Blame for script.js

// "In this article" contents box, built from the post's h2 headings.
(function () {
  var toc = document.querySelector(".toc");
  var heads = Array.prototype.filter.call(
    document.querySelectorAll(".article h2"),
    function (h) { return !h.closest(".stat"); }
  );
  if (!toc || heads.length < 2) { if (toc) toc.remove(); return; }

  var used = {};
  var items = heads.map(function (h) {
    var id = h.id || h.textContent.toLowerCase().replace(/[^a-z0-9]+/g, "-").replace(/^-|-$/g, "") || "section";
    while (used[id]) id += "-2";
    used[id] = true;
    h.id = id;
    return '<li><a href="#' + id + '">' + h.innerHTML.replace(/<[^>]+>/g, "") + "</a></li>";
  });

  toc.innerHTML =
    '<details open><summary>In this article</summary><ol>' + items.join("") + "</ol></details>";

  // Collapsed by default on narrow screens, where the box sits above the text.
  var details = toc.querySelector("details");
  if (window.matchMedia("(max-width: 72rem)").matches) details.removeAttribute("open");

  var links = toc.querySelectorAll("a");
  if ("IntersectionObserver" in window) {
    var io = new IntersectionObserver(function (entries) {
      entries.forEach(function (e) {
        if (!e.isIntersecting) return;
        links.forEach(function (a) {
          a.classList.toggle("on", a.getAttribute("href") === "#" + e.target.id);
        });
      });
    }, { rootMargin: "-80px 0px -70% 0px" });
    heads.forEach(function (h) { io.observe(h); });
  }
})();