diff --git a/app/index.html b/app/index.html
index d2324f7..f8dff1a 100644
--- a/app/index.html
+++ b/app/index.html
@@ -371,6 +371,33 @@ body {
 }
 .topbar-nav .nav-item.active { border-left: none; background: var(--nav-active-bg); }
 
+/* ─── Topbar grouped navigation (dropdowns) ─── */
+.topbar-nav .nav-group { position: relative; }
+.topbar-nav .nav-trigger {
+  display: flex; align-items: center; gap: 0.25rem;
+  padding: 0.35rem 0.75rem; border-radius: 5px;
+  background: none; border: none; cursor: pointer;
+  color: var(--font-colour); font-size: 0.85rem;
+  font-family: inherit; white-space: nowrap; text-decoration: none; line-height: inherit;
+}
+.topbar-nav .nav-trigger:hover,
+.topbar-nav .nav-group.open > .nav-trigger { background: var(--nav-hover-bg); }
+.topbar-nav .nav-group.has-active > .nav-trigger { background: var(--nav-active-bg); }
+.topbar-nav .nav-caret { font-size: 0.6rem; color: var(--font-colour-muted); opacity: 0.55; line-height: 1; }
+.topbar-nav .nav-dropdown {
+  display: none; position: absolute; top: calc(100% + 4px); left: 0;
+  background: var(--bg-nav); border: 1px solid var(--divider);
+  border-radius: 6px; box-shadow: 0 4px 16px rgba(0,0,0,0.1);
+  min-width: 160px; z-index: 200; padding: 0.25rem 0;
+}
+.topbar-nav .nav-group.open > .nav-dropdown { display: block; }
+.topbar-nav .nav-dropdown .nav-item {
+  display: block; padding: 0.45rem 1rem;
+  border-left: none; border-radius: 0; white-space: nowrap;
+}
+.topbar-nav .nav-dropdown .nav-item:hover { background: var(--nav-hover-bg); }
+.topbar-nav .nav-dropdown .nav-item.active { background: var(--nav-active-bg); font-weight: 600; }
+
 .topbar-actions { display: flex; align-items: center; gap: 0.5rem; flex-shrink: 0; }
 
 .topbar-search { position: relative; }
@@ -732,6 +759,19 @@ body {
     border-left: none;
   }
   .layout-topbar .mobile-nav-panel .nav-item.active { border-left: none; font-weight: 600; }
+  .layout-topbar .mobile-nav-panel .nav-group-row { display: flex; align-items: stretch; }
+  .layout-topbar .mobile-nav-panel .nav-group-row .nav-item { flex: 1; }
+  .layout-topbar .mobile-nav-panel .nav-section-label {
+    flex: 1; display: flex; align-items: center;
+    padding: 0.6rem 1.25rem; font-size: 1rem; color: var(--font-colour); font-weight: 500;
+  }
+  .layout-topbar .mobile-nav-panel .nav-expand-btn {
+    background: none; border: none; cursor: pointer;
+    color: var(--font-colour-muted); padding: 0 1.25rem; font-size: 1.2rem; line-height: 1; flex-shrink: 0;
+  }
+  .layout-topbar .mobile-nav-panel .nav-group-children { display: none; }
+  .layout-topbar .mobile-nav-panel .nav-group-children.open { display: block; }
+  .layout-topbar .mobile-nav-panel .nav-group-children .nav-item { padding-left: 2.5rem; }
 }
 
 @media (max-width: 480px) {
@@ -1800,6 +1840,10 @@ function fmtDatetime(dtStr) {
       if (panel) panel.classList.remove('open');
       hamburger.innerHTML = ICONS.menu;
     };
+
+    document.addEventListener('click', () => {
+      document.querySelectorAll('.topbar-nav .nav-group.open').forEach(g => g.classList.remove('open'));
+    });
   }
 
   function buildSearchWidget() {
@@ -2100,18 +2144,141 @@ function fmtDatetime(dtStr) {
     tree.forEach(root => renderTreeSection(container, root, 0, groups));
   }
 
-  function renderFlat(container) {
-    // Topbar inline: pages only, sorted by global sort, draft-section pages excluded.
+  function buildTopbarNavItems() {
     const byCode = {};
     navSections.forEach(s => { if (s.code) byCode[s.code] = s; });
-    const visible = navData.filter(p => {
-      const sid = p['section-id'];
-      return !sid || !isDraftSection(sid, byCode);
+    const items = [];
+
+    // Sections (non-draft), each becomes a dropdown trigger
+    navSections.forEach(s => {
+      if (!s.code || isDraftSection(s.code, byCode)) return;
+      const pages = navData.filter(p => p['section-id'] === s.code && pageShouldDisplay(p));
+      pages.sort((a, b) => ((a.sort ?? 999) - (b.sort ?? 999)) || a.file.localeCompare(b.file));
+      if (!pages.length) return;
+      items.push({ type: 'section', sort: s.sort ?? 999, section: s, pages });
     });
-    visible.sort((a, b) => ((a.sort ?? 999) - (b.sort ?? 999)) || a.file.localeCompare(b.file));
-    visible.forEach(p => {
-      const item = makeNavItem(p, 0);
-      if (item) container.appendChild(item);
+
+    // Unsectioned pages (or pages whose section isn't in nav), grouped by sort century
+    const unsectioned = navData.filter(p => {
+      if (!pageShouldDisplay(p)) return false;
+      const sid = p['section-id'];
+      return !sid || !byCode[sid];
+    });
+    unsectioned.sort((a, b) => ((a.sort ?? 999) - (b.sort ?? 999)) || a.file.localeCompare(b.file));
+    const centuryMap = new Map();
+    unsectioned.forEach(p => {
+      const c = Math.floor((p.sort ?? 999) / 100);
+      if (!centuryMap.has(c)) centuryMap.set(c, []);
+      centuryMap.get(c).push(p);
+    });
+    for (const [, pgs] of centuryMap) {
+      items.push({ type: 'group', sort: pgs[0].sort ?? 999, primary: pgs[0], children: pgs.slice(1) });
+    }
+
+    items.sort((a, b) => a.sort - b.sort);
+    return items;
+  }
+
+  function makeTopbarPageGroup({ primary, children }, isMobile) {
+    const group = el('div', { className: 'nav-group' });
+    const hasChildren = children.length > 0;
+
+    if (isMobile) {
+      const row = el('div', { className: 'nav-group-row' });
+      const link = makeNavItem(primary, 0);
+      if (link) row.appendChild(link);
+      if (hasChildren) {
+        const childrenEl = el('div', { className: 'nav-group-children' });
+        children.forEach(p => { const it = makeNavItem(p, 1); if (it) childrenEl.appendChild(it); });
+        const btn = el('button', { className: 'nav-expand-btn', 'aria-label': 'Expand', textContent: '+' });
+        btn.addEventListener('click', () => {
+          const open = childrenEl.classList.toggle('open');
+          btn.textContent = open ? '−' : '+';
+        });
+        row.appendChild(btn);
+        group.appendChild(row);
+        group.appendChild(childrenEl);
+      } else {
+        group.appendChild(row);
+      }
+    } else {
+      const title = pageDisplayTitle(primary);
+      const trigger = el('a', { className: 'nav-trigger', href: '#' + primary.file, 'data-file': primary.file });
+      trigger.appendChild(el('span', { textContent: title }));
+      if (hasChildren) trigger.appendChild(el('span', { className: 'nav-caret', textContent: '▾' }));
+      group.appendChild(trigger);
+
+      if (hasChildren) {
+        const dropdown = el('div', { className: 'nav-dropdown' });
+        children.forEach(p => { const it = makeNavItem(p, 0); if (it) dropdown.appendChild(it); });
+        group.appendChild(dropdown);
+        group.addEventListener('mouseenter', () => group.classList.add('open'));
+        group.addEventListener('mouseleave', () => group.classList.remove('open'));
+        trigger.addEventListener('click', e => {
+          e.preventDefault();
+          e.stopPropagation();
+          group.classList.toggle('open');
+          navigateTo(primary.file);
+          if (window._closeMobileMenu) window._closeMobileMenu();
+        });
+      } else {
+        trigger.addEventListener('click', e => {
+          e.preventDefault();
+          navigateTo(primary.file);
+          if (window._closeMobileMenu) window._closeMobileMenu();
+        });
+      }
+    }
+
+    return group;
+  }
+
+  function makeTopbarSection({ section, pages }, isMobile) {
+    const group = el('div', { className: 'nav-group' });
+    const isHidden = section.pagesvisibility === 'hidden';
+    const name = sectionDisplayName(section);
+
+    if (isMobile) {
+      const row = el('div', { className: 'nav-group-row' });
+      row.appendChild(el('span', { className: 'nav-section-label', textContent: name }));
+      const childrenEl = el('div', { className: 'nav-group-children' + (isHidden ? '' : ' open') });
+      pages.forEach(p => { const it = makeNavItem(p, 1); if (it) childrenEl.appendChild(it); });
+      const btn = el('button', { className: 'nav-expand-btn', 'aria-label': 'Expand', textContent: isHidden ? '+' : '−' });
+      btn.addEventListener('click', () => {
+        const open = childrenEl.classList.toggle('open');
+        btn.textContent = open ? '−' : '+';
+      });
+      row.appendChild(btn);
+      group.appendChild(row);
+      group.appendChild(childrenEl);
+    } else {
+      const trigger = el('button', { className: 'nav-trigger', type: 'button' });
+      trigger.appendChild(el('span', { textContent: name }));
+      trigger.appendChild(el('span', { className: 'nav-caret', textContent: '▾' }));
+      trigger.addEventListener('click', e => { e.stopPropagation(); group.classList.toggle('open'); });
+      group.appendChild(trigger);
+
+      const dropdown = el('div', { className: 'nav-dropdown' });
+      pages.forEach(p => { const it = makeNavItem(p, 0); if (it) dropdown.appendChild(it); });
+      group.appendChild(dropdown);
+
+      if (!isHidden) {
+        group.addEventListener('mouseenter', () => group.classList.add('open'));
+        group.addEventListener('mouseleave', () => group.classList.remove('open'));
+      }
+    }
+
+    return group;
+  }
+
+  function renderTopbarGrouped(container, isMobile) {
+    const items = buildTopbarNavItems();
+    items.forEach(item => {
+      container.appendChild(
+        item.type === 'group'
+          ? makeTopbarPageGroup(item, isMobile)
+          : makeTopbarSection(item, isMobile)
+      );
     });
   }
 
@@ -2121,19 +2288,23 @@ function fmtDatetime(dtStr) {
     const mobile = document.getElementById('mobileNavLinks');
     if (main) {
       main.innerHTML = '';
-      if (topbar) renderFlat(main);
+      if (topbar) renderTopbarGrouped(main, false);
       else renderTree(main);
     }
     if (mobile) {
       mobile.innerHTML = '';
-      renderTree(mobile);
+      if (topbar) renderTopbarGrouped(mobile, true);
+      else renderTree(mobile);
     }
   }
 
   function highlightNav(file) {
-    document.querySelectorAll('.nav-item').forEach(item => {
+    document.querySelectorAll('.nav-item, .nav-trigger[data-file]').forEach(item => {
       item.classList.toggle('active', item.getAttribute('data-file') === file);
     });
+    document.querySelectorAll('.topbar-nav .nav-group').forEach(group => {
+      group.classList.toggle('has-active', !!group.querySelector('[data-file].active'));
+    });
   }
 
   // ─── Page loading ─────────────────────────────────────────