fix terminal corruption that could happen when onboarding and update banner (#5269)

Instead of printing characters before booting the app, make the upgrade
banner a history cell so it's well-behaved.

<img width="771" height="586" alt="Screenshot 2025-10-16 at 4 20 51 PM"
src="https://github.com/user-attachments/assets/90629d47-2c3d-4970-a826-283795ab34e5"
/>

---------

Co-authored-by: Josh McKinney <joshka@openai.com>

codex-rs/tui/src/updates.rs

@@ -1,5 +1,3 @@
-#![cfg(any(not(debug_assertions), test))]
-
 use chrono::DateTime;
 use chrono::Duration;
 use chrono::Utc;
@@ -142,6 +140,53 @@ fn parse_version(v: &str) -> Option<(u64, u64, u64)> {
     Some((maj, min, pat))
 }
 
+/// Update action the CLI should perform after the TUI exits.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum UpdateAction {
+    /// Update via `npm install -g @openai/codex@latest`.
+    NpmGlobalLatest,
+    /// Update via `bun install -g @openai/codex@latest`.
+    BunGlobalLatest,
+    /// Update via `brew upgrade codex`.
+    BrewUpgrade,
+}
+
+#[cfg(any(not(debug_assertions), test))]
+pub(crate) fn get_update_action() -> Option<UpdateAction> {
+    let exe = std::env::current_exe().unwrap_or_default();
+    let managed_by_npm = std::env::var_os("CODEX_MANAGED_BY_NPM").is_some();
+    let managed_by_bun = std::env::var_os("CODEX_MANAGED_BY_BUN").is_some();
+    if managed_by_npm {
+        Some(UpdateAction::NpmGlobalLatest)
+    } else if managed_by_bun {
+        Some(UpdateAction::BunGlobalLatest)
+    } else if cfg!(target_os = "macos")
+        && (exe.starts_with("/opt/homebrew") || exe.starts_with("/usr/local"))
+    {
+        Some(UpdateAction::BrewUpgrade)
+    } else {
+        None
+    }
+}
+
+impl UpdateAction {
+    /// Returns the list of command-line arguments for invoking the update.
+    pub fn command_args(self) -> (&'static str, &'static [&'static str]) {
+        match self {
+            UpdateAction::NpmGlobalLatest => ("npm", &["install", "-g", "@openai/codex@latest"]),
+            UpdateAction::BunGlobalLatest => ("bun", &["install", "-g", "@openai/codex@latest"]),
+            UpdateAction::BrewUpgrade => ("brew", &["upgrade", "codex"]),
+        }
+    }
+
+    /// Returns string representation of the command-line arguments for invoking the update.
+    pub fn command_str(self) -> String {
+        let (command, args) = self.command_args();
+        let args_str = args.join(" ");
+        format!("{command} {args_str}")
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -165,4 +210,24 @@ mod tests {
         assert_eq!(parse_version(" 1.2.3 \n"), Some((1, 2, 3)));
         assert_eq!(is_newer(" 1.2.3 ", "1.2.2"), Some(true));
     }
+
+    #[test]
+    fn test_get_update_action() {
+        let prev = std::env::var_os("CODEX_MANAGED_BY_NPM");
+
+        // First: no npm var -> expect None (we do not run from brew in CI)
+        unsafe { std::env::remove_var("CODEX_MANAGED_BY_NPM") };
+        assert_eq!(get_update_action(), None);
+
+        // Then: with npm var -> expect NpmGlobalLatest
+        unsafe { std::env::set_var("CODEX_MANAGED_BY_NPM", "1") };
+        assert_eq!(get_update_action(), Some(UpdateAction::NpmGlobalLatest));
+
+        // Restore prior value to avoid leaking state
+        if let Some(v) = prev {
+            unsafe { std::env::set_var("CODEX_MANAGED_BY_NPM", v) };
+        } else {
+            unsafe { std::env::remove_var("CODEX_MANAGED_BY_NPM") };
+        }
+    }
 }