## Summary
This PR fixes TUI transcript-sync behavior for
`EventMsg::ThreadRolledBack` and makes rollback application order
deterministic.
Previously, rollback handling depended on `pending_rollback`:
- if `pending_rollback` was set (local backtrack), TUI trimmed correctly
- otherwise, replayed/external rollbacks were either ignored or could be
applied at the wrong time relative to queued transcript inserts
This change keeps the local backtrack path intact and routes non-pending
rollbacks through the app event queue so rollback trims are applied in
FIFO order with transcript cell inserts.
## What changed
- Added/used `trim_transcript_cells_drop_last_n_user_turns(...)` for
rollback-by-`num_turns` semantics.
- Renamed rollback app event:
- `AppEvent::ApplyReplayedThreadRollback` ->
`AppEvent::ApplyThreadRollback`
- Replay path (`ChatWidget`) now emits `ApplyThreadRollback`.
- Live non-pending rollback path (`App::handle_backtrack_event`) now
emits `ApplyThreadRollback` instead of trimming immediately.
- App-level event handler applies `ApplyThreadRollback` after queued
`InsertHistoryCell` events and schedules redraw only when a trim
occurred.
- When a trim occurs with an overlay open, TUI now syncs transcript
overlay committed cells, clamps backtrack preview selection, and clears
stale `deferred_history_lines` so closed overlays do not re-append
rolled-back lines.
- Clarified inline comments around the `pending_rollback` branch so
future readers can reason about why there are two paths.
## Why queueing matters
During resume/replay, transcript cells are populated via queued
`InsertHistoryCell` app events. If a rollback is applied immediately
outside that queue, it can run against an incomplete transcript and
under-trim. Queueing non-pending rollbacks ensures consistent ordering
and correct final transcript state.
## Behavior by rollback source
- `pending_rollback = Some(...)` (local backtrack requested by this
TUI):
- use `finish_pending_backtrack()` and the stored selection boundary
- `pending_rollback = None` (replay/external/non-local rollback):
- enqueue `AppEvent::ApplyThreadRollback { num_turns }` and trim in
app-event order
## Tests
Added/updated tests covering ordering and semantics:
-
`app_backtrack::tests::trim_drop_last_n_user_turns_applies_rollback_semantics`
- `app_backtrack::tests::trim_drop_last_n_user_turns_allows_overflow`
- `app::tests::replayed_initial_messages_apply_rollback_in_queue_order`
-
`app::tests::live_rollback_during_replay_is_applied_in_app_event_order`
-
`app::tests::queued_rollback_syncs_overlay_and_clears_deferred_history`
- `chatwidget::tests::replayed_thread_rollback_emits_ordered_app_event`
Validation run:
- `just fmt`
- `cargo test -p codex-tui`
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