fix: diff_buffers clear-to-end when deleting wide graphemes (#4921)

Fixes #4870  #4717 #3260 #4431 #2718 #4898 #5036

- Fix the chat composer “phantom space” bug that appeared when
backspacing CJK (and other double-width) characters after the composer
got a uniform background in 43b63ccae89c….
- Pull diff_buffers’s clear-to-end logic forward to iterate by display
width, so wide graphemes are counted correctly when computing the
trailing column.
- Keep modifier-aware detection so styled cells are still flushed, and
add a regression test (diff_buffers_clear_to_end_starts_after_wide_char)
that covers the CJK deletion scenario.

---------

Co-authored-by: Josh McKinney <joshka@openai.com>

codex-rs/tui/src/custom_terminal.rs

@@ -411,28 +411,35 @@ fn diff_buffers<'a>(a: &'a Buffer, b: &'a Buffer) -> Vec<DrawCommand<'a>> {
     let next_buffer = &b.content;
 
     let mut updates = vec![];
-    let mut last_nonblank_column = vec![0; a.area.height as usize];
+    let mut last_nonblank_columns = vec![0; a.area.height as usize];
     for y in 0..a.area.height {
         let row_start = y as usize * a.area.width as usize;
         let row_end = row_start + a.area.width as usize;
         let row = &next_buffer[row_start..row_end];
         let bg = row.last().map(|cell| cell.bg).unwrap_or(Color::Reset);
 
-        let x = row
-            .iter()
-            .rposition(|cell| {
-                cell.symbol() != " " || cell.bg != bg || cell.modifier != Modifier::empty()
-            })
-            .unwrap_or(0);
-        last_nonblank_column[y as usize] = x as u16;
-        if x < (a.area.width as usize).saturating_sub(1) {
-            let (x_abs, y_abs) = a.pos_of(row_start + x + 1);
-            updates.push(DrawCommand::ClearToEnd {
-                x: x_abs,
-                y: y_abs,
-                bg,
-            });
+        // Scan the row to find the rightmost column that still matters: any non-space glyph,
+        // any cell whose bg differs from the row’s trailing bg, or any cell with modifiers.
+        // Multi-width glyphs extend that region through their full displayed width.
+        // After that point the rest of the row can be cleared with a single ClearToEnd, a perf win
+        // versus emitting multiple space Put commands.
+        let mut last_nonblank_column = 0usize;
+        let mut column = 0usize;
+        while column < row.len() {
+            let cell = &row[column];
+            let width = cell.symbol().width();
+            if cell.symbol() != " " || cell.bg != bg || cell.modifier != Modifier::empty() {
+                last_nonblank_column = column + (width.saturating_sub(1));
+            }
+            column += width.max(1); // treat zero-width symbols as width 1
         }
+
+        if last_nonblank_column + 1 < row.len() {
+            let (x, y) = a.pos_of(row_start + last_nonblank_column + 1);
+            updates.push(DrawCommand::ClearToEnd { x, y, bg });
+        }
+
+        last_nonblank_columns[y as usize] = last_nonblank_column as u16;
     }
 
     // Cells invalidated by drawing/replacing preceding multi-width characters:
@@ -444,7 +451,7 @@ fn diff_buffers<'a>(a: &'a Buffer, b: &'a Buffer) -> Vec<DrawCommand<'a>> {
         if !current.skip && (current != previous || invalidated > 0) && to_skip == 0 {
             let (x, y) = a.pos_of(i);
             let row = i / a.area.width as usize;
-            if x <= last_nonblank_column[row] {
+            if x <= last_nonblank_columns[row] {
                 updates.push(DrawCommand::Put {
                     x,
                     y,
@@ -592,6 +599,7 @@ mod tests {
     use super::*;
     use pretty_assertions::assert_eq;
     use ratatui::layout::Rect;
+    use ratatui::style::Style;
 
     #[test]
     fn diff_buffers_does_not_emit_clear_to_end_for_full_width_row() {
@@ -620,4 +628,22 @@ mod tests {
             "expected diff_buffers to update the final cell; commands: {commands:?}",
         );
     }
+
+    #[test]
+    fn diff_buffers_clear_to_end_starts_after_wide_char() {
+        let area = Rect::new(0, 0, 10, 1);
+        let mut previous = Buffer::empty(area);
+        let mut next = Buffer::empty(area);
+
+        previous.set_string(0, 0, "中文", Style::default());
+        next.set_string(0, 0, "中", Style::default());
+
+        let commands = diff_buffers(&previous, &next);
+        assert!(
+            commands
+                .iter()
+                .any(|command| matches!(command, DrawCommand::ClearToEnd { x: 2, y: 0, .. })),
+            "expected clear-to-end to start after the remaining wide char; commands: {commands:?}"
+        );
+    }
 }