fix: add tui.alternate_screen config and --no-alt-screen CLI flag for Zellij scrollback (#8555)

Fixes #2558

Codex uses alternate screen mode (CSI 1049) which, per xterm spec,
doesn't support scrollback. Zellij follows this strictly, so users can't
scroll back through output.

**Changes:**
- Add `tui.alternate_screen` config: `auto` (default), `always`, `never`
- Add `--no-alt-screen` CLI flag
- Auto-detect Zellij and skip alt screen (uses existing `ZELLIJ` env var
detection)

**Usage:**
```bash
# CLI flag
codex --no-alt-screen

# Or in config.toml
[tui]
alternate_screen = "never"
```

With default `auto` mode, Zellij users get working scrollback without
any config changes.

---------

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

codex-rs/core/src/config/mod.rs

@@ -32,6 +32,7 @@ use crate::protocol::AskForApproval;
 use crate::protocol::SandboxPolicy;
 use codex_app_server_protocol::Tools;
 use codex_app_server_protocol::UserSavedConfig;
+use codex_protocol::config_types::AltScreenMode;
 use codex_protocol::config_types::ForcedLoginMethod;
 use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::config_types::SandboxMode;
@@ -236,6 +237,14 @@ pub struct Config {
     /// consistently to both mouse wheels and trackpads.
     pub tui_scroll_invert: bool,
 
+    /// Controls whether the TUI uses the terminal's alternate screen buffer.
+    ///
+    /// This is the same `tui.alternate_screen` value from `config.toml` (see [`Tui`]).
+    /// - `auto` (default): Disable alternate screen in Zellij, enable elsewhere.
+    /// - `always`: Always use alternate screen (original behavior).
+    /// - `never`: Never use alternate screen (inline mode, preserves scrollback).
+    pub tui_alternate_screen: AltScreenMode,
+
     /// The directory that should be treated as the current working directory
     /// for the session. All relative paths inside the business-logic layer are
     /// resolved against this path.
@@ -1443,6 +1452,11 @@ impl Config {
                 .as_ref()
                 .and_then(|t| t.scroll_wheel_like_max_duration_ms),
             tui_scroll_invert: cfg.tui.as_ref().map(|t| t.scroll_invert).unwrap_or(false),
+            tui_alternate_screen: cfg
+                .tui
+                .as_ref()
+                .map(|t| t.alternate_screen)
+                .unwrap_or_default(),
             otel: {
                 let t: OtelConfigToml = cfg.otel.unwrap_or_default();
                 let log_user_prompt = t.log_user_prompt.unwrap_or(false);
@@ -1641,6 +1655,7 @@ persistence = "none"
                 scroll_wheel_tick_detect_max_ms: None,
                 scroll_wheel_like_max_duration_ms: None,
                 scroll_invert: false,
+                alternate_screen: AltScreenMode::Auto,
             }
         );
     }
@@ -3276,6 +3291,7 @@ model_verbosity = "high"
                 tui_scroll_wheel_tick_detect_max_ms: None,
                 tui_scroll_wheel_like_max_duration_ms: None,
                 tui_scroll_invert: false,
+                tui_alternate_screen: AltScreenMode::Auto,
                 otel: OtelConfig::default(),
             },
             o3_profile_config
@@ -3361,6 +3377,7 @@ model_verbosity = "high"
             tui_scroll_wheel_tick_detect_max_ms: None,
             tui_scroll_wheel_like_max_duration_ms: None,
             tui_scroll_invert: false,
+            tui_alternate_screen: AltScreenMode::Auto,
             otel: OtelConfig::default(),
         };
 
@@ -3461,6 +3478,7 @@ model_verbosity = "high"
             tui_scroll_wheel_tick_detect_max_ms: None,
             tui_scroll_wheel_like_max_duration_ms: None,
             tui_scroll_invert: false,
+            tui_alternate_screen: AltScreenMode::Auto,
             otel: OtelConfig::default(),
         };
 
@@ -3547,6 +3565,7 @@ model_verbosity = "high"
             tui_scroll_wheel_tick_detect_max_ms: None,
             tui_scroll_wheel_like_max_duration_ms: None,
             tui_scroll_invert: false,
+            tui_alternate_screen: AltScreenMode::Auto,
             otel: OtelConfig::default(),
         };
 

codex-rs/core/src/config/types.rs

@@ -3,6 +3,7 @@
 // Note this file should generally be restricted to simple struct/enum
 // definitions that do not contain business logic.
 
+pub use codex_protocol::config_types::AltScreenMode;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use std::collections::BTreeMap;
 use std::collections::HashMap;
@@ -523,6 +524,17 @@ pub struct Tui {
     /// wheel and trackpad input.
     #[serde(default)]
     pub scroll_invert: bool,
+
+    /// Controls whether the TUI uses the terminal's alternate screen buffer.
+    ///
+    /// - `auto` (default): Disable alternate screen in Zellij, enable elsewhere.
+    /// - `always`: Always use alternate screen (original behavior).
+    /// - `never`: Never use alternate screen (inline mode only, preserves scrollback).
+    ///
+    /// Using alternate screen provides a cleaner fullscreen experience but prevents
+    /// scrollback in terminal multiplexers like Zellij that follow the xterm spec.
+    #[serde(default)]
+    pub alternate_screen: AltScreenMode,
 }
 
 const fn default_true() -> bool {

codex-rs/protocol/src/config_types.rs

@@ -80,3 +80,38 @@ pub enum TrustLevel {
     Trusted,
     Untrusted,
 }
+
+/// Controls whether the TUI uses the terminal's alternate screen buffer.
+///
+/// **Background:** The alternate screen buffer provides a cleaner fullscreen experience
+/// without polluting the terminal's scrollback history. However, it conflicts with terminal
+/// multiplexers like Zellij that strictly follow the xterm specification, which defines
+/// that alternate screen buffers should not have scrollback.
+///
+/// **Zellij's behavior:** Zellij intentionally disables scrollback in alternate screen mode
+/// (see https://github.com/zellij-org/zellij/pull/1032) to comply with the xterm spec. This
+/// is by design and not configurable in Zellij—there is no option to enable scrollback in
+/// alternate screen mode.
+///
+/// **Solution:** This setting provides a pragmatic workaround:
+/// - `auto` (default): Automatically detect the terminal multiplexer. If running in Zellij,
+///   disable alternate screen to preserve scrollback. Enable it everywhere else.
+/// - `always`: Always use alternate screen mode (original behavior before this fix).
+/// - `never`: Never use alternate screen mode. Runs in inline mode, preserving scrollback
+///   in all multiplexers.
+///
+/// The CLI flag `--no-alt-screen` can override this setting at runtime.
+#[derive(
+    Debug, Serialize, Deserialize, Default, Clone, Copy, PartialEq, Eq, Display, JsonSchema, TS,
+)]
+#[serde(rename_all = "lowercase")]
+#[strum(serialize_all = "lowercase")]
+pub enum AltScreenMode {
+    /// Auto-detect: disable alternate screen in Zellij, enable elsewhere.
+    #[default]
+    Auto,
+    /// Always use alternate screen (original behavior).
+    Always,
+    /// Never use alternate screen (inline mode only).
+    Never,
+}

codex-rs/tui/src/cli.rs

@@ -85,6 +85,14 @@ pub struct Cli {
     #[arg(long = "add-dir", value_name = "DIR", value_hint = ValueHint::DirPath)]
     pub add_dir: Vec<PathBuf>,
 
+    /// Disable alternate screen mode
+    ///
+    /// Runs the TUI in inline mode, preserving terminal scrollback history. This is useful
+    /// in terminal multiplexers like Zellij that follow the xterm spec strictly and disable
+    /// scrollback in alternate screen buffers.
+    #[arg(long = "no-alt-screen", default_value_t = false)]
+    pub no_alt_screen: bool,
+
     #[clap(skip)]
     pub config_overrides: CliConfigOverrides,
 }

codex-rs/tui/src/lib.rs

@@ -22,6 +22,8 @@ use codex_core::config::resolve_oss_provider;
 use codex_core::find_thread_path_by_id_str;
 use codex_core::get_platform_sandbox;
 use codex_core::protocol::AskForApproval;
+use codex_core::terminal::Multiplexer;
+use codex_protocol::config_types::AltScreenMode;
 use codex_protocol::config_types::SandboxMode;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use std::fs::OpenOptions;
@@ -493,7 +495,15 @@ async fn run_ratatui_app(
         resume_picker::ResumeSelection::StartFresh
     };
 
-    let Cli { prompt, images, .. } = cli;
+    let Cli {
+        prompt,
+        images,
+        no_alt_screen,
+        ..
+    } = cli;
+
+    let use_alt_screen = determine_alt_screen_mode(no_alt_screen, config.tui_alternate_screen);
+    tui.set_alt_screen_enabled(use_alt_screen);
 
     let app_result = App::run(
         &mut tui,
@@ -527,6 +537,37 @@ fn restore() {
     }
 }
 
+/// Determine whether to use the terminal's alternate screen buffer.
+///
+/// The alternate screen buffer provides a cleaner fullscreen experience without polluting
+/// the terminal's scrollback history. However, it conflicts with terminal multiplexers like
+/// Zellij that strictly follow the xterm spec, which disallows scrollback in alternate screen
+/// buffers. Zellij intentionally disables scrollback in alternate screen mode (see
+/// https://github.com/zellij-org/zellij/pull/1032) and offers no configuration option to
+/// change this behavior.
+///
+/// This function implements a pragmatic workaround:
+/// - If `--no-alt-screen` is explicitly passed, always disable alternate screen
+/// - Otherwise, respect the `tui.alternate_screen` config setting:
+///   - `always`: Use alternate screen everywhere (original behavior)
+///   - `never`: Inline mode only, preserves scrollback
+///   - `auto` (default): Auto-detect the terminal multiplexer and disable alternate screen
+///     only in Zellij, enabling it everywhere else
+fn determine_alt_screen_mode(no_alt_screen: bool, tui_alternate_screen: AltScreenMode) -> bool {
+    if no_alt_screen {
+        false
+    } else {
+        match tui_alternate_screen {
+            AltScreenMode::Always => true,
+            AltScreenMode::Never => false,
+            AltScreenMode::Auto => {
+                let terminal_info = codex_core::terminal::terminal_info();
+                !matches!(terminal_info.multiplexer, Some(Multiplexer::Zellij { .. }))
+            }
+        }
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum LoginStatus {
     AuthMode(AuthMode),

codex-rs/tui/src/tui.rs

@@ -247,6 +247,8 @@ pub struct Tui {
     terminal_focused: Arc<AtomicBool>,
     enhanced_keys_supported: bool,
     notification_backend: Option<DesktopNotificationBackend>,
+    // When false, enter_alt_screen() becomes a no-op (for Zellij scrollback support)
+    alt_screen_enabled: bool,
 }
 
 impl Tui {
@@ -274,9 +276,15 @@ impl Tui {
             terminal_focused: Arc::new(AtomicBool::new(true)),
             enhanced_keys_supported,
             notification_backend: Some(detect_backend()),
+            alt_screen_enabled: true,
         }
     }
 
+    /// Set whether alternate screen is enabled. When false, enter_alt_screen() becomes a no-op.
+    pub fn set_alt_screen_enabled(&mut self, enabled: bool) {
+        self.alt_screen_enabled = enabled;
+    }
+
     pub fn frame_requester(&self) -> FrameRequester {
         self.frame_requester.clone()
     }
@@ -407,6 +415,9 @@ impl Tui {
     /// Enter alternate screen and expand the viewport to full terminal size, saving the current
     /// inline viewport for restoration when leaving.
     pub fn enter_alt_screen(&mut self) -> Result<()> {
+        if !self.alt_screen_enabled {
+            return Ok(());
+        }
         let _ = execute!(self.terminal.backend_mut(), EnterAlternateScreen);
         // Enable "alternate scroll" so terminals may translate wheel to arrows
         let _ = execute!(self.terminal.backend_mut(), EnableAlternateScroll);
@@ -426,6 +437,9 @@ impl Tui {
 
     /// Leave alternate screen and restore the previously saved inline viewport, if any.
     pub fn leave_alt_screen(&mut self) -> Result<()> {
+        if !self.alt_screen_enabled {
+            return Ok(());
+        }
         // Disable alternate scroll when leaving alt-screen
         let _ = execute!(self.terminal.backend_mut(), DisableAlternateScroll);
         let _ = execute!(self.terminal.backend_mut(), LeaveAlternateScreen);

codex-rs/tui2/src/cli.rs

@@ -85,6 +85,11 @@ pub struct Cli {
     #[arg(long = "add-dir", value_name = "DIR", value_hint = ValueHint::DirPath)]
     pub add_dir: Vec<PathBuf>,
 
+    /// Disable alternate screen mode for better scrollback in terminal multiplexers like Zellij.
+    /// This runs the TUI in inline mode, preserving terminal scrollback history.
+    #[arg(long = "no-alt-screen", default_value_t = false)]
+    pub no_alt_screen: bool,
+
     #[clap(skip)]
     pub config_overrides: CliConfigOverrides,
 }
@@ -109,6 +114,7 @@ impl From<codex_tui::Cli> for Cli {
             cwd: cli.cwd,
             web_search: cli.web_search,
             add_dir: cli.add_dir,
+            no_alt_screen: cli.no_alt_screen,
             config_overrides: cli.config_overrides,
         }
     }

codex-rs/tui2/src/lib.rs

@@ -22,6 +22,8 @@ use codex_core::config::resolve_oss_provider;
 use codex_core::find_thread_path_by_id_str;
 use codex_core::get_platform_sandbox;
 use codex_core::protocol::AskForApproval;
+use codex_core::terminal::Multiplexer;
+use codex_protocol::config_types::AltScreenMode;
 use codex_protocol::config_types::SandboxMode;
 use codex_utils_absolute_path::AbsolutePathBuf;
 use std::fs::OpenOptions;
@@ -515,12 +517,39 @@ async fn run_ratatui_app(
         resume_picker::ResumeSelection::StartFresh
     };
 
-    let Cli { prompt, images, .. } = cli;
+    let Cli {
+        prompt,
+        images,
+        no_alt_screen,
+        ..
+    } = cli;
 
     // Run the main chat + transcript UI on the terminal's alternate screen so
     // the entire viewport can be used without polluting normal scrollback. This
     // mirrors the behavior of the legacy TUI but keeps inline mode available
     // for smaller prompts like onboarding and model migration.
+    //
+    // However, alternate screen prevents scrollback in terminal multiplexers like
+    // Zellij that strictly follow the xterm spec (which disallows scrollback in
+    // alternate screen buffers). This auto-detects the terminal and disables
+    // alternate screen in Zellij while keeping it enabled elsewhere.
+    let use_alt_screen = if no_alt_screen {
+        // CLI flag explicitly disables alternate screen
+        false
+    } else {
+        match config.tui_alternate_screen {
+            AltScreenMode::Always => true,
+            AltScreenMode::Never => false,
+            AltScreenMode::Auto => {
+                // Auto-detect: disable in Zellij, enable elsewhere
+                let terminal_info = codex_core::terminal::terminal_info();
+                !matches!(terminal_info.multiplexer, Some(Multiplexer::Zellij { .. }))
+            }
+        }
+    };
+
+    // Set flag on Tui so all enter_alt_screen() calls respect the setting
+    tui.set_alt_screen_enabled(use_alt_screen);
     let _ = tui.enter_alt_screen();
 
     let app_result = App::run(

codex-rs/tui2/src/tui.rs

@@ -143,6 +143,8 @@ pub struct Tui {
     terminal_focused: Arc<AtomicBool>,
     enhanced_keys_supported: bool,
     notification_backend: Option<DesktopNotificationBackend>,
+    // When false, enter_alt_screen() becomes a no-op (for Zellij scrollback support)
+    alt_screen_enabled: bool,
 }
 
 impl Tui {
@@ -170,9 +172,15 @@ impl Tui {
             terminal_focused: Arc::new(AtomicBool::new(true)),
             enhanced_keys_supported,
             notification_backend: Some(detect_backend()),
+            alt_screen_enabled: true,
         }
     }
 
+    /// Set whether alternate screen is enabled. When false, enter_alt_screen() becomes a no-op.
+    pub fn set_alt_screen_enabled(&mut self, enabled: bool) {
+        self.alt_screen_enabled = enabled;
+    }
+
     pub fn frame_requester(&self) -> FrameRequester {
         self.frame_requester.clone()
     }
@@ -309,6 +317,9 @@ impl Tui {
     /// Enter alternate screen and expand the viewport to full terminal size, saving the current
     /// inline viewport for restoration when leaving.
     pub fn enter_alt_screen(&mut self) -> Result<()> {
+        if !self.alt_screen_enabled {
+            return Ok(());
+        }
         if !self.alt_screen_nesting.enter() {
             self.alt_screen_active.store(true, Ordering::Relaxed);
             return Ok(());
@@ -330,6 +341,9 @@ impl Tui {
 
     /// Leave alternate screen and restore the previously saved inline viewport, if any.
     pub fn leave_alt_screen(&mut self) -> Result<()> {
+        if !self.alt_screen_enabled {
+            return Ok(());
+        }
         if !self.alt_screen_nesting.leave() {
             self.alt_screen_active
                 .store(self.alt_screen_nesting.is_active(), Ordering::Relaxed);