// entries are HTML (e.g. links)
sS hPopDownButton(S... entries) {
  S id = "dropdown-" + aGlobalID();
  ret hdiv(
      p(htmlEncode2(unicode_smallDownPointingTriangle()))
    + hdiv(ul(entries), id := jsQuote(id)),
    style := "position: relative; display: inline-block; cursor: context-menu",
    onclick := replaceDollarVars([[
      var x = document.getElementById($id);
      x.style.display = x.style.display === "none" ? "block" : "none";
    ]]));
}