Group posts by tag and sort alphabetically in Siteleaf

posts-by-tag.html

{% for tag in taxonomy['tags'] %}
    <h1>{{tag}}</h1>
    <ul>
    {% assign sorted_posts = tag.posts.all | sort:'title' %}
    {% for post in sorted_posts %}
      <li>{{post.title}}</li>
    {% endfor %}
    </ul>
{% endfor %}

Note: Change taxonomy['tags'] to site.pages['blog'].taxonomy['tags'] for tags outside of current page.