https://www.clientearth.org/assets/scripts/components/stickyPageAnchorNav.js

1(function($) {
2  $(function() {
3    initialize();
4  });
5
6  function initialize() {
7    $("#js-stickyPageNavMobBtn").on("click", function() {
8      $("#js-stickyPageNav").toggleClass("active");
9      $("#js-stickyPageNav .inner")
10        .stop()
11        .slideToggle("active");
12    });
13
14    var controller = new ScrollMagic.Controller();
15    $("section").each(function() {
16      var currentsection = this;
17      var duration = $(this).outerHeight();
18
19      new ScrollMagic.Scene({
20        triggerElement: currentsection,
21        offset: 200,
22        duration: duration
23      })
24        .addTo(controller)
25        .on("enter", function(e) {
26          var triggerID = $(this.triggerElement()).attr("id");
27
28          if (triggerID) {
29            var navtriggerID =
30              ".stickyPageNav.anchored .linkContainer a." + triggerID;
31            $("#js-stickyPageNav")
32              .find(".active")
33              .removeClass("active");
34            $(navtriggerID).addClass("active");
35          }
36        });
37    });
38  }
39})(window.jQuery);