Resume conversation from an earlier point in history (#2607)

Fixing merge conflict of this: #2588


https://github.com/user-attachments/assets/392c7c37-cf8f-4ed6-952e-8215e8c57bc4

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

@@ -1,5 +1,6 @@
 use std::sync::Arc;
 
+use codex_core::CodexConversation;
 use codex_core::ConversationManager;
 use codex_core::NewConversation;
 use codex_core::config::Config;
@@ -59,3 +60,40 @@ pub(crate) fn spawn_agent(
 
     codex_op_tx
 }
+
+/// Spawn agent loops for an existing conversation (e.g., a forked conversation).
+/// Sends the provided `SessionConfiguredEvent` immediately, then forwards subsequent
+/// events and accepts Ops for submission.
+pub(crate) fn spawn_agent_from_existing(
+    conversation: std::sync::Arc<CodexConversation>,
+    session_configured: codex_core::protocol::SessionConfiguredEvent,
+    app_event_tx: AppEventSender,
+) -> UnboundedSender<Op> {
+    let (codex_op_tx, mut codex_op_rx) = unbounded_channel::<Op>();
+
+    let app_event_tx_clone = app_event_tx.clone();
+    tokio::spawn(async move {
+        // Forward the captured `SessionConfigured` event so it can be rendered in the UI.
+        let ev = codex_core::protocol::Event {
+            id: "".to_string(),
+            msg: codex_core::protocol::EventMsg::SessionConfigured(session_configured),
+        };
+        app_event_tx_clone.send(AppEvent::CodexEvent(ev));
+
+        let conversation_clone = conversation.clone();
+        tokio::spawn(async move {
+            while let Some(op) = codex_op_rx.recv().await {
+                let id = conversation_clone.submit(op).await;
+                if let Err(e) = id {
+                    tracing::error!("failed to submit op: {e}");
+                }
+            }
+        });
+
+        while let Ok(event) = conversation.next_event().await {
+            app_event_tx_clone.send(AppEvent::CodexEvent(event));
+        }
+    });
+
+    codex_op_tx
+}

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

@@ -182,6 +182,7 @@ fn make_chatwidget_manual() -> (
         full_reasoning_buffer: String::new(),
         session_id: None,
         frame_requester: crate::tui::FrameRequester::test_dummy(),
+        show_welcome_banner: true,
         last_history_was_exec: false,
     };
     (widget, rx, op_rx)