Gtk.TextIter.prototype.set_line_offset

function set_line_offset(char_on_line:Number):void {
    // Gjs wrapper for gtk_text_iter_set_line_offset()
}

Moves iter within a line, to a new <emphasis>character</emphasis> (not byte) offset. The given character offset must be less than or equal to the number of characters in the line; if equal, iter moves to the start of the next line. See Gtk.TextIter.prototype.set_line_index if you have a byte index rather than a character offset.