I should mention though; FWIW, codex initially suggested rewriting the code to be as follows, as per RFC 3629:

  int fl_utf8len(char c) {
    const unsigned char u = (unsigned char)c;
    if (u <= 0x7f) return 1;
    if (u >= 0xc2 && u <= 0xdf) return 2;
    if (u >= 0xe0 && u <= 0xef) return 3;
    if (u >= 0xf0 && u <= 0xf4) return 4;
    return -1;
  }

  int fl_utf8len1(char c) {
    const int len = fl_utf8len(c);
    return len < 0 ? 1 : len;
  }
[..]

[..]

    OK, great, I'll commit it today, as it would seriously help the terminal tests.
    We can always revert if other devs makes a compelling argument to the contrary.