refactor transcript view to handle HistoryCells (#3538)

No (intended) functional change.

This refactors the transcript view to hold a list of HistoryCells
instead of a list of Lines. This simplifies and makes much of the logic
more robust, as well as laying the groundwork for future changes, e.g.
live-updating history cells in the transcript.

Similar to #2879 in goal. Fixes #2755.

Author: nornagon-openai

codex-rs/core/src/conversation_manager.rs

@@ -144,19 +144,19 @@ impl ConversationManager {
         self.conversations.write().await.remove(conversation_id)
     }
 
-    /// Fork an existing conversation by dropping the last `drop_last_messages`
-    /// user/assistant messages from its transcript and starting a new
+    /// Fork an existing conversation by taking messages up to the given position
+    /// (not including the message at the given position) and starting a new
     /// conversation with identical configuration (unless overridden by the
     /// caller's `config`). The new conversation will have a fresh id.
     pub async fn fork_conversation(
         &self,
-        num_messages_to_drop: usize,
+        nth_user_message: usize,
         config: Config,
         path: PathBuf,
     ) -> CodexResult<NewConversation> {
         // Compute the prefix up to the cut point.
         let history = RolloutRecorder::get_rollout_history(&path).await?;
-        let history = truncate_after_dropping_last_messages(history, num_messages_to_drop);
+        let history = truncate_after_nth_user_message(history, nth_user_message);
 
         // Spawn a new conversation with the computed initial history.
         let auth_manager = self.auth_manager.clone();
@@ -169,14 +169,10 @@ impl ConversationManager {
     }
 }
 
-/// Return a prefix of `items` obtained by dropping the last `n` user messages
-/// and all items that follow them.
-fn truncate_after_dropping_last_messages(history: InitialHistory, n: usize) -> InitialHistory {
-    if n == 0 {
-        return InitialHistory::Forked(history.get_rollout_items());
-    }
-
-    // Work directly on rollout items, and cut the vector at the nth-from-last user message input.
+/// Return a prefix of `items` obtained by cutting strictly before the nth user message
+/// (0-based) and all items that follow it.
+fn truncate_after_nth_user_message(history: InitialHistory, n: usize) -> InitialHistory {
+    // Work directly on rollout items, and cut the vector at the nth user message input.
     let items: Vec<RolloutItem> = history.get_rollout_items();
 
     // Find indices of user message inputs in rollout order.
@@ -189,13 +185,13 @@ fn truncate_after_dropping_last_messages(history: InitialHistory, n: usize) -> I
         }
     }
 
-    // If fewer than n user messages exist, treat as empty.
-    if user_positions.len() < n {
+    // If fewer than or equal to n user messages exist, treat as empty (out of range).
+    if user_positions.len() <= n {
         return InitialHistory::New;
     }
 
-    // Cut strictly before the nth-from-last user message (do not keep the nth itself).
-    let cut_idx = user_positions[user_positions.len() - n];
+    // Cut strictly before the nth user message (do not keep the nth itself).
+    let cut_idx = user_positions[n];
     let rolled: Vec<RolloutItem> = items.into_iter().take(cut_idx).collect();
 
     if rolled.is_empty() {
@@ -262,7 +258,7 @@ mod tests {
             .cloned()
             .map(RolloutItem::ResponseItem)
             .collect();
-        let truncated = truncate_after_dropping_last_messages(InitialHistory::Forked(initial), 1);
+        let truncated = truncate_after_nth_user_message(InitialHistory::Forked(initial), 1);
         let got_items = truncated.get_rollout_items();
         let expected_items = vec![
             RolloutItem::ResponseItem(items[0].clone()),
@@ -279,7 +275,7 @@ mod tests {
             .cloned()
             .map(RolloutItem::ResponseItem)
             .collect();
-        let truncated2 = truncate_after_dropping_last_messages(InitialHistory::Forked(initial2), 2);
+        let truncated2 = truncate_after_nth_user_message(InitialHistory::Forked(initial2), 2);
         assert!(matches!(truncated2, InitialHistory::New));
     }
 }

codex-rs/core/tests/suite/fork_conversation.rs

@@ -104,7 +104,8 @@ async fn fork_conversation_twice_drops_to_first_message() {
         items
     };
 
-    // Compute expected prefixes after each fork by truncating base rollout at nth-from-last user input.
+    // Compute expected prefixes after each fork by truncating base rollout
+    // strictly before the nth user input (0-based).
     let base_items = read_items(&base_path);
     let find_user_input_positions = |items: &[RolloutItem]| -> Vec<usize> {
         let mut pos = Vec::new();
@@ -126,11 +127,8 @@ async fn fork_conversation_twice_drops_to_first_message() {
     };
     let user_inputs = find_user_input_positions(&base_items);
 
-    // After dropping last user input (n=1), cut strictly before that input if present, else empty.
-    let cut1 = user_inputs
-        .get(user_inputs.len().saturating_sub(1))
-        .copied()
-        .unwrap_or(0);
+    // After cutting at nth user input (n=1 → second user message), cut strictly before that input.
+    let cut1 = user_inputs.get(1).copied().unwrap_or(0);
     let expected_after_first: Vec<RolloutItem> = base_items[..cut1].to_vec();
 
     // After dropping again (n=1 on fork1), compute expected relative to fork1's rollout.
@@ -161,12 +159,12 @@ async fn fork_conversation_twice_drops_to_first_message() {
         serde_json::to_value(&expected_after_first).unwrap()
     );
 
-    // Fork again with n=1 → drops the (new) last user message, leaving only the first.
+    // Fork again with n=0 → drops the (new) last user message, leaving only the first.
     let NewConversation {
         conversation: codex_fork2,
         ..
     } = conversation_manager
-        .fork_conversation(1, config_for_fork.clone(), fork1_path.clone())
+        .fork_conversation(0, config_for_fork.clone(), fork1_path.clone())
         .await
         .expect("fork 2");
 

codex-rs/tui/src/app.rs

@@ -3,6 +3,7 @@ use crate::app_event::AppEvent;
 use crate::app_event_sender::AppEventSender;
 use crate::chatwidget::ChatWidget;
 use crate::file_search::FileSearchManager;
+use crate::history_cell::HistoryCell;
 use crate::pager_overlay::Overlay;
 use crate::resume_picker::ResumeSelection;
 use crate::tui;
@@ -46,7 +47,7 @@ pub(crate) struct App {
 
     pub(crate) file_search: FileSearchManager,
 
-    pub(crate) transcript_lines: Vec<Line<'static>>,
+    pub(crate) transcript_cells: Vec<Arc<dyn HistoryCell>>,
 
     // Pager overlay state (Transcript or Static like Diff)
     pub(crate) overlay: Option<Overlay>,
@@ -131,7 +132,7 @@ impl App {
             model_saved_to_global: false,
             file_search,
             enhanced_keys_supported,
-            transcript_lines: Vec::new(),
+            transcript_cells: Vec::new(),
             overlay: None,
             deferred_history_lines: Vec::new(),
             has_emitted_history_lines: false,
@@ -213,15 +214,12 @@ impl App {
                 tui.frame_requester().schedule_frame();
             }
             AppEvent::InsertHistoryCell(cell) => {
-                let mut cell_transcript = cell.transcript_lines();
-                if !cell.is_stream_continuation() && !self.transcript_lines.is_empty() {
-                    cell_transcript.insert(0, Line::from(""));
-                }
+                let cell: Arc<dyn HistoryCell> = cell.into();
                 if let Some(Overlay::Transcript(t)) = &mut self.overlay {
-                    t.insert_lines(cell_transcript.clone());
+                    t.insert_cell(cell.clone());
                     tui.frame_requester().schedule_frame();
                 }
-                self.transcript_lines.extend(cell_transcript.clone());
+                self.transcript_cells.push(cell.clone());
                 let mut display = cell.display_lines(tui.terminal.last_known_screen_size.width);
                 if !display.is_empty() {
                     // Only insert a separating blank line for new cells that are not
@@ -437,7 +435,7 @@ impl App {
             } => {
                 // Enter alternate screen and set viewport to full size.
                 let _ = tui.enter_alt_screen();
-                self.overlay = Some(Overlay::new_transcript(self.transcript_lines.clone()));
+                self.overlay = Some(Overlay::new_transcript(self.transcript_cells.clone()));
                 tui.frame_requester().schedule_frame();
             }
             KeyEvent {
@@ -470,7 +468,7 @@ impl App {
                 kind: KeyEventKind::Press,
                 ..
             } if self.backtrack.primed
-                && self.backtrack.count > 0
+                && self.backtrack.nth_user_message != usize::MAX
                 && self.chat_widget.composer_is_empty() =>
             {
                 // Delegate to helper for clarity; preserves behavior.
@@ -503,7 +501,6 @@ mod tests {
     use crate::file_search::FileSearchManager;
     use codex_core::CodexAuth;
     use codex_core::ConversationManager;
-    use ratatui::text::Line;
     use std::sync::Arc;
     use std::sync::atomic::AtomicBool;
 
@@ -525,7 +522,7 @@ mod tests {
             model_saved_to_profile: false,
             model_saved_to_global: false,
             file_search,
-            transcript_lines: Vec::<Line<'static>>::new(),
+            transcript_cells: Vec::new(),
             overlay: None,
             deferred_history_lines: Vec::new(),
             has_emitted_history_lines: false,