Fix TUI layout for halfwidth Japanese sound marks (#35962)

## What changed

- Use Ratatui-compatible terminal widths throughout TUI layout, preserving
  full-width counters for long lines.
- Wrap and truncate on grapheme boundaries so halfwidth kana sound marks and
  emoji sequences stay intact.
- Apply the corrected sizing to text input, selection popups, status and
  history cells, Markdown, hyperlinks, and scrollable cloud diffs.

## Testing

- Add focused unit and snapshot coverage for wrapping, truncation, cursor and
  mask alignment, popup columns, status output, session headers, Markdown
  tables, process output, and scrollable diffs.

GitOrigin-RevId: 294ec0bcff743c6bb16d55ec26308524ce741879

codex-rs/tui/src/width.rs

@@ -1,16 +1,38 @@
-//! Width guards for transcript rendering with fixed prefix columns.
+//! Terminal display-width helpers and guards for fixed prefix columns.
 //!
 //! Several rendering paths reserve a fixed number of columns for bullets,
 //! gutters, or labels before laying out content.  When the terminal is very
 //! narrow, those reserved columns can consume the entire width, leaving zero
 //! or negative space for content.
 //!
-//! These helpers centralise the subtraction and enforce a strict-positive
+//! The display-width helpers match Ratatui's terminal-cell semantics while retaining `usize`
+//! precision for long lines. The guards centralise subtraction and enforce a strict-positive
 //! contract: they return `Some(n)` where `n > 0`, or `None` when no usable
 //! content width remains.  Callers treat `None` as "render prefix-only
 //! fallback" rather than attempting wrapped rendering at zero width, which
 //! would produce empty or unstable output.
 
+use unicode_width::UnicodeWidthChar;
+use unicode_width::UnicodeWidthStr;
+
+/// Returns the display width Ratatui uses for terminal text without its `u16` limit.
+pub(crate) fn display_width(text: &str) -> usize {
+    UnicodeWidthStr::width(text)
+        + text
+            .chars()
+            .filter(|ch| matches!(ch, '\u{FF9E}' | '\u{FF9F}'))
+            .count()
+}
+
+/// Returns a scalar's terminal width, treating halfwidth sound marks as visible cells.
+pub(crate) fn char_width(ch: char) -> usize {
+    if matches!(ch, '\u{FF9E}' | '\u{FF9F}') {
+        1
+    } else {
+        UnicodeWidthChar::width(ch).unwrap_or(0)
+    }
+}
+
 /// Returns usable content width after reserving fixed columns.
 ///
 /// Guarantees a strict positive width (`Some(n)` where `n > 0`) or `None` when
@@ -36,7 +58,22 @@ pub(crate) fn usable_content_width_u16(total_width: u16, reserved_cols: u16) ->
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::line_truncation::line_width;
     use pretty_assertions::assert_eq;
+    use ratatui::text::Line;
+
+    #[test]
+    fn display_width_matches_ratatui_halfwidth_sound_marks_without_overflow() {
+        assert_eq!(display_width("ｶﾞﾊﾟ"), 4);
+        assert_eq!(display_width("ｶﾞﾞ"), 3);
+        assert_eq!(display_width("界ﾞ"), 3);
+        assert_eq!(char_width('\u{FF9E}'), 1);
+        assert_eq!(char_width('\u{FF9F}'), 1);
+
+        let text = "a".repeat(65_536);
+        assert_eq!(display_width(&text), 65_536);
+        assert_eq!(line_width(&Line::from(text)), 65_536);
+    }
 
     #[test]
     fn usable_content_width_returns_none_when_reserved_exhausts_width() {