This is an advanced trick, but I've successfully done this as part of a binary search to find the glyph at the visual mid-point of the string. UTF-8 is self-synchronizing so you can detect if you cut a code point in half. You still need to handle things like ligatures which even UTF-32 won't speak to directly.

Doesn't work well with BIDI languages, or Mongolian. Seriously if you haven't tested with Mongolian your code is probably wrong.