tui: allow forward navigation in backtrack preview (#9059)

Fixes #9058

## Summary
When the transcript backtrack preview is armed (press `Esc`), allow
navigating to newer user messages with the `→` arrow, in addition to
navigating backwards with `Esc`/`←`, before confirming with `Enter`.

## Changes
- Backtrack preview navigation: `Esc`/`←` steps to older user messages,
`→` steps to newer ones, `Enter` edits the selected message (clamped at
bounds, no wrap-around).
- Transcript overlay footer hints updated to advertise `esc/←`, `→`, and
`enter` when a message is highlighted.

## Related
- WSL shortcut-overlay snapshot determinism: #9359

## Testing
- `just fmt`
- `just fix -p codex-tui`
- `just fix -p codex-tui2`
- `cargo test -p codex-tui app_backtrack::`
- `cargo test -p codex-tui pager_overlay::`
- `cargo test -p codex-tui2 app_backtrack::`
- `cargo test -p codex-tui2 pager_overlay::`

---------

Co-authored-by: Josh McKinney <joshka@openai.com>

codex-rs/tui/src/app_backtrack.rs

@@ -93,8 +93,9 @@ pub(crate) struct PendingBacktrackRollback {
 impl App {
     /// Route overlay events while the transcript overlay is active.
     ///
-    /// If backtrack preview is active, Esc steps the selection and Enter confirms it.
-    /// Otherwise, Esc begins preview mode and all other events are forwarded to the overlay.
+    /// If backtrack preview is active, Esc / Left steps selection, Right steps forward, Enter
+    /// confirms. Otherwise, Esc begins preview mode and all other events are forwarded to the
+    /// overlay.
     pub(crate) async fn handle_backtrack_overlay_event(
         &mut self,
         tui: &mut tui::Tui,
@@ -110,6 +111,22 @@ impl App {
                     self.overlay_step_backtrack(tui, event)?;
                     Ok(true)
                 }
+                TuiEvent::Key(KeyEvent {
+                    code: KeyCode::Left,
+                    kind: KeyEventKind::Press | KeyEventKind::Repeat,
+                    ..
+                }) => {
+                    self.overlay_step_backtrack(tui, event)?;
+                    Ok(true)
+                }
+                TuiEvent::Key(KeyEvent {
+                    code: KeyCode::Right,
+                    kind: KeyEventKind::Press | KeyEventKind::Repeat,
+                    ..
+                }) => {
+                    self.overlay_step_backtrack_forward(tui, event)?;
+                    Ok(true)
+                }
                 TuiEvent::Key(KeyEvent {
                     code: KeyCode::Enter,
                     kind: KeyEventKind::Press,
@@ -277,6 +294,27 @@ impl App {
         tui.frame_requester().schedule_frame();
     }
 
+    /// Step selection to the next newer user message and update overlay.
+    fn step_forward_backtrack_and_highlight(&mut self, tui: &mut tui::Tui) {
+        let count = user_count(&self.transcript_cells);
+        if count == 0 {
+            return;
+        }
+
+        let last_index = count.saturating_sub(1);
+        let next_selection = if self.backtrack.nth_user_message == usize::MAX {
+            last_index
+        } else {
+            self.backtrack
+                .nth_user_message
+                .saturating_add(1)
+                .min(last_index)
+        };
+
+        self.apply_backtrack_selection_internal(next_selection);
+        tui.frame_requester().schedule_frame();
+    }
+
     /// Apply a computed backtrack selection to the overlay and internal counter.
     fn apply_backtrack_selection_internal(&mut self, nth_user_message: usize) {
         if let Some(cell_idx) = nth_user_position(&self.transcript_cells, nth_user_message) {
@@ -364,6 +402,20 @@ impl App {
         Ok(())
     }
 
+    /// Handle Right in overlay backtrack preview: step selection forward if armed, else forward.
+    fn overlay_step_backtrack_forward(
+        &mut self,
+        tui: &mut tui::Tui,
+        event: TuiEvent,
+    ) -> Result<()> {
+        if self.backtrack.base_id.is_some() {
+            self.step_forward_backtrack_and_highlight(tui);
+        } else {
+            self.overlay_forward_event(tui, event)?;
+        }
+        Ok(())
+    }
+
     /// Confirm a primed backtrack from the main view (no overlay visible).
     /// Computes the prefill from the selected user message for rollback.
     pub(crate) fn confirm_backtrack_from_main(&mut self) -> Option<BacktrackSelection> {