Speed up /side parent restore replay

codex-rs/tui/src/app.rs

@@ -423,6 +423,7 @@ struct SessionSummary {
 #[derive(Debug, Default)]
 struct InitialHistoryReplayBuffer {
     retained_lines: VecDeque<Line<'static>>,
+    render_from_transcript_tail: bool,
 }
 
 pub(crate) struct App {

codex-rs/tui/src/app/event_dispatch.rs

@@ -182,6 +182,9 @@ impl App {
             AppEvent::BeginInitialHistoryReplayBuffer => {
                 self.begin_initial_history_replay_buffer();
             }
+            AppEvent::BeginThreadSwitchHistoryReplayBuffer => {
+                self.begin_thread_switch_history_replay_buffer();
+            }
             AppEvent::InsertHistoryCell(cell) => {
                 let cell: Arc<dyn HistoryCell> = cell.into();
                 if let Some(Overlay::Transcript(t)) = &mut self.overlay {

codex-rs/tui/src/app/resize_reflow.rs

@@ -119,6 +119,23 @@ impl App {
         }
     }
 
+    /// Start retaining a thread-switch transcript replay without rendering each historical cell.
+    ///
+    /// Thread switches already rebuild `transcript_cells` from source. When a row cap exists, we can
+    /// defer terminal writes until the replay is complete and reuse the resize-reflow tail renderer
+    /// so only the rows the terminal would retain are formatted and inserted.
+    pub(super) fn begin_thread_switch_history_replay_buffer(&mut self) {
+        if self.terminal_resize_reflow_enabled()
+            && self.resize_reflow_max_rows().is_some()
+            && self.overlay.is_none()
+        {
+            self.initial_history_replay_buffer = Some(InitialHistoryReplayBuffer {
+                retained_lines: VecDeque::new(),
+                render_from_transcript_tail: true,
+            });
+        }
+    }
+
     /// Flush retained initial resume replay rows into terminal scrollback.
     ///
     /// The buffer stores display lines, not cells, because the cap is measured in terminal rows.
@@ -130,6 +147,13 @@ impl App {
         };
 
         if buffer.retained_lines.is_empty() {
+            if buffer.render_from_transcript_tail {
+                let width = tui.terminal.last_known_screen_size.width;
+                let reflowed_lines = self.render_transcript_lines_for_reflow(width).lines;
+                if !reflowed_lines.is_empty() {
+                    tui.insert_history_lines(reflowed_lines);
+                }
+            }
             return;
         }
 
@@ -143,6 +167,14 @@ impl App {
         cell: &dyn HistoryCell,
         width: u16,
     ) {
+        if self
+            .initial_history_replay_buffer
+            .as_ref()
+            .is_some_and(|buffer| buffer.render_from_transcript_tail)
+        {
+            return;
+        }
+
         let display = self.display_lines_for_history_insert(cell, width);
 
         if display.is_empty() {

codex-rs/tui/src/app/tests.rs

@@ -3986,6 +3986,33 @@ async fn initial_replay_buffer_keeps_recent_rows_when_row_cap_present() {
     );
 }
 
+#[tokio::test]
+async fn thread_switch_replay_buffer_uses_transcript_tail_mode_when_row_cap_present() {
+    let (mut app, _rx, _op_rx) = make_test_app_with_channels().await;
+    enable_terminal_resize_reflow(&mut app);
+    app.config.terminal_resize_reflow.max_rows = TerminalResizeReflowMaxRows::Limit(3);
+
+    app.begin_thread_switch_history_replay_buffer();
+
+    let buffer = app
+        .initial_history_replay_buffer
+        .as_ref()
+        .expect("thread switch replay buffer should be active");
+    assert!(buffer.render_from_transcript_tail);
+    assert!(buffer.retained_lines.is_empty());
+}
+
+#[tokio::test]
+async fn thread_switch_replay_buffer_is_disabled_without_row_cap() {
+    let (mut app, _rx, _op_rx) = make_test_app_with_channels().await;
+    enable_terminal_resize_reflow(&mut app);
+    app.config.terminal_resize_reflow.max_rows = TerminalResizeReflowMaxRows::Disabled;
+
+    app.begin_thread_switch_history_replay_buffer();
+
+    assert!(app.initial_history_replay_buffer.is_none());
+}
+
 #[tokio::test]
 async fn height_shrink_schedules_resize_reflow() {
     let (mut app, _rx, _op_rx) = make_test_app_with_channels().await;

codex-rs/tui/src/app/thread_routing.rs

@@ -1238,6 +1238,12 @@ impl App {
         snapshot: ThreadEventSnapshot,
         resume_restored_queue: bool,
     ) {
+        let should_buffer_replay = self.terminal_resize_reflow_enabled()
+            && (!snapshot.turns.is_empty() || !snapshot.events.is_empty());
+        if should_buffer_replay {
+            self.app_event_tx
+                .send(AppEvent::BeginThreadSwitchHistoryReplayBuffer);
+        }
         let suppress_replay_notices =
             replay_filter::snapshot_has_pending_interactive_request(&snapshot);
         if let Some(session) = snapshot.session {
@@ -1263,6 +1269,10 @@ impl App {
             }
             self.handle_thread_event_replay(event);
         }
+        if should_buffer_replay {
+            self.app_event_tx
+                .send(AppEvent::EndInitialHistoryReplayBuffer);
+        }
         self.chat_widget
             .set_queue_autosend_suppressed(/*suppressed*/ false);
         self.chat_widget

codex-rs/tui/src/app_event.rs

@@ -485,6 +485,10 @@ pub(crate) enum AppEvent {
     /// Begin buffering initial resume replay rows before they are written to scrollback.
     BeginInitialHistoryReplayBuffer,
 
+    /// Begin buffering thread-switch replay cells so the final scrollback write can reuse the
+    /// resize-reflow tail renderer.
+    BeginThreadSwitchHistoryReplayBuffer,
+
     InsertHistoryCell(Box<dyn HistoryCell>),
 
     /// Finish buffering initial resume replay after all replay events have been queued.