<ul class="current">
    {% for nav_item in nav %}
    {% for nav_item in nav_item.children %}
        <li class="toctree-l1 {% if nav_item.active%}current{%endif%}">
            <a class="{% if nav_item.active%}current{%endif%}" href="{{ nav_item.url }}">{{ nav_item.title }}</a>
            {% if nav_item == current_page %}
                <ul>
                {% for toc_item in toc %}
                    <li class="toctree-l2"><a href="{{ toc_item.url }}">{{ toc_item.title }}</a></li>
                    {% for toc_item in toc_item.children %}
                        <li><a class="toctree-l3" href="{{ toc_item.url }}">{{ toc_item.title }}</a></li>
                    {% endfor %}
                {% endfor %}
                </ul>
            {% endif %}
        </li>
    {% endfor %}{% endfor %}
</ul>

<ul class="current">
    {% for nav_item in nav %}
        {% if nav_item.children %}
            {% for nav_item in nav_item.children %}
                <li class="toctree-l1 {% if nav_item.active%}current{%endif%}">
                    <a class="{% if nav_item.active%}current{%endif%}" href="{{ nav_item.url }}">{{ nav_item.title }}</a>
                    {% if nav_item == current_page %}
                        <ul>
                        {% for toc_item in toc %}
                            <li class="toctree-l2"><a href="{{ toc_item.url }}">{{ toc_item.title }}</a></li>
                            {% for toc_item in toc_item.children %}
                                <li><a class="toctree-l3" href="{{ toc_item.url }}">{{ toc_item.title }}</a></li>
                            {% endfor %}
                        {% endfor %}
                        </ul>
                    {% endif %}
                </li>
            {% endfor %}
        {% else %}
            <li class="toctree-l1 {% if nav_item.active%}current{%endif%}">
                <a class="{% if nav_item.active%}current{%endif%}" href="{{ nav_item.url }}">{{ nav_item.title }}</a>
            </li>
        {% endif %}

    {% endfor %}
</ul>

Fix RTD TOC on mkdocs
Fix RTD TOC on mkdocs