fix(ui): preserve last-message preview during channel-state reloads

[VelikovPetar]

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Linear: FLU-549

Github Issue: #

CLA

• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

Backport of master commit 36d8a1c94ab74448680c67531460ac10977f7d8a — "fix(ui): preserve last-message preview during channel-state reloads (#2774)".

Why

The same last-message-preview flicker exists on v9. When opening a channel with unreads, Channel.query(idAround:) runs state.truncate() (emits an empty messages list) immediately followed by updateChannelState() (emits the populated state), so the channel-list cell briefly flashes the empty-state text ("There are no messages currently") before snapping to the real preview.

The preview widget already had a _currentLastMessage cache field meant to absorb that transient empty emission, but in _ChannelLastMessageTextState it was declared and never assigned — making the absorber a no-op, so the flash was visible.

The fix

Assign _currentLastMessage = message on every build, gated on channelState.isUpToDate, and compute it before .latest so the freshly-stored value participates in the selector:

• While isUpToDate is false (the idAround truncate mid-load), fall back to the cached message so the preview keeps showing the real latest message.
• A real channel.truncated event keeps isUpToDate == true, so the cache correctly clears and the empty-state surfaces.

Manual port (not a clean cherry-pick)

• The source file was renamed on master (stream_channel_list_item.dart) vs v9 (stream_channel_list_tile.dart).
• Master applied the same fix to two widgets (_ChannelLastMessageWithStatus and ChannelLastMessageText). _ChannelLastMessageWithStatus does not exist on v9, so only the ChannelLastMessageText hunk was ported (there is no cache to fix in v9's inline sending-indicator path).
• The regression tests were extended into the existing v9 stream_channel_list_tile_test.dart (which already covers this widget) rather than the standalone file master added. Three scenarios were added: preserves the last-known message while not up-to-date and emitting empty, rebinding to a different channel shows that channel's state, and shows the empty-state on truncate while up-to-date.
• The empty-state string differs on v9 ("There are no messages currently" vs master's "No messages yet"), so the test constant was adapted accordingly.

Verification

• melos run format → 0 changed
• dart analyze --fatal-infos → no issues
• 8/8 tests pass in stream_channel_list_tile_test.dart (5 existing + 3 new)

Screenshots / Videos

No UI changes. (Backport of an already-reviewed fix; behaviour verified via the added regression tests.)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (v9@1d7a5a7). Learn more about missing BASE report.

Additional details and impacted files

@@          Coverage Diff          @@
##             v9    #2811   +/-   ##
=====================================
  Coverage      ?   66.80%           
=====================================
  Files         ?      429           
  Lines         ?    27100           
  Branches      ?        0           
=====================================
  Hits          ?    18105           
  Misses        ?     8995           
  Partials      ?        0           

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