Use relative_url filter

_includes/nav_list

@@ -10,12 +10,12 @@
         {% if nav.url %}
           {% comment %} internal/external URL check {% endcomment %}
           {% if nav.url contains "://" %}
-            {% assign domain = "" %}
+            {% assign nav_url = nav.url %}
           {% else %}
-            {% assign domain = site.url | append: site.baseurl %}
+            {% assign nav_url = nav.url | relative_url %}
           {% endif %}
 
-          <a href="{{ domain }}{{ nav.url }}"><span class="nav__sub-title">{{ nav.title }}</span></a>
+          <a href="{{ nav_url }}"><span class="nav__sub-title">{{ nav.title }}</span></a>
         {% else %}
           <span class="nav__sub-title">{{ nav.title }}</span>
         {% endif %}
@@ -25,9 +25,9 @@
           {% for child in nav.children %}
             {% comment %} internal/external URL check {% endcomment %}
             {% if child.url contains "://" %}
-              {% assign domain = "" %}
+              {% assign child_url = child.url %}
             {% else %}
-              {% assign domain = site.url | append: site.baseurl %}
+              {% assign child_url = child.url | relative_url %}
             {% endif %}
 
             {% comment %} set "active" class on current page {% endcomment %}
@@ -37,7 +37,7 @@
               {% assign active = "" %}
             {% endif %}
 
-            <li><a href="{{ domain }}{{ child.url }}" class="{{ active }}">{{ child.title }}</a></li>
+            <li><a href="{{ child_url }}" class="{{ active }}">{{ child.title }}</a></li>
           {% endfor %}
         </ul>
         {% endif %}