src/main/org/freemarker/docgen/statics/page-menu.js

(function() {
  function isAtTop(node) {
    var parent = node.offsetParent;
    var nodeOffsetTop = node.offsetTop + node.offsetParent.offsetTop - 5;
    var windowOffsetTop = window.pageYOffset;

    // @todo: figure out why less than isn't working when they are equal
    return (nodeOffsetTop === windowOffsetTop || nodeOffsetTop < windowOffsetTop);
  }

  // remove highlight class so animation can be repeated on same node again
  function unHighlightNode(nodeId) {
    var node = document.getElementById(nodeId);

    if (node) {
      node.classList.remove('active');
    }
  }

  function highlightNode(nodeId) {
    var node = document.getElementById(nodeId);

    // wrap in a setTimeout so that window.scrollY is accurate when we poll it
    window.setTimeout(function() {
      if (node && !isAtTop(node)) {
        node.classList.add('active');

        window.setTimeout(function() {
          unHighlightNode(nodeId);
        }, 1000);
      }
    }, 1);
  }

  function onPageMenuClick(e) {
    var node = e.target;

    if (node.classList.contains('page-menu-link')) {
      highlightNode(node.getAttribute('data-menu-target'));
    }
  }

  function getNodeIdFromHash() {
    if (window.location.hash) {
      return window.location.hash.substring(1);
    } else {
      return '';
    }
  }

  function onDocReady() {
    var nodeId = getNodeIdFromHash();

    if (nodeId !== '') {
      highlightNode(nodeId);
    }
  }

  function init() {
    document.addEventListener('click', onPageMenuClick);
    document.addEventListener('DOMContentLoaded', onDocReady);
  }

  init();

})();