fix(list): saturating_sub to fix highlight_symbol overflow (#949)

An overflow (pedantically an underflow) can occur if the
highlight_symbol is a multi-byte char, and area is reduced to a size
less than that char length.

src/widgets/list.rs

@@ -973,7 +973,7 @@ impl StatefulWidgetRef for List<'_> {
                 let highlight_symbol_width = self.highlight_symbol.unwrap_or("").width() as u16;
                 Rect {
                     x: row_area.x + highlight_symbol_width,
-                    width: row_area.width - highlight_symbol_width,
+                    width: row_area.width.saturating_sub(highlight_symbol_width),
                     ..row_area
                 }
             } else {
@@ -1046,7 +1046,7 @@ mod tests {
     use std::borrow::Cow;
 
     use pretty_assertions::assert_eq;
-    use rstest::rstest;
+    use rstest::{fixture, rstest};
 
     use super::*;
     use crate::{assert_buffer_eq, widgets::Borders};
@@ -2234,4 +2234,30 @@ mod tests {
             "          ",
         ]));
     }
+
+    #[fixture]
+    fn single_line_buf() -> Buffer {
+        Buffer::empty(Rect::new(0, 0, 10, 1))
+    }
+
+    /// Regression test for a bug where highlight symbol being greater than width caused a panic due
+    /// to subtraction with underflow.
+    ///
+    /// See [#949](https://github.com/ratatui-org/ratatui/pull/949) for details
+    #[rstest]
+    #[case::under(">>>>", "Item1", ">>>>Item1 ")] // enough space to render the highlight symbol
+    #[case::exact(">>>>>", "Item1", ">>>>>Item1")] // exact space to render the highlight symbol
+    #[case::overflow(">>>>>>", "Item1", ">>>>>>Item")] // not enough space
+    fn highlight_symbol_overflow(
+        #[case] highlight_symbol: &str,
+        #[case] item: &str,
+        #[case] expected: &str,
+        mut single_line_buf: Buffer,
+    ) {
+        let list = List::new(vec![item]).highlight_symbol(highlight_symbol);
+        let mut state = ListState::default();
+        state.select(Some(0));
+        StatefulWidget::render(list, single_line_buf.area, &mut single_line_buf, &mut state);
+        assert_buffer_eq!(single_line_buf, Buffer::with_lines(vec![expected]));
+    }
 }