diff --git a/src/diff.rs b/src/diff.rs
index b05988f..971dfba 100644
--- a/src/diff.rs
+++ b/src/diff.rs
@@ -1,5 +1,7 @@
 use similar::{ChangeTag, TextDiff};
 
+use crate::text::{Alignment, pad_to_width, truncate_to_width};
+
 pub struct DiffArgs {
     pub file_a: String,
     pub file_b: String,
@@ -63,9 +65,13 @@ fn print_side_by_side<'a>(diff: &TextDiff<'a, 'a, 'a, str>, name_a: &str, name_b
 
     // Header
     println!(
-        "\x1b[1m{:<col_width$}\x1b[0m | \x1b[1m{}\x1b[0m",
-        truncate(name_a, col_width),
-        truncate(name_b, col_width),
+        "\x1b[1m{}\x1b[0m | \x1b[1m{}\x1b[0m",
+        pad_to_width(
+            &truncate_to_width(name_a, col_width),
+            col_width,
+            Alignment::Left
+        ),
+        truncate_to_width(name_b, col_width),
     );
     println!("{}", "-".repeat(term_width.min(col_width * 2 + 3)));
 
@@ -75,34 +81,27 @@ fn print_side_by_side<'a>(diff: &TextDiff<'a, 'a, 'a, str>, name_a: &str, name_b
 
         match change.tag() {
             ChangeTag::Equal => {
-                let left = truncate(line, col_width);
-                let right = truncate(line, col_width);
+                let left = truncate_to_width(line, col_width);
                 println!(
-                    "\x1b[90m{:<col_width$}\x1b[0m | \x1b[90m{}\x1b[0m",
-                    left, right
+                    "\x1b[90m{}\x1b[0m | \x1b[90m{}\x1b[0m",
+                    pad_to_width(&left, col_width, Alignment::Left),
+                    left
                 );
             }
             ChangeTag::Delete => {
-                let left = truncate(line, col_width);
-                println!("\x1b[31m{:<col_width$}\x1b[0m | ", left);
+                let left = truncate_to_width(line, col_width);
+                println!(
+                    "\x1b[31m{}\x1b[0m | ",
+                    pad_to_width(&left, col_width, Alignment::Left)
+                );
             }
             ChangeTag::Insert => {
                 println!(
-                    "{:<col_width$} | \x1b[32m{}\x1b[0m",
-                    "",
-                    truncate(line, col_width)
+                    "{} | \x1b[32m{}\x1b[0m",
+                    " ".repeat(col_width),
+                    truncate_to_width(line, col_width)
                 );
             }
         }
     }
 }
-
-fn truncate(s: &str, max: usize) -> String {
-    if s.len() <= max {
-        s.to_string()
-    } else if max > 3 {
-        format!("{}...", &s[..max - 3])
-    } else {
-        s[..max].to_string()
-    }
-}
diff --git a/src/text.rs b/src/text.rs
index dbb5887..5aa9198 100644
--- a/src/text.rs
+++ b/src/text.rs
@@ -18,18 +18,18 @@ pub fn repeat_char(ch: char, count: usize) -> String {
     std::iter::repeat_n(ch, count).collect()
 }
 
-/// Truncate a URL to fit within `max_width` display columns.
-/// If the URL fits, it is returned as-is.
-/// Otherwise it is cut and an `…` (U+2026) is appended.
-pub fn truncate_url(url: &str, max_width: usize) -> String {
-    let width = display_width(url);
+/// Truncate to fit within `max_width` display columns, appending `…` (U+2026)
+/// if anything was cut. Counts display columns, not bytes or chars, so a line
+/// of CJK or emoji lands in the right column.
+pub fn truncate_to_width(s: &str, max_width: usize) -> String {
+    let width = display_width(s);
     if width <= max_width || max_width < 4 {
-        return url.to_string();
+        return s.to_string();
     }
     let mut result = String::new();
     let mut w = 0;
     let limit = max_width - 1; // reserve 1 column for …
-    for ch in url.chars() {
+    for ch in s.chars() {
         let cw = unicode_width::UnicodeWidthChar::width(ch).unwrap_or(0);
         if w + cw > limit {
             break;
@@ -41,6 +41,11 @@ pub fn truncate_url(url: &str, max_width: usize) -> String {
     result
 }
 
+/// Truncate a URL to fit within `max_width` display columns.
+pub fn truncate_url(url: &str, max_width: usize) -> String {
+    truncate_to_width(url, max_width)
+}
+
 /// Pad a string to a given display width with spaces.
 pub fn pad_to_width(s: &str, width: usize, align: Alignment) -> String {
     let current = display_width(s);
@@ -147,3 +152,43 @@ mod fence_tests {
         assert!(!f.feed("text"));
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_truncate_to_width_leaves_short_strings_alone() {
+        assert_eq!(truncate_to_width("abc", 10), "abc");
+        assert_eq!(truncate_to_width("", 10), "");
+    }
+
+    /// Byte slicing here used to abort the process: `mdx diff` on any document
+    /// containing a wide character panicked on a non-char-boundary index.
+    #[test]
+    fn test_truncate_to_width_never_splits_a_character() {
+        for width in 0..12 {
+            let out = truncate_to_width("✅✅✅✅✅✅", width);
+            assert!(
+                out.chars().count() * 3 >= out.len(),
+                "produced invalid UTF-8"
+            );
+        }
+        assert_eq!(truncate_to_width("日本語テキスト", 6), "日本\u{2026}");
+    }
+
+    /// Wide characters occupy two columns, so counting chars or bytes puts the
+    /// next column in the wrong place.
+    #[test]
+    fn test_truncate_to_width_counts_display_columns() {
+        assert!(display_width(&truncate_to_width("✅✅✅✅✅✅", 8)) <= 8);
+        assert!(display_width(&truncate_to_width("abcdefghij", 8)) <= 8);
+        assert!(display_width(&truncate_to_width("日本語テキスト日本語", 10)) <= 10);
+    }
+
+    #[test]
+    fn test_truncate_to_width_marks_the_cut() {
+        assert!(truncate_to_width("abcdefghij", 5).ends_with('\u{2026}'));
+        assert!(!truncate_to_width("abc", 5).ends_with('\u{2026}'));
+    }
+}
diff --git a/tests/basic_test.rs b/tests/basic_test.rs
index 81574d7..6effe55 100644
--- a/tests/basic_test.rs
+++ b/tests/basic_test.rs
@@ -2066,3 +2066,64 @@ fn test_export_pdf_square_backgrounds_are_not_rasterized() {
     );
     assert!(bytes.windows(4).any(|w| w == b"%PDF"));
 }
+
+// ── diff ─────────────────────────────────────────────────────────────
+
+fn run_diff(a: &str, b: &str, extra: &[&str]) -> std::process::Output {
+    let fa = write_tmp("diff-a.md", a);
+    let fb = write_tmp("diff-b.md", b);
+    let out = Command::new(env!("CARGO_BIN_EXE_mdx"))
+        .arg("diff")
+        .args(extra)
+        .arg(&fa)
+        .arg(&fb)
+        .env("NO_COLOR", "1")
+        .output()
+        .expect("Failed to execute mdx");
+    let _ = std::fs::remove_file(&fa);
+    let _ = std::fs::remove_file(&fb);
+    out
+}
+
+/// Side-by-side truncation byte-sliced its input, so a wide character crossing
+/// the column boundary aborted the process -- and `panic = "abort"` makes that
+/// a SIGABRT with a core dump, not a clean error.
+#[test]
+fn test_diff_survives_wide_characters() {
+    for (a, b) in [
+        (
+            "# A\n\n\u{2705} \u{65e5}\u{672c}\u{8a9e} text\n",
+            "# B\n\n\u{2705} other \u{1f389}\n",
+        ),
+        (&"\u{2705}".repeat(200), "plain\n"),
+        (
+            "caf\u{e9} \u{3a9}\u{3bc}\u{3ad}\u{3b3}\u{3b1}\n",
+            "\u{41f}\u{440}\u{438}\u{432}\u{435}\u{442}\n",
+        ),
+    ] {
+        let out = run_diff(a, b, &[]);
+        assert!(
+            out.status.success(),
+            "diff aborted on wide characters: {}",
+            String::from_utf8_lossy(&out.stderr)
+        );
+    }
+}
+
+#[test]
+fn test_diff_handles_empty_and_identical_files() {
+    assert!(run_diff("", "# B\n", &[]).status.success());
+    assert!(run_diff("# Same\n", "# Same\n", &[]).status.success());
+    assert!(run_diff("", "", &[]).status.success());
+}
+
+#[test]
+fn test_diff_unified_mode_works() {
+    let out = run_diff(
+        "# A\n\n\u{2705} wide\n",
+        "# B\n\n\u{2705} wide\n",
+        &["--unified"],
+    );
+    assert!(out.status.success());
+    assert!(!String::from_utf8_lossy(&out.stdout).is_empty());
+}