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/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),