fix: make enter_alt_screen() respect alt_screen_enabled flag

Addresses review comments:
- Resume picker now respects --no-alt-screen flag
- Legacy TUI now properly wires up the flag

The fix adds an alt_screen_enabled flag to the Tui struct that makes
enter_alt_screen()/leave_alt_screen() no-ops when false. This ensures
all call sites (resume picker, overlays, etc.) respect the setting.

Author: hjanuschka

codex-rs/tui/src/lib.rs

@@ -22,6 +22,8 @@ use codex_core::config::resolve_oss_provider;
 use codex_core::find_conversation_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;
@@ -497,7 +499,27 @@ async fn run_ratatui_app(
         resume_picker::ResumeSelection::StartFresh
     };
 
-    let Cli { prompt, images, .. } = cli;
+    let Cli {
+        prompt,
+        images,
+        no_alt_screen,
+        ..
+    } = cli;
+
+    // Determine whether to use alternate screen based on CLI flag and config.
+    let use_alt_screen = if no_alt_screen {
+        false
+    } else {
+        match config.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 { .. }))
+            }
+        }
+    };
+    tui.set_alt_screen_enabled(use_alt_screen);
 
     let app_result = App::run(
         &mut tui,

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/lib.rs

@@ -541,9 +541,9 @@ async fn run_ratatui_app(
         }
     };
 
-    if use_alt_screen {
-        let _ = tui.enter_alt_screen();
-    }
+    // 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(
         &mut tui,
@@ -558,9 +558,7 @@ async fn run_ratatui_app(
     )
     .await;
 
-    if use_alt_screen {
-        let _ = tui.leave_alt_screen();
-    }
+    let _ = tui.leave_alt_screen();
     restore();
     if let Ok(exit_info) = &app_result {
         let mut stdout = std::io::stdout();

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);