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function determineDropDirection(){
  $(".dropdown-menu").each( function(){

    // Invisibly expand the dropdown menu so its true height can be calculated
    $(this).css({
      visibility: "hidden",
      display: "block"
    });

    // Necessary to remove class each time so we don't unwantedly use dropup's offset top
    $(this).parent().removeClass("dropup");

    // Determine whether bottom of menu will be below window at current scroll position
    if ($(this).offset().top + $(this).outerHeight() > $(window).innerHeight() + $(window).scrollTop()){
      $(this).parent().addClass("dropup");
    }

    // Return dropdown menu to fully hidden state
    $(this).removeAttr("style");
  });
}

determineDropDirection();

$(window).scroll(determineDropDirection);