adding fork information (UI) when forking (#10246)

- shows `/fork` command that ran in prev session
- shows `session forked from name (uuid) || uuid (if name is not set)` as an event in new session

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

@@ -246,6 +246,70 @@ async fn replayed_user_message_preserves_text_elements_and_local_images() {
     assert_eq!(stored_images, local_images);
 }
 
+#[tokio::test]
+async fn forked_thread_history_line_includes_name_and_id_snapshot() {
+    let (chat, mut rx, _op_rx) = make_chatwidget_manual(None).await;
+    let mut chat = chat;
+    let temp = tempdir().expect("tempdir");
+    chat.config.codex_home = temp.path().to_path_buf();
+
+    let forked_from_id =
+        ThreadId::from_string("e9f18a88-8081-4e51-9d4e-8af5cde2d8dd").expect("forked id");
+    let session_index_entry = format!(
+        "{{\"id\":\"{forked_from_id}\",\"thread_name\":\"named-thread\",\"updated_at\":\"2024-01-02T00:00:00Z\"}}\n"
+    );
+    std::fs::write(temp.path().join("session_index.jsonl"), session_index_entry)
+        .expect("write session index");
+
+    chat.emit_forked_thread_event(forked_from_id);
+
+    let history_cell = tokio::time::timeout(std::time::Duration::from_secs(2), async {
+        loop {
+            match rx.recv().await {
+                Some(AppEvent::InsertHistoryCell(cell)) => break cell,
+                Some(_) => continue,
+                None => panic!("app event channel closed before forked thread history was emitted"),
+            }
+        }
+    })
+    .await
+    .expect("timed out waiting for forked thread history");
+    let combined = lines_to_single_string(&history_cell.display_lines(80));
+
+    assert!(
+        combined.contains("Thread forked from"),
+        "expected forked thread message in history"
+    );
+    assert_snapshot!("forked_thread_history_line", combined);
+}
+
+#[tokio::test]
+async fn forked_thread_history_line_without_name_shows_id_once_snapshot() {
+    let (chat, mut rx, _op_rx) = make_chatwidget_manual(None).await;
+    let mut chat = chat;
+    let temp = tempdir().expect("tempdir");
+    chat.config.codex_home = temp.path().to_path_buf();
+
+    let forked_from_id =
+        ThreadId::from_string("019c2d47-4935-7423-a190-05691f566092").expect("forked id");
+    chat.emit_forked_thread_event(forked_from_id);
+
+    let history_cell = tokio::time::timeout(std::time::Duration::from_secs(2), async {
+        loop {
+            match rx.recv().await {
+                Some(AppEvent::InsertHistoryCell(cell)) => break cell,
+                Some(_) => continue,
+                None => panic!("app event channel closed before forked thread history was emitted"),
+            }
+        }
+    })
+    .await
+    .expect("timed out waiting for forked thread history");
+    let combined = lines_to_single_string(&history_cell.display_lines(80));
+
+    assert_snapshot!("forked_thread_history_line_without_name", combined);
+}
+
 #[tokio::test]
 async fn submission_preserves_text_elements_and_local_images() {
     let (mut chat, mut rx, mut op_rx) = make_chatwidget_manual(None).await;