feat: add a /mention slash command (#2114)

- To help people discover @mentions.
- Command just places a @ in the composer.
- #2115 then improves the behavior of @mentions with empty queries.

codex-rs/tui/src/app.rs

@@ -366,6 +366,11 @@ impl App<'_> {
                             widget.add_diff_output(text);
                         }
                     }
+                    SlashCommand::Mention => {
+                        if let AppState::Chat { widget } = &mut self.app_state {
+                            widget.insert_str("@");
+                        }
+                    }
                     SlashCommand::Status => {
                         if let AppState::Chat { widget } = &mut self.app_state {
                             widget.add_status_output();

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

@@ -198,6 +198,12 @@ impl ChatComposer {
         self.set_has_focus(has_focus);
     }
 
+    pub(crate) fn insert_str(&mut self, text: &str) {
+        self.textarea.insert_str(text);
+        self.sync_command_popup();
+        self.sync_file_search_popup();
+    }
+
     /// Handle a key event coming from the main UI.
     pub fn handle_key_event(&mut self, key_event: KeyEvent) -> (InputResult, bool) {
         let result = match &mut self.active_popup {
@@ -1078,6 +1084,46 @@ mod tests {
         }
     }
 
+    #[test]
+    fn slash_mention_dispatches_command_and_inserts_at() {
+        use crossterm::event::KeyCode;
+        use crossterm::event::KeyEvent;
+        use crossterm::event::KeyModifiers;
+        use std::sync::mpsc::TryRecvError;
+
+        let (tx, rx) = std::sync::mpsc::channel();
+        let sender = AppEventSender::new(tx);
+        let mut composer = ChatComposer::new(true, sender, false);
+
+        for ch in ['/', 'm', 'e', 'n', 't', 'i', 'o', 'n'] {
+            let _ = composer.handle_key_event(KeyEvent::new(KeyCode::Char(ch), KeyModifiers::NONE));
+        }
+
+        let (result, _needs_redraw) =
+            composer.handle_key_event(KeyEvent::new(KeyCode::Enter, KeyModifiers::NONE));
+
+        match result {
+            InputResult::None => {}
+            InputResult::Submitted(text) => {
+                panic!("expected command dispatch, but composer submitted literal text: {text}")
+            }
+        }
+        assert!(composer.textarea.is_empty(), "composer should be cleared");
+
+        match rx.try_recv() {
+            Ok(AppEvent::DispatchCommand(cmd)) => {
+                assert_eq!(cmd.command(), "mention");
+                composer.insert_str("@");
+            }
+            Ok(_other) => panic!("unexpected app event"),
+            Err(TryRecvError::Empty) => panic!("expected a DispatchCommand event for '/mention'"),
+            Err(TryRecvError::Disconnected) => {
+                panic!("app event channel disconnected")
+            }
+        }
+        assert_eq!(composer.textarea.text(), "@");
+    }
+
     #[test]
     fn test_multiple_pastes_submission() {
         use crossterm::event::KeyCode;

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

@@ -196,6 +196,11 @@ impl BottomPane<'_> {
         }
     }
 
+    pub(crate) fn insert_str(&mut self, text: &str) {
+        self.composer.insert_str(text);
+        self.request_redraw();
+    }
+
     /// Update the status indicator text. Prefer replacing the composer with
     /// the StatusIndicatorView so the input pane shows a single-line status
     /// like: `▌ Working waiting for model`.

codex-rs/tui/src/chatwidget.rs

@@ -676,6 +676,10 @@ impl ChatWidget<'_> {
         self.submit_user_message(text.into());
     }
 
+    pub(crate) fn insert_str(&mut self, text: &str) {
+        self.bottom_pane.insert_str(text);
+    }
+
     pub(crate) fn token_usage(&self) -> &TokenUsage {
         &self.total_token_usage
     }

codex-rs/tui/src/slash_command.rs

@@ -16,6 +16,7 @@ pub enum SlashCommand {
     Init,
     Compact,
     Diff,
+    Mention,
     Status,
     Prompts,
     Logout,
@@ -33,6 +34,7 @@ impl SlashCommand {
             SlashCommand::Compact => "summarize conversation to prevent hitting the context limit",
             SlashCommand::Quit => "exit Codex",
             SlashCommand::Diff => "show git diff (including untracked files)",
+            SlashCommand::Mention => "mention a file",
             SlashCommand::Status => "show current session configuration and token usage",
             SlashCommand::Prompts => "show example prompts",
             SlashCommand::Logout => "log out of Codex",