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

src/side_diff.rs

@@ -285,6 +285,8 @@ 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.
 
+        // Pad to the middle of the gutter so the marker doesn't drift on wider gutters (#269).
+        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 +1005,26 @@ mod tests {
             assert_eq!(contains_string(&output, "equal"), 2)
         }
 
+        // Regression for #269: the `-y` marker must sit at the gutter middle at any width.
+        #[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();

tests/integration.rs

@@ -343,6 +343,54 @@ mod diff {
 
         Ok(())
     }
+
+    // Visual (tab-aware) column of the `-y` gutter marker on the first output line.
+    fn side_marker_visual_column(output: &[u8], tabsize: usize) -> Option<usize> {
+        let text = String::from_utf8_lossy(output);
+        let line = text.lines().next()?;
+        let mut col = 0;
+        for c in line.chars() {
+            match c {
+                '<' | '|' | '>' => return Some(col),
+                '\t' => col += tabsize - (col % tabsize),
+                _ => col += 1,
+            }
+        }
+        None
+    }
+
+    // Regression for #269: the `-y` gutter marker must sit at the middle of the gutter.
+    #[test]
+    fn sdiff_gutter_marker_column() -> Result<(), Box<dyn std::error::Error>> {
+        let mut file1 = NamedTempFile::new()?;
+        file1.write_all("aa\n".as_bytes())?;
+        let mut file2 = NamedTempFile::new()?;
+        file2.write_all("bb\n".as_bytes())?;
+
+        // --expand-tabs: padding is spaces, marker lands at visual column 19.
+        let mut cmd = cargo_bin_cmd!("diffutils");
+        cmd.arg("diff")
+            .arg("-y")
+            .arg("--width=40")
+            .arg("--expand-tabs")
+            .arg(file1.path())
+            .arg(file2.path());
+        let output = cmd.output().unwrap().stdout;
+        assert_eq!(side_marker_visual_column(&output, 8), Some(19));
+
+        // Default (expand_tabs: false): padding goes through the tab path, but the
+        // marker must still land at the same visual column.
+        let mut cmd = cargo_bin_cmd!("diffutils");
+        cmd.arg("diff")
+            .arg("-y")
+            .arg("--width=40")
+            .arg(file1.path())
+            .arg(file2.path());
+        let output = cmd.output().unwrap().stdout;
+        assert_eq!(side_marker_visual_column(&output, 8), Some(19));
+
+        Ok(())
+    }
 }
 
 mod cmp {