Use sessionStorage instead of localStorage for sidebar open-state persistence

Co-authored-by: sarzhann <32760183+sarzhann@users.noreply.github.com>
Agent-Logs-Url: https://github.com/vespa-engine/documentation/sessions/13bf9147-a40d-41bc-a44a-297ec880667b

Author: Copilot

_includes/sidebar.html

@@ -94,7 +94,7 @@
       {%- endif -%}
     {%- endfor -%}
     <li class="top">
-      <details{% if item_contains_current %} open{% endif %}><!-- open so current page is visible when landing via direct link -->
+      <details data-id="{{ item.title }}"{% if item_contains_current %} open{% endif %}><!-- open so current page is visible when landing via direct link -->
         <summary>{{ item.title }}</summary>
         {% if item.documents %}
         <ul class="child">
@@ -112,7 +112,7 @@
                     {%- endfor -%}
                   {%- endif -%}
                 {%- endfor -%}
-                <details{% if child_contains_current %} open{% endif %}><!-- open so current page is visible when landing via direct link -->
+                <details data-id="{{ item.title }}.{{ child.title }}"{% if child_contains_current %} open{% endif %}><!-- open so current page is visible when landing via direct link -->
                   <summary>{{child.title}}</summary>
                   <ul class="child">
                     {% for subchild in child.sub %}
@@ -124,7 +124,7 @@
                         {%- for ssd in subchild.sub -%}
                           {%- if ssd.url == page.url -%}{%- assign subchild_contains_current = true -%}{%- endif -%}
                         {%- endfor -%}
-                        <details{% if subchild_contains_current %} open{% endif %}><!-- open so current page is visible when landing via direct link -->
+                        <details data-id="{{ item.title }}.{{ child.title }}.{{ subchild.title }}"{% if subchild_contains_current %} open{% endif %}><!-- open so current page is visible when landing via direct link -->
                           <summary>{{subchild.title}}</summary>
                           <ul class="child">
                             {% for subsubchild in subchild.sub %}
@@ -155,4 +155,16 @@
     {% endfor %}
   </ul>
 </div>
+
+<script type="application/javascript">
+    const saved = new Set(JSON.parse(sessionStorage.getItem('expanded') || '[]'));
+
+    document.querySelectorAll('details[data-id]').forEach(d => {
+        if (saved.has(d.dataset.id)) d.open = true;
+        d.addEventListener('toggle', () => {
+            if (d.open) saved.add(d.dataset.id); else saved.delete(d.dataset.id);
+            sessionStorage.setItem('expanded', JSON.stringify([...saved]));
+        });
+    });
+</script>