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    var didScroll;
    var lastScrollTop = 0;
    var delta = 5;
    var navbarHeight = $("header").outerHeight();

    $(window).scroll(function(event){
      didScroll = true;
    });

    setInterval(function() {
      if (didScroll) {
        hasScrolled();
        didScroll = false;
      }
    }, 250);

    function hasScrolled() {
      var st = $(this).scrollTop();

      // Make sure they scroll more than delta
      if(Math.abs(lastScrollTop - st) <= delta)
          return;

      // If they scrolled down and are past the navbar, add class .nav-up.
      // This is necessary so you never see what is "behind" the navbar.
      if (st > lastScrollTop && st > navbarHeight){
        // Scroll Down
        $("header").removeClass("nav-down").addClass("nav-up");
      } else {
        // Scroll Up
        if(st + $(window).height() < $(document).height()) {
          $("header").removeClass("nav-up").addClass("nav-down");
        }
      }

      lastScrollTop = st;
    }