fix(cli): draw grapheme clusters whole in the theme preview

The +list-themes interactive preview dropped the second emoji of ZWJ
clusters (e.g. woman + ZWJ + rocket) in the search box. Zig's libvaxis
enables Unicode grapheme mode 2027 for its session and sizes text per
grapheme cluster; the Rust preview did neither, so the terminal split
the cluster at the ZWJ and the cursor column was computed from summed
per-codepoint widths (4 instead of 2).
- The preview sends ?2027h on entry and ?2027l on exit like vaxis.
- tui::gwidth follows vaxis's unicode width rules per grapheme cluster
  (VS15/VS16, emoji presentation, regional-indicator pairs) using the
  ghostty-unicode emoji table; the preview's print path uses it.
+list-colors and +list-keybinds pretty output (also using gwidth) stays
byte-identical to /usr/bin/ghostty at widths 40/80/132.

Test Plan:
- cargo test -p ghostty-cli --test list_themes_pty
  zwj_emoji_cluster_is_drawn_whole_in_search_box
- cargo test --workspace --no-fail-fast

Cargo.lock

@@ -1436,6 +1436,7 @@ dependencies = [
  "ghostty-helpgen",
  "ghostty-input",
  "ghostty-os",
+ "ghostty-unicode",
  "ghostty-vt",
  "log",
  "ratatui",

crates/ghostty-cli/Cargo.toml

@@ -18,6 +18,7 @@ ghostty-crash.workspace = true
 ghostty-font.workspace = true
 ghostty-input.workspace = true
 ghostty-os.workspace = true
+ghostty-unicode.workspace = true
 ghostty-vt.workspace = true
 log = "0.4.34"
 ratatui = "0.30.2"

crates/ghostty-cli/src/list_themes/preview.rs

@@ -1570,7 +1570,7 @@ impl Win<'_, '_> {
         let x = self.area.x + col;
         let y = self.area.y + row;
         let max = usize::from(self.area.width - col);
-        let (nx, _) = self.buf.borrow_mut().set_stringn(x, y, text, max, style);
+        let nx = self.set_graphemes(x, y, text, max, style);
         if (link.is_some() || ul.is_some()) && nx > x {
             self.decos.borrow_mut().push(Deco {
                 x,
@@ -1583,6 +1583,33 @@ impl Win<'_, '_> {
         nx - self.area.x
     }
 
+    /// Write `text` grapheme by grapheme, each sized with `vaxis.gwidth`
+    /// (not ratatui's width), stopping when a grapheme no longer fits.
+    fn set_graphemes(&self, x: u16, y: u16, text: &str, max: usize, style: Style) -> u16 {
+        let mut buf = self.buf.borrow_mut();
+        let mut remaining = max.min(usize::from(buf.area.right().saturating_sub(x)));
+        let mut cx = x;
+        for g in text.graphemes(true) {
+            if g.contains(char::is_control) {
+                continue;
+            }
+            let w = gw(g);
+            if w == 0 {
+                continue;
+            }
+            let Some(rest) = remaining.checked_sub(usize::from(w)) else {
+                break;
+            };
+            remaining = rest;
+            buf[(cx, y)].set_symbol(g).set_style(style);
+            for i in 1..w {
+                buf[(cx + i, y)].reset();
+            }
+            cx += w;
+        }
+        cx
+    }
+
     /// `Window.print`: several segments one after another.
     fn print_segments(&self, segments: &[(&str, Style)], row: u16, col: u16) -> u16 {
         let mut col = col;
@@ -1711,7 +1738,7 @@ fn run_loop(app: &mut Preview) -> Result<(), Error> {
     // Alternate screen, title, mouse reporting.
     write!(
         tty,
-        "\x1b[?1049h\x1b]2;\u{1f47b} Ghostty Theme Preview \u{1f47b}\x1b\\\
+        "\x1b[?1049h\x1b[?2027h\x1b]2;\u{1f47b} Ghostty Theme Preview \u{1f47b}\x1b\\\
          \x1b[?1000h\x1b[?1002h\x1b[?1003h\x1b[?1006h"
     )?;
 
@@ -1767,7 +1794,7 @@ fn run_loop(app: &mut Preview) -> Result<(), Error> {
     let _ = terminal.show_cursor();
     let _ = write!(
         tty,
-        "{}\x1b[?1006l\x1b[?1003l\x1b[?1002l\x1b[?1000l\x1b[?1049l",
+        "{}\x1b[?1006l\x1b[?1003l\x1b[?1002l\x1b[?1000l\x1b[?2027l\x1b[?1049l",
         Csi::Mode(Mode::ResetDecPrivateMode(theme_mode)),
     );
     let _ = tty.flush();

crates/ghostty-cli/src/tui.rs

@@ -64,10 +64,49 @@ pub struct Row {
     rows: Vec<Vec<Cell>>,
 }
 
-/// Display width of a string (`Window.gwidth` with unicode mode on).
+/// Display width of one grapheme (`vaxis.gwidth` `.unicode` method).
+fn grapheme_width(g: &str) -> u16 {
+    let mut width = 0u16;
+    let (mut vs16, mut vs15, mut emoji, mut ri) = (false, false, false, 0u8);
+    for c in g.chars() {
+        match c {
+            '\u{fe0f}' => {
+                vs16 = true;
+                continue;
+            }
+            '\u{fe0e}' => {
+                vs15 = true;
+                continue;
+            }
+            _ => {}
+        }
+        if ghostty_unicode::emoji_table::table().get(u32::from(c)) {
+            emoji = true;
+        }
+        if ('\u{1F1E6}'..='\u{1F1FF}').contains(&c) {
+            ri += 1;
+        }
+        // 0x1f3fb..=0x1f3ff are wide in vaxis' EAW table too.
+        let w = c.width().unwrap_or(0);
+        width = width.max(u16::try_from(w).unwrap_or(2));
+    }
+    if vs15 {
+        width = width.max(1);
+    } else if vs16 || emoji || ri == 2 {
+        width = width.max(2);
+    }
+    width
+}
+
+/// Display width of a string (`Window.gwidth` with unicode mode on): the sum
+/// of the widths of its grapheme clusters, as `vaxis.gwidth` computes it.
 #[must_use]
 pub fn gwidth(s: &str) -> u16 {
-    let w: usize = s.chars().map(|c| c.width().unwrap_or(0)).sum();
+    use unicode_segmentation::UnicodeSegmentation;
+    let w: usize = s
+        .graphemes(true)
+        .map(|g| usize::from(grapheme_width(g)))
+        .sum();
     u16::try_from(w).unwrap_or(u16::MAX)
 }
 
@@ -432,6 +471,10 @@ mod tests {
         let r = row.print_segment("a\u{4e2d}b", Style::default(), 0);
         assert_eq!(4, r.col);
         assert_eq!(2, gwidth("\u{4e2d}"));
+        assert_eq!(2, gwidth("\u{1F469}\u{200D}\u{1F680}"));
+        assert_eq!(2, gwidth("\u{2764}\u{FE0F}"));
+        assert_eq!(2, gwidth("\u{1F1FA}\u{1F1F8}"));
+        assert_eq!(0, gwidth("\u{200B}"));
         let r = row.print_segment("zz", Style::default(), 3);
         assert!(!r.overflow);
     }

tests/list_themes_pty.rs

@@ -380,3 +380,10 @@ fn load_file_error_exits_the_preview() {
     }
     assert!(code.is_some(), "the preview must have exited");
 }
+
+#[test]
+fn zwj_emoji_cluster_is_drawn_whole_in_search_box() {
+    let cluster = "\u{1F469}\u{200D}\u{1F680}";
+    let typed = format!("a{cluster}b");
+    check_trace(&[typed.as_bytes()], &[&typed]);
+}