Function display_to_grapheme_col 

pub fn display_to_grapheme_col(
    line: &str,
    display_col: usize,
    tab_width: usize,
) -> usize

Grapheme index containing display_col.

Clicking anywhere inside a wide grapheme selects that grapheme, so the right half of a CJK character does not land on the following one. Clicks past the end of the line clamp to the line length.