{#-
  Tabs with submenus.

  The stock partial renders a section as a single link to its index page and
  drops the children, so on a wide screen the whole top navigation is six
  words and every page below the first level is reachable only from the
  sidebar.  This renders the children as a menu under their tab.

  `navigation.indexes` promotes a section's index page to the section
  itself, exactly as the sidebar does, so the tab link and the sidebar entry
  lead to the same page and the index is not repeated inside the menu.
  Sections nested one level deeper (the rule categories) become a labelled
  group rather than being flattened, so the menu keeps the sidebar's shape.

  The index is tracked as an *item*, never as its URL: the site root's index
  page has an empty URL, so testing the URL for truthiness silently treats
  the root section as having no index and promotes its second page into the
  tab.  That is how the Overview tab came to point at `/security/`.
-#}

{% macro render_content(nav_item, ref) %}
  {% set ref = ref or nav_item %}
  {{ ref.title or nav_item.title }}
{% endmacro %}

{#- One entry in a tab's menu: a page, or a nested section as a group. -#}
{% macro render_entry(item) %}
  {% if item.children %}
    {% set inner = namespace(index = none) %}
    {% if "navigation.indexes" in features %}
      {% for child in item.children %}
        {% if child.is_index and inner.index is none %}
          {% set inner.index = child %}
        {% endif %}
      {% endfor %}
    {% endif %}
    <li class="md-tabs__menu-item md-tabs__menu-item--group">
      {% if inner.index %}
        <a href="{{ inner.index.url | url }}" class="md-tabs__menu-title">
          {{ item.title }}
        </a>
      {% else %}
        <span class="md-tabs__menu-title">{{ item.title }}</span>
      {% endif %}
      <ul class="md-tabs__menu-group">
        {% for child in item.children %}
          {% if child is not sameas inner.index %}
            {{ render_entry(child) }}
          {% endif %}
        {% endfor %}
      </ul>
    </li>
  {% elif item.url is not none %}
    <li class="md-tabs__menu-item">
      <a href="{{ item.url | url }}" class="md-tabs__menu-link">
        {{ item.title }}
      </a>
    </li>
  {% endif %}
{% endmacro %}

{% macro render(nav_item, ref) %}
  {% set ref = ref or nav_item %}
  {% set class = "md-tabs__item" %}
  {% if ref.active %}
    {% set class = class ~ " md-tabs__item--active" %}
  {% endif %}

  {% if nav_item.children %}
    {% set _ = namespace(index = none) %}
    {% if "navigation.indexes" in features %}
      {% for item in nav_item.children %}
        {% if item.is_index and _.index is none %}
          {% set _.index = item %}
        {% endif %}
      {% endfor %}
    {% endif %}

    {#- Without an index page the tab points at the first child, which is
        what the stock partial does; that child is then not repeated as the
        menu's first entry. -#}
    {% set head = _.index or (nav_item.children | first) %}
    {% if head.children %}
      {{ render(head, ref) }}
    {% else %}
      {% set _rest = namespace(items = []) %}
      {% for item in nav_item.children %}
        {% if item is not sameas head %}
          {% set _rest.items = _rest.items + [item] %}
        {% endif %}
      {% endfor %}
      <li class="{{ class }}{% if _rest.items %} md-tabs__item--nested{% endif %}">
        <a href="{{ head.url | url }}" class="md-tabs__link"
           {% if _rest.items %}aria-haspopup="true"{% endif %}>
          {{ render_content(nav_item, ref) }}
        </a>
        {% if _rest.items %}
          <ul class="md-tabs__menu" aria-label="{{ nav_item.title }}">
            {% for item in _rest.items %}
              {{ render_entry(item) }}
            {% endfor %}
          </ul>
        {% endif %}
      </li>
    {% endif %}
  {% else %}
    <li class="{{ class }}">
      <a href="{{ nav_item.url | url }}" class="md-tabs__link">
        {{ render_content(nav_item) }}
      </a>
    </li>
  {% endif %}
{% endmacro %}