blog/_includes/sidebar/article-menu.html

<style type="text/css" media="screen">
.post-menu ul {
  list-style: none;
  padding: 0;
  margin: 0;
}
</style>

<div class="post-menu">
  <div class="post-menu-title">Contents</div>
  <div class="post-menu-content"></div>
</div>

<script>
  function generateContent() {
    var menu = document.querySelector(".post-menu");
    var menuContent =  menu.querySelector(".post-menu-content");
    var headings = document.querySelector(".post-content").querySelectorAll("h2, h3, h4, h5, h6");

    // Hide menu when no headings
    if (headings.length === 0) {
      return menu.style.display = "none";
    }

    // Generate post menu
    var menuHTML = '';
    for (var i = 0; i < headings.length; i++) {
      var h = headings[i];
      menuHTML += (
        '<li class="h-' + h.tagName.toLowerCase() + '">'
        + '<a href="#h-' + h.getAttribute('id') + '">' + h.textContent + '</a></li>');
    }

    menuContent.innerHTML = '<ul>' + menuHTML + '</ul>';

    // The header element
    var header = document.querySelector('header.site-header');

    // Active the menu item
    window.addEventListener('scroll', function (event) {
      var lastActive = menuContent.querySelector('.active');
      var changed = true;
      for (var i = headings.length - 1; i >= 0; i--) {
        var h = headings[i];
        var headingRect = h.getBoundingClientRect();
        var headerRect = header.getBoundingClientRect();
        var headerTop = Math.floor(headerRect.top);
        var headerHeight = Math.floor(headerRect.height);
        var headerHeight = headerTop + headerHeight + 20;
        if (headingRect.top <= headerHeight) {
          var id = 'h-' + h.getAttribute('id');
          var curActive = menuContent.querySelector('a[href="#' + id  + '"]');
          if (curActive) {
            curActive.classList.add('active');
          }
          if (lastActive == curActive) {
            changed = false;
          }
          break;
        }
      }
      if (lastActive && changed) {
        lastActive.classList.remove('active');
      }
      event.preventDefault();
    });
  }
  generateContent();
</script>