Tweak a few things for clarity / future understanding

Author: joshka-oai

codex-rs/tui/src/custom_terminal.rs

@@ -411,33 +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 mut last_nonblank: Option<usize> = None;
-        let mut col = 0usize;
-        while col < row.len() {
-            let cell = &row[col];
-            let width = cell.symbol().width().max(1);
+        // 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 = Some(col + width.saturating_sub(1));
+                last_nonblank_column = column + (width.saturating_sub(1));
             }
-            col += width;
+            column += width.max(1); // treat zero-width symbols as width 1
         }
-        let x = last_nonblank.unwrap_or(0);
-        last_nonblank_column[y as usize] = x as u16;
-        if x + 1 < row.len() {
-            let (x_abs, y_abs) = a.pos_of(row_start + x + 1);
-            updates.push(DrawCommand::ClearToEnd {
-                x: x_abs,
-                y: y_abs,
-                bg,
-            });
+
+        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:
@@ -449,7 +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,