feat(terminal): add synchronized output (DEC mode ?2026)

CHANGELOG.md

@@ -6,6 +6,7 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 ## [Unreleased]
 
 ### Added
+- synchronized output (DEC mode `?2026`): frames are held while a program is mid-update, eliminating tearing
 - add `--maximized` and `--fullscreen` flags to start the window in that mode
 
 ### Changed

src/renderer/render_ops.rs

@@ -86,12 +86,27 @@ pub(super) fn draw_overlays(
 }
 
 impl App {
+    /// True while the active pane's program holds synchronized output (?2026h).
+    fn active_grid_synchronized(&self) -> bool {
+        let tab = &self.state.tabs[self.state.active_tab];
+        tab.panes
+            .get(&tab.active)
+            .and_then(|e| e.pane.grid_read().map(|g| g.synchronized_output))
+            .unwrap_or(false)
+    }
+
     pub(crate) fn redraw(&mut self) {
         // No tabs means startup failed to spawn any pane and an exit is pending;
         // skip the frame rather than indexing an empty `tabs`.
         if self.state.tabs.is_empty() {
             return;
         }
+        // Synchronized output (?2026): the running program is mid-update, so skip
+        // the frame entirely — the window keeps the last presented buffer and never
+        // shows a partial screen. The next PTY byte (the ?2026l reset) redraws.
+        if self.active_grid_synchronized() {
+            return;
+        }
         if self.state.blink_last.elapsed()
             >= Duration::from_millis(self.state.config.window.cursor_blink_ms as u64)
         {

src/terminal/grid.rs

@@ -197,6 +197,8 @@ pub struct Grid {
     pub cursor_shape: CursorShape,
     // Bracketed paste mode (?2004)
     pub bracketed_paste: bool,
+    // Synchronized output (?2026): while true, the renderer skips presenting frames
+    pub synchronized_output: bool,
     // Mouse reporting mode: 0=off, 1000=click, 1002=button-motion, 1003=any-motion
     pub mouse_mode: u16,
     // SGR extended mouse encoding (?1006)
@@ -281,6 +283,7 @@ impl Grid {
             cursor_visible: true,
             cursor_shape: CursorShape::Block,
             bracketed_paste: false,
+            synchronized_output: false,
             mouse_mode: 0,
             mouse_sgr: false,
             alternate_saved: None,
@@ -1033,6 +1036,7 @@ impl Grid {
         self.cursor_visible = true;
         self.cursor_shape = CursorShape::Block;
         self.bracketed_paste = false;
+        self.synchronized_output = false;
         self.mouse_mode = 0;
         self.mouse_sgr = false;
         self.application_cursor_keys = false;

src/terminal/parser.rs

@@ -66,27 +66,20 @@ struct Performer<'a> {
 
 impl Performer<'_> {
     fn handle_dec_private_modes(&mut self, action: char, p0: u16) {
+        // Only DECSET (h) / DECRST (l) toggle modes; the `('h' | 'l', _)` patterns
+        // keep other actions out of these arms, so `on` is only used when relevant.
+        let on = action == 'h';
         match (action, p0) {
-            ('h', 1) => self.grid.application_cursor_keys = true,
-            ('l', 1) => self.grid.application_cursor_keys = false,
-            ('h', 7) => self.grid.autowrap = true,
-            ('l', 7) => self.grid.autowrap = false,
-            ('h', 25) => self.grid.cursor_visible = true,
-            ('l', 25) => self.grid.cursor_visible = false,
-            ('h', 1000) => self.grid.mouse_mode = 1000,
-            ('l', 1000) => self.grid.mouse_mode = 0,
-            ('h', 1002) => self.grid.mouse_mode = 1002,
-            ('l', 1002) => self.grid.mouse_mode = 0,
-            ('h', 1003) => self.grid.mouse_mode = 1003,
-            ('l', 1003) => self.grid.mouse_mode = 0,
-            ('h', 1004) => self.grid.focus_report = true,
-            ('l', 1004) => self.grid.focus_report = false,
-            ('h', 1006) => self.grid.mouse_sgr = true,
-            ('l', 1006) => self.grid.mouse_sgr = false,
+            ('h' | 'l', 1) => self.grid.application_cursor_keys = on,
+            ('h' | 'l', 7) => self.grid.autowrap = on,
+            ('h' | 'l', 25) => self.grid.cursor_visible = on,
+            ('h' | 'l', 1000 | 1002 | 1003) => self.grid.mouse_mode = if on { p0 } else { 0 },
+            ('h' | 'l', 1004) => self.grid.focus_report = on,
+            ('h' | 'l', 1006) => self.grid.mouse_sgr = on,
+            ('h' | 'l', 2004) => self.grid.bracketed_paste = on,
+            ('h' | 'l', 2026) => self.grid.synchronized_output = on,
             ('h', 1049) => self.grid.enter_alternate_screen(),
             ('l', 1049) => self.grid.exit_alternate_screen(),
-            ('h', 2004) => self.grid.bracketed_paste = true,
-            ('l', 2004) => self.grid.bracketed_paste = false,
             _ => {}
         }
     }

src/terminal/parser_test.rs

@@ -290,6 +290,15 @@ fn bracketed_paste_mode() {
     assert!(!p.grid.bracketed_paste);
 }
 
+#[test]
+fn synchronized_output_mode() {
+    let mut p = make_parser(10, 5);
+    p.process(b"\x1b[?2026h");
+    assert!(p.grid.synchronized_output);
+    p.process(b"\x1b[?2026l");
+    assert!(!p.grid.synchronized_output);
+}
+
 #[test]
 fn scroll_region_sets_top_and_bottom() {
     let mut p = make_parser(10, 10);