Support one level of dropdown menus in the navbar

A `main` menu entry with child entries (others with `parent:` set) is
now rendered as a Bootstrap dropdown; flat entries are unchanged.
Active-state highlighting is trailing-slash tolerant and marks the
dropdown parent active when one of its children is the current page.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -34,6 +34,37 @@ Optional params: `title`, `height` (a CSS length such as `60vh`, which
 crops the image with `object-fit: cover`) and `class`.
 
 
+## Navigation
+
+The navbar renders the `main` menu. A top-level entry that has child
+entries (other entries pointing at it with `parent:`) is rendered as a
+Bootstrap dropdown; entries without children stay as plain links. Only
+one level of nesting is supported (Bootstrap 5 has no nested submenus).
+
+```yaml
+# hugo.yaml / menus.yaml
+menu:
+  main:
+    - name: Home
+      pageRef: /
+      weight: 1
+    - name: Research
+      weight: 2
+    - name: Proposals
+      parent: Research
+      pageRef: /proposals
+      weight: 1
+    - name: History
+      parent: Research
+      pageRef: /history
+      weight: 2
+```
+
+The entry matching the current page (and its parent, for a dropdown
+child) gets Bootstrap's `active` class. A parent entry acts only as the
+dropdown toggle, so it does not need its own `url`/`pageRef`.
+
+
 ## Custom error page
 
 The theme has a custom 404 error page.

layouts/_partials/header.html

@@ -14,35 +14,40 @@
 
     <div class="collapse navbar-collapse" id="navbarNav">
       <ul class="navbar-nav ms-auto">
-        {{ range .Site.Menus.main }}
+        {{- /* Current page, without a trailing slash, for menu-item highlighting. */ -}}
-          {{ if .HasChildren }}
+        {{- $current := strings.TrimSuffix "/" .RelPermalink -}}
-            <!-- Dropdown menu for items with children -->
+        {{- range .Site.Menus.main -}}
+          {{- $isActive := and .URL (eq (strings.TrimSuffix "/" .URL) $current) -}}
+          {{- if .HasChildren -}}
+            {{- /* A menu entry with sub-entries (set `parent:` on them) becomes a dropdown. */ -}}
+            {{- range .Children -}}
+              {{- if and .URL (eq (strings.TrimSuffix "/" .URL) $current) }}{{ $isActive = true }}{{ end -}}
+            {{- end -}}
+            {{- $id := printf "navbar-%s" (.Identifier | default (.Name | urlize)) -}}
             <li class="nav-item dropdown">
-              <a class="nav-link dropdown-toggle" href="#" id="navbar{{ .Name | urlize }}" 
+              <a class="nav-link dropdown-toggle{{ if $isActive }} active{{ end }}" href="#"
-                 role="button" data-bs-toggle="dropdown" aria-expanded="false">
+                 id="{{ $id }}" role="button" data-bs-toggle="dropdown" aria-expanded="false">
                 {{ .Name }}
               </a>
-              <ul class="dropdown-menu" aria-labelledby="navbar{{ .Name | urlize }}">
+              <ul class="dropdown-menu" aria-labelledby="{{ $id }}">
-                {{ range .Children }}
+                {{- range .Children }}
                   <li>
-                    <a class="dropdown-item {{ if eq .URL ($.RelPermalink) }}active{{ end }}" 
+                    <a class="dropdown-item{{ if and .URL (eq (strings.TrimSuffix "/" .URL) $current) }} active{{ end }}"
-                       href="{{ .URL | relURL }}">
+                       href="{{ .URL }}">
                       {{ .Name }}
                     </a>
                   </li>
-                {{ end }}
+                {{- end }}
               </ul>
             </li>
-          {{ else }}
+          {{- else -}}
-            <!-- Regular menu item -->
             <li class="nav-item">
-              <a class="nav-link {{ if eq .URL ($.RelPermalink) }}active{{ end }}" 
+              <a class="nav-link{{ if $isActive }} active{{ end }}" href="{{ .URL }}">
-                 href="{{ .URL | relURL }}">
                 {{ .Name }}
               </a>
             </li>
-          {{ end }}
+          {{- end -}}
-        {{ end }}
+        {{- end }}
       </ul>
     </div>
   </div>