Replace chr() by mb_chr() and remove the workaround

program/lib/Roundcube/rcube_utils.php

@@ -724,14 +724,7 @@ class rcube_utils
     public static function xss_entity_decode(string $content): string
     {
         $callback = static function ($matches) {
-            $bytevalue = hexdec((string) $matches[1]);
-            // chr() only covers values between 0 and 255. The following 4 lines are from the former default behaviour
-            // to ensure that, which is now deprecated, so we now explicitly do the shifting here.
-            while ($bytevalue < 0) {
-                $bytevalue += 256;
-            }
-            $bytevalue %= 256;
-            return chr($bytevalue);
+            return strval(mb_chr(hexdec((string) $matches[1])));
         };
 
         $out = html_entity_decode(html_entity_decode($content));