Gtk.TextIter.prototype.set_line_index

function set_line_index(byte_on_line:Number):void {
    // Gjs wrapper for gtk_text_iter_set_line_index()
}

Same as Gtk.set_line_offset, but works with a <emphasis>byte</emphasis> index. The given byte index must be at the start of a character, it can't be in the middle of a UTF-8 encoded character.