Mirror/docs/_includes/f_sidebar_nav.html

{% if include.item.children %}

    <a href="#submenu_{{include.id}}" aria-controls="submenu_{{include.id}}" data-toggle="collapse" aria-expanded="false" class="bd-toc-link dropdown-toggle">{{include.item.title}}</a>

    {% assign pageurl = page.url | remove: '.html' %}

    {% capture capturechild %}
        {% for child in include.item.children %}

        {% assign active = "" %}
        {% assign childurlfolder = child.url | append: '/' %}
        {% if child.url and (pageurl == child.url or pageurl == childurlfolder) %}
            {% assign active = "active bd-sidenav-active" %}
        {% endif %}

        <li class="{{active}}">
            {% assign childid = include.id | append: '_' | append: forloop.index %}
            {% include f_sidebar_nav.html item=child id=childid %}
        </li>

        {% endfor %}
    {% endcapture %}

    {% assign show = "" %}
    {% if capturechild contains 'bd-sidenav-active' %}
        {% assign show = "show" %}
    {% endif %}

    <ul class="collapse nav bd-sidenav {{show}}" id="submenu_{{include.id}}">
        {{ capturechild }}
    </ul>
{% else %}

    <a href="{{include.item.url | relative_url}}">{{include.item.title}}</a>
{% endif %}