Fix 1x1 grid scrolling

src/grid.rs

@@ -680,7 +680,7 @@ impl Grid {
             let mut prev_pos = self.pos;
             self.pos.col = 0;
             let scrolled = self.row_inc_scroll(1);
-            prev_pos.row -= scrolled;
+            prev_pos.row = prev_pos.row.saturating_sub(scrolled);
             let new_pos = self.pos;
             self.drawing_row_mut(prev_pos.row)
                 // we assume self.pos.row is always valid, and so prev_pos.row

tests/scroll.rs

@@ -176,6 +176,15 @@ fn edge_of_screen() {
     );
 }
 
+#[test]
+fn one_row_wrap_does_not_panic() {
+    let mut parser = vt100::Parser::new(1, 1, 10);
+
+    parser.process(b"ab");
+
+    assert_eq!(parser.screen().contents(), "b");
+}
+
 #[test]
 fn scrollback_larger_than_rows() {
     let mut parser = vt100::Parser::new(3, 20, 10);