fix(llc): handle per-channel notification.mark_read events again

[xsahil03x]

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Linear: FLU-693

Fixes: #2886

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Description of the pull request

Backport of master commit a9b048261 — "fix(llc): handle per-channel notification.mark_read events again" (#2890).

v9 is affected: the regression landed in 9.20.0, so every v9 release from 9.20.0 onward carries it.

ChannelClientState._listenReadEvents subscribed to both message.read and notification.mark_read until the delivery-receipts refactor (#2429, 3b5e410bb) split them into separate listeners and did not carry notification.mark_read over.

That distinction is load-bearing:

• message.read is a channel event — delivered to watchers only.
• notification.mark_read is delivered on the reading user's own connection — so it reached the channel whether watched or not.

Channel.markRead() writes no local state itself, and channels are easily non-watched (watch is silently downgraded when there is no connection id). So marking a non-watched channel as read succeeded server-side while unreadCount stayed stale forever — the unread pill and divider survived the dismiss X, markReadWhenAtTheBottom, and every reopen, with no user action able to clear them.

Port notes

Effectively a clean cherry-pick. v9's _listenReadEvents is byte-identical to master's pre-fix version — including the thread guard and the delivery-field preservation — so the fix applies unchanged:

_channel
    .on(EventType.messageRead, EventType.notificationMarkRead)
    .listen(

Two mechanical differences from the master commit:

1. Formatting. v9 formats at 80 cols with Dart 3.6's short style (pinned to Flutter 3.27.4 via .fvmrc), where master is 120/tall. The single-line .on(...).listen( from master is 82 chars, so it wraps across three lines. Formatted with the pinned 3.6.2 SDK rather than a newer local one, which would have restyled the entire file.
2. CHANGELOG.md resolved manually into v9's ## 9.27.0 section (the only cherry-pick conflict).

The test file applied without conflict.

Testing

Four tests in channel_test.dart. On master I verified they actually catch the regression by reverting the one-line fix and re-running — three go red (the fourth is a negative test asserting thread-scoped reads are ignored, so it passes either way):

Test	Without the fix
resets unread count on a non-watched channel	fails: Expected: <0> Actual: <10>
preserves delivery info	fails: Expected: <0> Actual: <10>
reconciles delivery for the current user	fails: No matching calls
ignores thread-scoped mark-reads	passes (guards the guard)

Verified on v9 with the pinned toolchain (Dart 3.6.2 / Flutter 3.27.4):

dart test                                   1306 passed / 2 skipped
dart analyze --fatal-infos lib test         No issues found
dart format --set-exit-if-changed lib test  226 files, 0 changed

Out of scope

Same as the master PR:

• Client-level counters are not broken. The issue reports totalUnreadCount / unreadChannels going stale too, and the reporter shipped a client-level listener for it. That part isn't needed — ClientState.subscribeToEvents applies both from any event carrying them.
• Persistence package causes double unread count to appear in scroll to bottom button and no unread count on the back button when opened from message notification #2202 (reconnect sync() replay re-incrementing unreadCount via the non-idempotent addNewMessage) is real and related, but a separate bug.

Screenshots / Videos

No UI changes — beyond the unread pill and divider now clearing, which is the bug itself.

🤖 Generated with Claude Code

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (v9@36bde74). Learn more about missing BASE report.

Additional details and impacted files

@@          Coverage Diff          @@
##             v9    #2891   +/-   ##
=====================================
  Coverage      ?   67.30%           
=====================================
  Files         ?      431           
  Lines         ?    27390           
  Branches      ?        0           
=====================================
  Hits          ?    18434           
  Misses        ?     8956           
  Partials      ?        0           

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