diff: place -y gutter marker at the middle of the gutter

src/side_diff.rs

@@ -285,6 +285,11 @@ fn push_output<T: Write>(
         // the diff always want to put all tabs possible in the usable are,
         // even in the middle space between the gutters if possible.
 
+        // `process_half_line` left the cursor one column past the half width. The gutter marker
+        // belongs at the middle of the gutter (`separator_pos`), which is only `half_width + 1`
+        // when the gutter is 3 or 4 columns wide; pad up to it for wider gutters so the marker
+        // doesn't drift (see #269). `format_tabs_and_spaces` is a no-op when already at/past it.
+        format_tabs_and_spaces(half_width + 1, separator_pos, config, output)?;
         output.write_all(&[symbol])?;
         if !right_ln.is_empty() {
             format_tabs_and_spaces(separator_pos + 1, column_two_offset, config, output)?;
@@ -1003,6 +1008,29 @@ mod tests {
             assert_eq!(contains_string(&output, "equal"), 2)
         }
 
+        // Regression test for #269: the `-y` gutter marker must sit at the middle of the gutter
+        // (separator_pos) regardless of gutter width. At the default width (gutter 3) the marker
+        // was already correct; at other widths it drifted. Here width=40 yields a wider gutter, so
+        // the marker must land at column 19 (matching GNU), not the old `half_width + 1`.
+        #[test]
+        fn test_gutter_marker_column_wide_gutter() {
+            let params = Params {
+                tabsize: 8,
+                expand_tabs: true,
+                width: 40,
+                ..Default::default()
+            };
+            let mut output = vec![];
+            diff(b"aa\n", b"", &mut output, &params);
+            let text = String::from_utf8(output).unwrap();
+            let line = text.lines().next().unwrap();
+            assert_eq!(
+                line.find('<'),
+                Some(19),
+                "gutter marker '<' should be at column 19 for --width=40, got: {line:?}"
+            );
+        }
+
         #[test]
         fn test_different_lines() {
             let params = generate_params();