strimertul-website/static/script/toc.js

const toc = document.getElementById("TableOfContents");

const getId = (el) => {
  do {
    const id = el.getAttribute("id");
    if (id) {
      return id;
    }
    el = el.previousElementSibling;
  } while (el);
};
if (toc) {
  if (toc.children.length < 1) {
    toc.parentElement.remove();
  } else {
    const observer = new IntersectionObserver((entries) => {
      const filtered = entries.filter(
        (i) => getId(i.target) && i.intersectionRatio > 0
      );
      if (filtered.length > 0) {
        toc
          .querySelectorAll(`li.active`)
          .forEach((entry) => entry.classList.remove("active"));
        filtered.forEach((entry) => {
          const id = getId(entry.target);
          toc
            .querySelector(`li a[href="#${id}"]`)
            .parentElement.classList.add("active");
        });
      }
    });

    // Track all sections that have an `id` applied
    document.querySelectorAll(".copy *").forEach((section) => {
      observer.observe(section);
    });
  }
}