStorage but each
cache the unread stores in module state and re-serialise that cache wholesale.
With no storage listener for either key, a stale client overwrites the other's
writes: a session's viewed count can roll backwards (re-lighting a cleared
unread dot via message_count > viewed_count) and a cleared completion-unread
marker can be resurrected.
Fix the class:
- max-merge viewed counts on save so a count is monotonic and additive across
clients, never dropping another client's entries or lowering a value;
- keep the in-cache viewed count monotonic on set;
- delete a viewed count directly from the persisted map (removals must still
leave disk, unlike the additive set path);
- invalidate both unread caches on storage events so a concurrent write is
re-read instead of overwritten.
Adds tests/test_unread_cross_client_sync.py (node-driven, observable behavior):
the viewed-count rollback fails before the fix and passes after; the clear path
keeps pruning; the storage-event handler prevents marker resurrection.
Michael Snoyman
ef31e3ac43
