site/_includes/news_contents_mobile.html

<div class="docs-nav-mobile unit whole show-on-mobiles">
  <select onchange="if (this.value) window.location.href=this.value">
    <option value="">Navigate the blog…</option>
    <option value="{{ site.baseurl }}/news/">Home</option>
    <optgroup label="v1.x">
      {% for post in site.posts %}
      <option value="{{ post.url }}">{{ post.title }}</option>
      {% endfor %}
    </optgroup>
  </select>
</div>