just-the-docs/_includes/nav.html

<ul class="nav-list">
  {%- assign titled_pages = include.pages
        | where_exp:"item", "item.title != nil" -%}

  {%- comment -%}
    The values of `title` and `nav_order` can be numbers or strings.
    Jekyll gives build failures when sorting on mixtures of different types,
    so numbers and strings need to be sorted separately.

    Here, numbers are sorted by their values, and come before all strings.
    An omitted `nav_order` value is equivalent to the page's `title` value
    (except that a numerical `title` value is treated as a string).

    The case-sensitivity of string sorting is determined by `site.nav_sort`.
  {%- endcomment -%}
  
  {%- assign string_ordered_pages = titled_pages
        | where_exp:"item", "item.nav_order == nil" -%}
  {%- assign nav_ordered_pages = titled_pages
        | where_exp:"item", "item.nav_order != nil"  -%}

  {%- comment -%}
    The nav_ordered_pages have to be added to number_ordered_pages and
    string_ordered_pages, depending on the nav_order value.
    The first character of the jsonify result is `"` only for strings.
  {%- endcomment -%}
  {%- assign nav_ordered_groups = nav_ordered_pages
        | group_by_exp:"item", "item.nav_order | jsonify | slice: 0" -%}
  {%- assign number_ordered_pages = "" | split:"X" -%}
  {%- for group in nav_ordered_groups -%}
    {%- if group.name == '"' -%}
      {%- assign string_ordered_pages = string_ordered_pages | concat: group.items -%}
    {%- else -%}
      {%- assign number_ordered_pages = number_ordered_pages | concat: group.items -%}
    {%- endif -%}
  {%- endfor -%}
  
  {%- assign sorted_number_ordered_pages = number_ordered_pages | sort:"nav_order" -%}
  
  {%- comment -%}
    The string_ordered_pages have to be sorted by nav_order, and otherwise title
    (where appending the empty string to a numeric title converts it to a string).
    After grouping them by those values, the groups are sorted, then the items
    of each group are concatenated.
  {%- endcomment -%}
  {%- assign string_ordered_groups = string_ordered_pages
        | group_by_exp:"item", "item.nav_order | default: item.title | append:''" -%}
  {%- if site.nav_sort == 'case_insensitive' -%}
    {%- assign sorted_string_ordered_groups = string_ordered_groups | sort_natural:"name" -%}
  {%- else -%}
    {%- assign sorted_string_ordered_groups = string_ordered_groups | sort:"name" -%}
  {%- endif -%}
  {%- assign sorted_string_ordered_pages = "" | split:"X" -%}
  {%- for group in sorted_string_ordered_groups -%}
    {%- assign sorted_string_ordered_pages = sorted_string_ordered_pages | concat: group.items -%}
  {%- endfor -%}

  {%- assign pages_list = sorted_number_ordered_pages | concat: sorted_string_ordered_pages -%}
  
  {%- for node in pages_list -%}
    {%- if node.parent == nil -%}
      {%- unless node.nav_exclude -%}
      <li class="nav-list-item{% if page.collection == include.key and page.url == node.url or page.parent == node.title or page.grand_parent == node.title %} active{% endif %}">
        {%- if node.has_children -%}
          <a href="#" class="nav-list-expander"><svg viewBox="0 0 24 24"><use xlink:href="#svg-arrow-right"></use></svg></a>
        {%- endif -%}
        <a href="{{ node.url | relative_url }}" class="nav-list-link{% if page.url == node.url %} active{% endif %}">{{ node.title }}</a>
        {%- if node.has_children -%}
          {%- assign children_list = pages_list | where: "parent", node.title -%}
          <ul class="nav-list ">
          {%- for child in children_list -%}
            {%- unless child.nav_exclude -%}
            <li class="nav-list-item {% if page.url == child.url or page.parent == child.title %} active{% endif %}">
              {%- if child.has_children -%}
                <a href="#" class="nav-list-expander"><svg viewBox="0 0 24 24"><use xlink:href="#svg-arrow-right"></use></svg></a>
              {%- endif -%}
              <a href="{{ child.url | relative_url }}" class="nav-list-link{% if page.url == child.url %} active{% endif %}">{{ child.title }}</a>
              {%- if child.has_children -%}
                {%- assign grand_children_list = pages_list | where: "parent", child.title | where: "grand_parent", node.title -%}
                <ul class="nav-list">
                {%- for grand_child in grand_children_list -%}
                  {%- unless grand_child.nav_exclude -%}
                  <li class="nav-list-item {% if page.url == grand_child.url %} active{% endif %}">
                    <a href="{{ grand_child.url | relative_url }}" class="nav-list-link{% if page.url == grand_child.url %} active{% endif %}">{{ grand_child.title }}</a>
                  </li>
                  {%- endunless -%}
                {%- endfor -%}
                </ul>
              {%- endif -%}
            </li>
            {%- endunless -%}
          {%- endfor -%}
          </ul>
        {%- endif -%}
      </li>
      {%- endunless -%}
    {%- endif -%}
  {%- endfor -%}
</ul>

{%- if page.collection == include.key -%}

  {%- for node in pages_list -%}
    {%- if node.parent == nil -%}
      {%- if page.parent == node.title or page.grand_parent == node.title -%}
        {%- assign first_level_url = node.url | relative_url -%}
      {%- endif -%}
      {%- if node.has_children -%}
        {%- assign children_list = pages_list | where: "parent", node.title -%}
        {%- for child in children_list -%}
          {%- if child.has_children -%}
            {%- if page.url == child.url or page.parent == child.title and page.grand_parent == child.parent -%}
              {%- assign second_level_url = child.url | relative_url -%}
            {%- endif -%}
          {%- endif -%}
        {%- endfor -%}
      {%- endif -%}
    {%- endif -%}
  {%- endfor -%}

  {% if page.has_children == true and page.has_toc != false %}
    {%- assign toc_list = pages_list | where: "parent", page.title | where: "grand_parent", page.parent -%}
  {%- endif -%}

{%- endif -%}