docs/_includes/treebuilder.html

{% for section in site.data.toc.horizontalnav %}
  {% assign topicFound="false" %}
  {% assign activeCSS = "" %}
  {% assign tree = site.data.toc[section.node] %}
  <!-- path comparison: item.path: {{ item.path }} == page.path: {{ page.url }} -->
  {% capture thisSection %}{% include tree.html %}{% endcapture %}
  {% if section.path == page.url or thisSection contains "active currentPage" %}
    {% capture leftnav %}{{ thisSection }}{% endcapture %}
    {% capture activeCSS %} class="active"{% endcapture %}
  {% endif %}
  <li id="{{ section.node }}"{{ activeCSS }}><a href="{{ section.path }}">{{ section.title }}</a></li>
{% endfor %}