Prefill rename prompt with current thread name (#18057)

Addresses #12178

Problem: The TUI /rename prompt opened blank even when the current
thread already had a custom name, making small edits awkward.

Solution: Let custom prompts receive initial text and prefill /rename
with the existing thread name while preserving the empty prompt for
unnamed threads.

Testing: Manually verified that the feature works by using `/rename`
with unnamed and already-named threads.

codex-rs/tui/src/bottom_pane/custom_prompt_view.rs

@@ -41,15 +41,22 @@ impl CustomPromptView {
     pub(crate) fn new(
         title: String,
         placeholder: String,
+        initial_text: String,
         context_label: Option<String>,
         on_submit: PromptSubmitted,
     ) -> Self {
+        let mut textarea = TextArea::new();
+        if !initial_text.is_empty() {
+            textarea.set_text_clearing_elements(&initial_text);
+            textarea.set_cursor(initial_text.len());
+        }
+
         Self {
             title,
             placeholder,
             context_label,
             on_submit,
-            textarea: TextArea::new(),
+            textarea,
             textarea_state: RefCell::new(TextAreaState::default()),
             complete: false,
         }

codex-rs/tui/src/chatwidget.rs

@@ -5279,11 +5279,8 @@ impl ChatWidget {
 
     fn show_rename_prompt(&mut self) {
         let tx = self.app_event_tx.clone();
-        let has_name = self
-            .thread_name
-            .as_ref()
-            .is_some_and(|name| !name.is_empty());
-        let title = if has_name {
+        let existing_name = self.thread_name.as_deref().filter(|name| !name.is_empty());
+        let title = if existing_name.is_some() {
             "Rename thread"
         } else {
             "Name thread"
@@ -5291,6 +5288,7 @@ impl ChatWidget {
         let view = CustomPromptView::new(
             title.to_string(),
             "Type a name and press Enter".to_string(),
+            /*initial_text*/ existing_name.unwrap_or_default().to_string(),
             /*context_label*/ None,
             Box::new(move |name: String| {
                 let Some(name) = crate::legacy_core::util::normalize_thread_name(&name) else {
@@ -10599,6 +10597,7 @@ impl ChatWidget {
         let view = CustomPromptView::new(
             "Custom review instructions".to_string(),
             "Type instructions and press Enter".to_string(),
+            /*initial_text*/ String::new(),
             /*context_label*/ None,
             Box::new(move |prompt: String| {
                 let trimmed = prompt.trim().to_string();

codex-rs/tui/src/chatwidget/snapshots/codex_tui__chatwidget__tests__slash_rename_prefilled_prompt.snap

@@ -0,0 +1,10 @@
+---
+source: tui/src/chatwidget/tests/slash_commands.rs
+assertion_line: 135
+expression: popup
+---
+▌ Rename thread
+▌
+▌ Current project title
+
+Press enter to confirm or esc to go back

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

@@ -124,6 +124,39 @@ async fn inline_slash_command_is_available_from_local_recall_after_dispatch() {
     assert_eq!(chat.bottom_pane.composer_text(), "/rename Better title");
 }
 
+#[tokio::test]
+async fn slash_rename_prefills_existing_thread_name() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+    chat.thread_name = Some("Current project title".to_string());
+
+    chat.dispatch_command(SlashCommand::Rename);
+
+    let popup = render_bottom_popup(&chat, /*width*/ 80);
+    assert_chatwidget_snapshot!("slash_rename_prefilled_prompt", popup);
+
+    chat.handle_key_event(KeyEvent::new(KeyCode::Enter, KeyModifiers::NONE));
+
+    assert_matches!(
+        rx.try_recv(),
+        Ok(AppEvent::CodexOp(Op::SetThreadName { name })) if name == "Current project title"
+    );
+}
+
+#[tokio::test]
+async fn slash_rename_without_existing_thread_name_starts_empty() {
+    let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(/*model_override*/ None).await;
+
+    chat.dispatch_command(SlashCommand::Rename);
+
+    let popup = render_bottom_popup(&chat, /*width*/ 80);
+    assert!(popup.contains("Name thread"));
+    assert!(popup.contains("Type a name and press Enter"));
+
+    chat.handle_key_event(KeyEvent::new(KeyCode::Enter, KeyModifiers::NONE));
+
+    assert_matches!(rx.try_recv(), Err(TryRecvError::Empty));
+}
+
 #[tokio::test]
 async fn usage_error_slash_command_is_available_from_local_recall() {
     let (mut chat, mut rx, _op_rx) = make_chatwidget_manual(Some("gpt-5.3-codex")).await;