Move TUI on top of app server (parallel code) (#14717)

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.

Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.

codex-rs/tui_app_server/src/notifications/bel.rs

@@ -0,0 +1,37 @@
+use std::fmt;
+use std::io;
+use std::io::stdout;
+
+use crossterm::Command;
+use ratatui::crossterm::execute;
+
+#[derive(Debug, Default)]
+pub struct BelBackend;
+
+impl BelBackend {
+    pub fn notify(&mut self, _message: &str) -> io::Result<()> {
+        execute!(stdout(), PostNotification)
+    }
+}
+
+/// Command that emits a BEL desktop notification.
+#[derive(Debug, Clone)]
+pub struct PostNotification;
+
+impl Command for PostNotification {
+    fn write_ansi(&self, f: &mut impl fmt::Write) -> fmt::Result {
+        write!(f, "\x07")
+    }
+
+    #[cfg(windows)]
+    fn execute_winapi(&self) -> io::Result<()> {
+        Err(std::io::Error::other(
+            "tried to execute PostNotification using WinAPI; use ANSI instead",
+        ))
+    }
+
+    #[cfg(windows)]
+    fn is_ansi_code_supported(&self) -> bool {
+        true
+    }
+}

codex-rs/tui_app_server/src/notifications/mod.rs

@@ -0,0 +1,156 @@
+mod bel;
+mod osc9;
+
+use std::env;
+use std::io;
+
+use bel::BelBackend;
+use codex_core::config::types::NotificationMethod;
+use osc9::Osc9Backend;
+
+#[derive(Debug)]
+pub enum DesktopNotificationBackend {
+    Osc9(Osc9Backend),
+    Bel(BelBackend),
+}
+
+impl DesktopNotificationBackend {
+    pub fn for_method(method: NotificationMethod) -> Self {
+        match method {
+            NotificationMethod::Auto => {
+                if supports_osc9() {
+                    Self::Osc9(Osc9Backend)
+                } else {
+                    Self::Bel(BelBackend)
+                }
+            }
+            NotificationMethod::Osc9 => Self::Osc9(Osc9Backend),
+            NotificationMethod::Bel => Self::Bel(BelBackend),
+        }
+    }
+
+    pub fn method(&self) -> NotificationMethod {
+        match self {
+            DesktopNotificationBackend::Osc9(_) => NotificationMethod::Osc9,
+            DesktopNotificationBackend::Bel(_) => NotificationMethod::Bel,
+        }
+    }
+
+    pub fn notify(&mut self, message: &str) -> io::Result<()> {
+        match self {
+            DesktopNotificationBackend::Osc9(backend) => backend.notify(message),
+            DesktopNotificationBackend::Bel(backend) => backend.notify(message),
+        }
+    }
+}
+
+pub fn detect_backend(method: NotificationMethod) -> DesktopNotificationBackend {
+    DesktopNotificationBackend::for_method(method)
+}
+
+fn supports_osc9() -> bool {
+    if env::var_os("WT_SESSION").is_some() {
+        return false;
+    }
+    // Prefer TERM_PROGRAM when present, but keep fallbacks for shells/launchers
+    // that don't set it (e.g., tmux/ssh) to avoid regressing OSC 9 support.
+    if matches!(
+        env::var("TERM_PROGRAM").ok().as_deref(),
+        Some("WezTerm" | "ghostty")
+    ) {
+        return true;
+    }
+    // iTerm still provides a strong session signal even when TERM_PROGRAM is missing.
+    if env::var_os("ITERM_SESSION_ID").is_some() {
+        return true;
+    }
+    // TERM-based hints cover kitty/wezterm setups without TERM_PROGRAM.
+    matches!(
+        env::var("TERM").ok().as_deref(),
+        Some("xterm-kitty" | "wezterm" | "wezterm-mux")
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use super::detect_backend;
+    use codex_core::config::types::NotificationMethod;
+    use serial_test::serial;
+    use std::ffi::OsString;
+
+    struct EnvVarGuard {
+        key: &'static str,
+        original: Option<OsString>,
+    }
+
+    impl EnvVarGuard {
+        fn set(key: &'static str, value: &str) -> Self {
+            let original = std::env::var_os(key);
+            unsafe {
+                std::env::set_var(key, value);
+            }
+            Self { key, original }
+        }
+
+        fn remove(key: &'static str) -> Self {
+            let original = std::env::var_os(key);
+            unsafe {
+                std::env::remove_var(key);
+            }
+            Self { key, original }
+        }
+    }
+
+    impl Drop for EnvVarGuard {
+        fn drop(&mut self) {
+            unsafe {
+                match &self.original {
+                    Some(value) => std::env::set_var(self.key, value),
+                    None => std::env::remove_var(self.key),
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn selects_osc9_method() {
+        assert!(matches!(
+            detect_backend(NotificationMethod::Osc9),
+            super::DesktopNotificationBackend::Osc9(_)
+        ));
+    }
+
+    #[test]
+    fn selects_bel_method() {
+        assert!(matches!(
+            detect_backend(NotificationMethod::Bel),
+            super::DesktopNotificationBackend::Bel(_)
+        ));
+    }
+
+    #[test]
+    #[serial]
+    fn auto_prefers_bel_without_hints() {
+        let _term = EnvVarGuard::remove("TERM");
+        let _term_program = EnvVarGuard::remove("TERM_PROGRAM");
+        let _iterm = EnvVarGuard::remove("ITERM_SESSION_ID");
+        let _wt = EnvVarGuard::remove("WT_SESSION");
+        assert!(matches!(
+            detect_backend(NotificationMethod::Auto),
+            super::DesktopNotificationBackend::Bel(_)
+        ));
+    }
+
+    #[test]
+    #[serial]
+    fn auto_uses_osc9_for_iterm() {
+        let _term = EnvVarGuard::remove("TERM");
+        let _term_program = EnvVarGuard::remove("TERM_PROGRAM");
+        let _iterm = EnvVarGuard::set("ITERM_SESSION_ID", "abc");
+        let _wt = EnvVarGuard::remove("WT_SESSION");
+        assert!(matches!(
+            detect_backend(NotificationMethod::Auto),
+            super::DesktopNotificationBackend::Osc9(_)
+        ));
+    }
+}

codex-rs/tui_app_server/src/notifications/osc9.rs

@@ -0,0 +1,37 @@
+use std::fmt;
+use std::io;
+use std::io::stdout;
+
+use crossterm::Command;
+use ratatui::crossterm::execute;
+
+#[derive(Debug, Default)]
+pub struct Osc9Backend;
+
+impl Osc9Backend {
+    pub fn notify(&mut self, message: &str) -> io::Result<()> {
+        execute!(stdout(), PostNotification(message.to_string()))
+    }
+}
+
+/// Command that emits an OSC 9 desktop notification with a message.
+#[derive(Debug, Clone)]
+pub struct PostNotification(pub String);
+
+impl Command for PostNotification {
+    fn write_ansi(&self, f: &mut impl fmt::Write) -> fmt::Result {
+        write!(f, "\x1b]9;{}\x07", self.0)
+    }
+
+    #[cfg(windows)]
+    fn execute_winapi(&self) -> io::Result<()> {
+        Err(std::io::Error::other(
+            "tried to execute PostNotification using WinAPI; use ANSI instead",
+        ))
+    }
+
+    #[cfg(windows)]
+    fn is_ansi_code_supported(&self) -> bool {
+        true
+    }
+}