diff --git a/crates/app-iced/src/pages/appearance/section/accent.rs b/crates/app-iced/src/pages/appearance/section/accent.rs
index 676cad5..3944737 100644
--- a/crates/app-iced/src/pages/appearance/section/accent.rs
+++ b/crates/app-iced/src/pages/appearance/section/accent.rs
@@ -212,3 +212,24 @@ fn visible(ch: char) -> String {
         format!("U+{:04X}", u32::from(ch))
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::visible;
+
+    /// 错行里的字符必须**看得见**：不可见字符写成 `U+XXXX`，可见字符原样（r1 小事 2）。
+    ///
+    /// 抓得住：把 `visible` 改回 `ch.to_string()`——`U+00A0` / `U+200B` 那两条立刻红（错行会变成
+    /// 「强调色里有非十六进制字符：」后面一片空白，用户不知道自己错在哪）。
+    /// 抓不住：句子本身（词表在 `unlucky-i18n`）。
+    #[test]
+    fn invisible_characters_are_shown_as_code_points() {
+        assert_eq!(visible('\u{a0}'), "U+00A0", "不换行空格看不见");
+        assert_eq!(visible('\u{200b}'), "U+200B", "零宽空格看不见");
+        assert_eq!(visible('\u{202e}'), "U+202E", "RTL 覆盖符看不见");
+        assert_eq!(visible(' '), "U+0020", "普通空格也看不见");
+        assert_eq!(visible('z'), "z", "可见字符原样");
+        assert_eq!(visible('#'), "#", "ASCII 标点原样");
+        assert_eq!(visible('中'), "中", "非 ASCII 可见字符原样");
+    }
+}