0024056 and 0025002 introduced use of mb_strimwidth() to ensure that History API does not attempt to insert bigger data than the underlying old_value and new_value database columns can accept.

This works just fine with Western alphabets, but a string's width is NOT necessarily the same as its length. In Chinese, Japanese and Korean, some characters can be represented as full or half width, which may lead to unexpected results. In this case, if the string contains full-width chars, then it will be truncated to less than the expected 255 chars.

Example:

$s = 'スイス';                 # Original string (full-width katakana) - 9 bytes, 3 chars, width=6
echo mb_substr($s, 0, 3);     # スイス
echo mb_strimwidth($s, 0, 3); # ス