{#
/**
 * @file
 * Default theme implementation to display a table of contents as a select menu.
 *
 * Returns HTML for a Table of contents select menu.
 *
 * Available variables:
 * - index: List of header items. Each header item contains:
 *   - prefix: The header option prefix. Either indentation or the complete path.
 *   - title: The header title.
 *   - url: The header fragrment (ie hash) URL, instance of \Drupal\Core\Url.
 *
 * @ingroup themeable
 */
#}
{% set classes = ['toc', 'toc-menu'] %}
<form {{ attributes.addClass(classes) }}>
  <select>
    <option value="">{{ title }}</option>
    {% for item in index %}
      <option value="{{ item.url }}">{{ item.prefix }}{{ item.title }}</option>
    {% endfor %}
  </select>
</form>