Web ListBox header separator renders double-thick with grid lines

With GridLinesVisible enabled, the separator below a Web ListBox's header row rendered thicker than the other horizontal grid lines: the header element already draws a 1px bottom separator, and the header cells stacked their own 1px grid-line bottom border directly onto it. The header cells now draw no bottom border of their own, so the separator stays 1px — matching the designer preview and the other row separators.