Preserve inline TUI scrollback in Windows Terminal (#39619)

## Why

Windows Terminal can discard rows scrolled through a partial DEC scroll region instead of retaining them in terminal scrollback.

## What changed

- Detect Windows Terminal, including via `WT_SESSION`, and use full-screen scrolling for inline viewport growth and history insertion.
- Keep the existing terminal-wrapped history behavior for Zellij while centralizing terminal-specific scrollback selection.

## Testing

Add VT100-backed tests covering strategy selection and preservation of scrollback during history insertion and viewport growth.

GitOrigin-RevId: 209a7a8053a1fcf911d6e0aee764e563b8725bd1

codex-rs/tui/src/tui.rs

@@ -48,7 +48,6 @@ use self::input_boundary::terminal_input_is_readable;
 use crate::custom_terminal;
 use crate::custom_terminal::Terminal as CustomTerminal;
 use crate::insert_history::HistoryLineWrapPolicy;
-use crate::insert_history::InsertHistoryMode;
 use crate::notifications::DesktopNotificationBackend;
 use crate::notifications::detect_backend;
 use crate::terminal_hyperlinks::HyperlinkLine;
@@ -58,6 +57,7 @@ use crate::tui::event_stream::TuiEventStream;
 #[cfg(unix)]
 use crate::tui::job_control::SuspendContext;
 use crate::tui::screen_size::ScreenSizePolicy;
+use crate::tui::scrollback::ScrollbackStrategy;
 use codex_config::types::NotificationCondition;
 use codex_config::types::NotificationMethod;
 
@@ -70,6 +70,7 @@ mod input_boundary;
 mod job_control;
 mod keyboard_modes;
 mod screen_size;
+mod scrollback;
 #[cfg(all(test, unix))]
 #[path = "tui_startup_tests.rs"]
 mod startup_tests;
@@ -594,8 +595,7 @@ pub struct Tui {
     enhanced_keys_supported: bool,
     notification_backend: Option<DesktopNotificationBackend>,
     notification_condition: NotificationCondition,
-    // Raw terminal-wrapped history needs a non-scroll-region insertion path in Zellij.
-    is_zellij: bool,
+    scrollback: ScrollbackStrategy,
     // When false, enter_alt_screen() becomes a no-op.
     alt_screen_enabled: bool,
     // Keeps unmanaged process stderr writes out of the inline viewport.
@@ -631,7 +631,7 @@ impl Tui {
         // Cache this to avoid contention with the event reader.
         supports_color::on_cached(supports_color::Stream::Stdout);
         let _ = crate::terminal_palette::default_colors();
-        let is_zellij = codex_terminal_detection::terminal_info().is_zellij();
+        let scrollback = ScrollbackStrategy::detect(&codex_terminal_detection::terminal_info());
 
         Self {
             frame_requester,
@@ -650,7 +650,7 @@ impl Tui {
             enhanced_keys_supported,
             notification_backend: Some(detect_backend(NotificationMethod::default())),
             notification_condition: NotificationCondition::default(),
-            is_zellij,
+            scrollback,
             alt_screen_enabled: true,
             _stderr_guard: stderr_guard,
         }
@@ -893,6 +893,7 @@ impl Tui {
         terminal: &mut Terminal,
         height: u16,
         screen_size: Size,
+        scrollback: ScrollbackStrategy,
     ) -> Result<bool> {
         let terminal_height_shrank = screen_size.height < terminal.last_known_screen_size.height;
         let terminal_height_grew = screen_size.height > terminal.last_known_screen_size.height;
@@ -908,9 +909,7 @@ impl Tui {
         if area.bottom() > screen_size.height {
             let scroll_by = area.bottom() - screen_size.height;
             if !terminal_height_shrank {
-                terminal
-                    .backend_mut()
-                    .scroll_region_up(0..area.top(), scroll_by)?;
+                scrollback.grow_viewport(terminal, area.top(), screen_size, scroll_by)?;
             }
             area.y = screen_size.height - area.height;
         } else if terminal_height_grew && viewport_was_bottom_aligned {
@@ -931,7 +930,7 @@ impl Tui {
     fn flush_pending_history_lines(
         terminal: &mut Terminal,
         pending_history_lines: &mut Vec<PendingHistoryLines>,
-        is_zellij: bool,
+        scrollback: ScrollbackStrategy,
         screen_size: Size,
     ) -> Result<()> {
         if pending_history_lines.is_empty() {
@@ -939,11 +938,7 @@ impl Tui {
         }
 
         for batch in pending_history_lines.iter() {
-            let mode = if is_zellij && batch.wrap_policy == HistoryLineWrapPolicy::Terminal {
-                InsertHistoryMode::ZellijRaw
-            } else {
-                InsertHistoryMode::Standard
-            };
+            let mode = scrollback.history_insertion_mode(batch.wrap_policy);
             crate::insert_history::insert_history_hyperlink_lines_with_mode_and_wrap_policy(
                 terminal,
                 &batch.lines,
@@ -992,9 +987,12 @@ impl Tui {
             area.width = screen_size.width;
             // If the viewport has expanded, scroll everything else up to make room.
             if area.bottom() > screen_size.height {
-                terminal
-                    .backend_mut()
-                    .scroll_region_up(0..area.top(), area.bottom() - screen_size.height)?;
+                self.scrollback.grow_viewport(
+                    terminal,
+                    area.top(),
+                    screen_size,
+                    area.bottom() - screen_size.height,
+                )?;
                 area.y = screen_size.height - area.height;
             }
             if area != terminal.viewport_area {
@@ -1007,7 +1005,7 @@ impl Tui {
             Self::flush_pending_history_lines(
                 terminal,
                 &mut self.pending_history_lines,
-                self.is_zellij,
+                self.scrollback,
                 screen_size,
             )?;
 
@@ -1114,8 +1112,12 @@ impl Tui {
             }
 
             let terminal = &mut self.terminal;
-            let needs_full_repaint =
-                Self::update_inline_viewport_for_resize_reflow(terminal, height, screen_size)?;
+            let needs_full_repaint = Self::update_inline_viewport_for_resize_reflow(
+                terminal,
+                height,
+                screen_size,
+                self.scrollback,
+            )?;
             // A zero- or one-row history region cannot isolate raw history writes from the
             // viewport, so replayed rows can leave stale cells inside the composer.
             let history_can_overlap_viewport =
@@ -1123,7 +1125,7 @@ impl Tui {
             Self::flush_pending_history_lines(
                 terminal,
                 &mut self.pending_history_lines,
-                self.is_zellij,
+                self.scrollback,
                 screen_size,
             )?;