Fix Row::clear_wide panic on truncated wide character

src/row.rs

@@ -56,7 +56,11 @@ impl Row {
         let wide = self.cells[usize::from(i)].is_wide();
         self.clear_wide(i);
         self.cells[usize::from(i)].clear(attrs);
-        if i == self.cols() - if wide { 2 } else { 1 } {
+        // `wide` is captured pre-erase on purpose: a 2-wide char occupies the
+        // logical last column at cols() - 2. saturating_sub guards the
+        // degenerate case of an orphaned wide cell in a one-column row, where
+        // cols() - 2 would underflow.
+        if i == self.cols().saturating_sub(if wide { 2 } else { 1 }) {
             self.wrapped = false;
         }
     }
@@ -71,8 +75,19 @@ impl Row {
     }
 
     pub fn resize(&mut self, len: u16, cell: crate::Cell) {
+        let old_len = self.cells.len();
         self.cells.resize(usize::from(len), cell);
         self.wrapped = false;
+        // When shrinking, a wide char at the new last position may
+        // have lost its continuation cell — clear it to avoid
+        // dangling state. Matches what Row::truncate already does.
+        let new_len = usize::from(len);
+        if new_len > 0 && new_len < old_len {
+            let last_cell = &mut self.cells[new_len - 1];
+            if last_cell.is_wide() {
+                last_cell.clear(*last_cell.attrs());
+            }
+        }
     }
 
     pub fn wrap(&mut self, wrap: bool) {
@@ -84,9 +99,20 @@ impl Row {
     }
 
     pub fn clear_wide(&mut self, col: u16) {
-        let cell = &self.cells[usize::from(col)];
+        let col_idx = usize::from(col);
+        let cell = &self.cells[col_idx];
         let other = if cell.is_wide() {
-            &mut self.cells[usize::from(col + 1)]
+            let next = col_idx + 1;
+            if next >= self.cells.len() {
+                // Wide char at the last column with no continuation
+                // (e.g. a shrinking resize that split the pair).
+                // Clear the cell in place instead of indexing past
+                // the end.
+                let attrs = *self.cells[col_idx].attrs();
+                self.cells[col_idx].clear(attrs);
+                return;
+            }
+            &mut self.cells[next]
         } else if cell.is_wide_continuation() {
             &mut self.cells[usize::from(col - 1)]
         } else {
@@ -472,3 +498,29 @@ impl Row {
         (prev_pos, prev_attrs)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    // An orphaned wide cell (wide flag set, no continuation) in a
+    // one-column row must not make Row::erase underflow when it derives the
+    // logical last column as cols() - 2. Covers the Copilot review note on
+    // the clear_wide OOB fix: clear_wide itself was hardened, but its caller
+    // erase computed cols() - 2 == 1 - 2 from the captured `wide` value.
+    #[test]
+    fn erase_wide_cell_in_one_column_row_does_not_underflow() {
+        let attrs = crate::attrs::Attrs::default();
+        let mut row = Row::new(1);
+        // A 2-wide char in the only column: a wide cell with no room for a
+        // continuation cell.
+        row.get_mut(0).unwrap().set('中', attrs);
+        assert!(row.get(0).unwrap().is_wide());
+
+        row.erase(0, attrs);
+
+        let cell = row.get(0).unwrap();
+        assert!(!cell.is_wide(), "erased cell must no longer be wide");
+        assert!(!cell.has_contents(), "erased cell must be empty");
+    }
+}

tests/basic.rs

@@ -122,3 +122,16 @@ fn cell_attrs() {
 
     assert!(parser.screen().cell(0, 4).unwrap().italic());
 }
+
+// Regression test for the panic where shrinking a screen truncates the
+// continuation cell of a wide character, then erasing the line tries
+// to clear past the end of the row. The repro is from
+// https://github.com/doy/vt100-rust/issues/28.
+#[test]
+fn resize_truncating_wide_char_then_erase_line_does_not_panic() {
+    let mut parser = vt100::Parser::new(2, 4, 0);
+    parser.process("你".as_bytes());
+    parser.screen_mut().set_size(2, 1);
+    parser.process(b"\x1b[K");
+    assert_eq!(parser.screen().size(), (2, 1));
+}