Improve current menu item detection

templates/macros/menu.html

@@ -5,20 +5,30 @@
         <nav class="menu">
             <ul class="menu__inner">
             {%- for item in menu_items %}
-                {%- set rel_item_url = item.url | replace(from="$BASE_URL", to="") -%}
-                {%- set current = rel_item_url == current_path
-                    or rel_item_url == "/" ~ current_path
-                    or rel_item_url ~ "/" == "/" ~ current_path
-                    or rel_item_url == "" and current_path == "/"
+                {%- set abs_item_url = item.url | replace(from="$BASE_URL", to=config.base_url) -%}
+                {%- set is_current = current_url == abs_item_url ~ "/"
+                                     or current_url is starting_with(abs_item_url)
                 -%}
-                {%- if current %}
+                {%- set is_base = abs_item_url == config.base_url
+                                  or abs_item_url == config.base_url ~ "/"
+                -%}
+
+                {%- if is_base %}
+                    {%- set_global base_item = item -%}
+                {% endif -%}
+
+                {%- if is_current and not is_base %}
                     {%- set_global current_item = item -%}
                 {% endif -%}
             {% endfor -%}
 
+            {%- if not current_item and base_item %}
+                {# Did not match any menu URLs -- assume it's a blog post #}
+                {%- set current_item = base_item -%}
+            {% endif -%}
+
             {%- for item in menu_items %}
-                {%- set blog_post = not current_item and item.url == "$BASE_URL" -%}
-                <li {%- if current_item and item == current_item or blog_post %} class="active" {%- endif %}>
+                <li {%- if current_item and current_item == item %} class="active" {%- endif %}>
                     {%- if item.newtab -%}
                         <a href="{{ item.url | replace(from="$BASE_URL", to=config.base_url) | safe }}" target="_blank" rel="noopener noreferrer">{{ item.name | safe }}</a>
                     {%- else -%}