diff --git a/lib/rdoc/generator/template/aliki/_head.rhtml b/lib/rdoc/generator/template/aliki/_head.rhtml
index 96ba1d65e5..d1ca4b7ad0 100644
--- a/lib/rdoc/generator/template/aliki/_head.rhtml
+++ b/lib/rdoc/generator/template/aliki/_head.rhtml
@@ -125,6 +125,11 @@
   defer
 ></script>
 
+<script
+  src="<%= h asset_rel_prefix %>/js/navigation.js?v=<%= h VERSION %>"
+  defer
+></script>
+
 <script
   src="<%= h asset_rel_prefix %>/js/search_ranker.js?v=<%= h VERSION %>"
   defer
diff --git a/lib/rdoc/generator/template/aliki/_sidebar_classes.rhtml b/lib/rdoc/generator/template/aliki/_sidebar_classes.rhtml
index 11ff53e1de..c4efc3403b 100644
--- a/lib/rdoc/generator/template/aliki/_sidebar_classes.rhtml
+++ b/lib/rdoc/generator/template/aliki/_sidebar_classes.rhtml
@@ -10,6 +10,6 @@
       </span>
     </summary>
 
-    <%= generate_class_index_content(@classes, rel_prefix) %>
+    <ul id="namespace-navigation" class="link-list nav-list"></ul>
   </details>
 </div>
diff --git a/lib/rdoc/generator/template/aliki/js/navigation.js b/lib/rdoc/generator/template/aliki/js/navigation.js
new file mode 100644
index 0000000000..8d030fe913
--- /dev/null
+++ b/lib/rdoc/generator/template/aliki/js/navigation.js
@@ -0,0 +1,70 @@
+'use strict';
+
+// Reuse the search index's visible classes and modules, adding unlinked
+// ancestors where a namespace is hidden but has visible descendants.
+function buildClassTree(index) {
+  const roots = [];
+  const byName = new Map();
+
+  index.forEach((entry) => {
+    if (entry.type !== 'class' && entry.type !== 'module') return;
+
+    let children = roots;
+    let fullName = '';
+    entry.full_name.split('::').forEach((name) => {
+      fullName = fullName ? fullName + '::' + name : name;
+      let node = byName.get(fullName);
+      if (!node) {
+        node = {name, path: null, children: []};
+        byName.set(fullName, node);
+        children.push(node);
+      }
+      children = node.children;
+    });
+    byName.get(fullName).path = entry.path;
+  });
+
+  return roots;
+}
+
+// Share the class tree across pages without embedding it in every page's HTML.
+function buildClassNavigation(container, nodes, prefix) {
+  function renderBranch(entries, list, expandSingleRoot = false) {
+    list.className = 'link-list nav-list';
+
+    entries.forEach((node) => {
+      const item = document.createElement('li');
+      const code = document.createElement('code');
+      if (node.path) {
+        const link = document.createElement('a');
+        link.href = prefix + node.path;
+        link.textContent = node.name;
+        code.appendChild(link);
+      } else {
+        code.textContent = node.name;
+      }
+
+      if (node.children.length) {
+        const details = document.createElement('details');
+        const summary = document.createElement('summary');
+        summary.appendChild(code);
+        details.appendChild(summary);
+        details.appendChild(renderBranch(node.children, document.createElement('ul')));
+        details.open = expandSingleRoot;
+        item.appendChild(details);
+      } else {
+        item.appendChild(code);
+      }
+      list.appendChild(item);
+    });
+    return list;
+  }
+
+  renderBranch(nodes, container, nodes.length === 1);
+}
+
+document.addEventListener('DOMContentLoaded', () => {
+  const container = document.getElementById('namespace-navigation');
+  if (!container || typeof search_data === 'undefined') return;
+  buildClassNavigation(container, buildClassTree(search_data.index), index_rel_prefix);
+});
diff --git a/test/rdoc/generator/aliki_test.rb b/test/rdoc/generator/aliki_test.rb
index f38f27d1dd..068fb92ebf 100644
--- a/test/rdoc/generator/aliki_test.rb
+++ b/test/rdoc/generator/aliki_test.rb
@@ -84,6 +84,7 @@ def test_write_style_sheet_copies_css_and_js_only
     # Aliki should have these assets
     assert_file 'css/rdoc.css'
     assert_file 'js/aliki.js'
+    assert_file 'js/navigation.js'
     assert_file 'js/search_controller.js'
     assert_file 'js/search_navigation.js'
     assert_file 'js/search_ranker.js'
@@ -241,6 +242,37 @@ def test_generate
     assert_match %r{<main role="main">}, index
   end
 
+  def test_shared_class_navigation
+    @klass.add_class RDoc::NormalClass, 'Inner'
+    @g.generate
+
+    data = JSON.parse(File.read('js/search_data.js').delete_prefix('var search_data = ').delete_suffix(';'))['index']
+    assert_include data.map { |entry| entry['full_name'] }, 'Klass::Inner'
+
+    page = File.read('Klass/Inner.html')
+    assert_include page, '<ul id="namespace-navigation" class="link-list nav-list"></ul>'
+    assert_include page, '../js/search_data.js?v='
+    sidebar = page[/<div id="classindex-section".*?<\/nav>/m]
+    assert_not_include sidebar, '<a href="../Klass.html">Klass</a>'
+  end
+
+  def test_navigation_preserves_hidden_namespace_and_omits_hidden_leaf
+    @klass = @top_level.add_class RDoc::NormalClass, 'HiddenRoot'
+    inner = @klass.add_class RDoc::NormalClass, 'Inner'
+    leaf = inner.add_class RDoc::NormalClass, 'Leaf'
+    hidden = @klass.add_class RDoc::NormalClass, 'Hidden'
+    hidden.document_self = false
+    inner.document_self = false
+    @klass.document_self = false
+
+    data = @g.build_search_index
+    names = data.map { |entry| entry[:full_name] }
+    assert_include names, leaf.full_name
+    assert_not_include names, @klass.full_name
+    assert_not_include names, inner.full_name
+    assert_not_include names, hidden.full_name
+  end
+
   def test_canonical_url
     @klass.add_class RDoc::NormalClass, 'Inner'
     @store.options.canonical_root = @options.canonical_root = "https://example.com/docs/"
diff --git a/test/rdoc/rdoc_server_test.rb b/test/rdoc/rdoc_server_test.rb
index 1a8a0f9909..9f9a715713 100644
--- a/test/rdoc/rdoc_server_test.rb
+++ b/test/rdoc/rdoc_server_test.rb
@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 require_relative 'support/test_case'
+require 'net/http'
 
 class RDocServerTest < RDoc::TestCase
 
@@ -79,52 +80,44 @@ def test_route_returns_404_for_missing_page
   end
 
   def test_check_for_changes_parses_and_reloads_rbs_signatures
-    @server.instance_variable_set(:@file_mtimes, @rdoc.last_modified.keys.to_h { |file|
-      [file, File.mtime(file)]
-    })
+    with_running_server do |port|
+      sig_dir = File.join @dir, 'sig'
+      FileUtils.mkdir_p sig_dir
+      File.write File.join(sig_dir, 'example.rbs'), <<~RBS
+        class Example
+          # RBS method docs.
+          def greet: () -> String
+        end
+      RBS
 
-    sig_dir = File.join @dir, 'sig'
-    FileUtils.mkdir_p sig_dir
-    File.write File.join(sig_dir, 'example.rbs'), <<~RBS
-      class Example
-        # RBS method docs.
-        def greet: () -> String
+      wait_for('class page to include the RBS method documentation') do
+        get(port, '/Example.html').body.include?('RBS method docs.')
       end
-    RBS
 
-    _out, err = capture_output do
-      assert @server.send(:check_for_changes)
+      example = @rdoc.store.find_class_or_module 'Example'
+      greet = example.find_method 'greet', false
+      assert_equal "RBS method docs.", greet.comment.to_s.strip
+      assert_equal ['() -> String'], greet.type_signature_lines
+      assert_equal ['() -> String'], @rdoc.store.rbs_signature_for(greet)
     end
-
-    assert_not_include err, 'Error parsing'
-
-    example = @rdoc.store.find_class_or_module 'Example'
-    greet = example.find_method 'greet', false
-    assert_equal "RBS method docs.", greet.comment.to_s.strip
-    assert_equal ['() -> String'], greet.type_signature_lines
-    assert_equal ['() -> String'], @rdoc.store.rbs_signature_for(greet)
   end
 
   def test_check_for_changes_parses_rbs_sources
-    @server.instance_variable_set(:@file_mtimes, @rdoc.last_modified.keys.to_h { |file|
-      [file, File.mtime(file)]
-    })
+    with_running_server do |port|
+      File.write File.join(@dir, 'sample.rbs'), <<~RBS
+        class Sample
+          def greet: () -> String
+        end
+      RBS
 
-    File.write File.join(@dir, 'sample.rbs'), <<~RBS
-      class Sample
-        def greet: () -> String
+      wait_for('class page to include the new RBS source') do
+        get(port, '/Sample.html').code == '200'
       end
-    RBS
 
-    _out, err = capture_output do
-      assert @server.send(:check_for_changes)
+      sample = @rdoc.store.find_class_or_module 'Sample'
+      greet = sample.find_method 'greet', false
+      assert_equal ['() -> String'], greet.type_signature_lines
     end
-
-    assert_not_include err, 'Error parsing'
-
-    sample = @rdoc.store.find_class_or_module 'Sample'
-    greet = sample.find_method 'greet', false
-    assert_equal ['() -> String'], greet.type_signature_lines
   end
 
   def test_current_watch_files_deduplicates_symlinked_source_tree
@@ -140,4 +133,36 @@ def test_current_watch_files_deduplicates_symlinked_source_tree
   rescue NotImplementedError, Errno::EACCES, Errno::EPERM
     omit 'symlinks are not supported'
   end
+
+  private
+
+  def with_running_server
+    port = TCPServer.open('127.0.0.1', 0) { |socket| socket.addr[1] }
+    @server = RDoc::Server.new(@rdoc, port)
+    server_thread = Thread.new { @server.start }
+
+    wait_for('server to start') do
+      get(port, '/__status').code == '200'
+    rescue Errno::ECONNREFUSED
+      false
+    end
+
+    yield port
+  ensure
+    server_thread&.raise(Interrupt) if server_thread&.alive?
+    server_thread&.join
+  end
+
+  def get(port, path)
+    Net::HTTP.start('127.0.0.1', port) { |http| http.get(path) }
+  end
+
+  def wait_for(description)
+    deadline = Process.clock_gettime(Process::CLOCK_MONOTONIC) + 10
+
+    until yield
+      flunk "Timed out waiting for #{description}" if Process.clock_gettime(Process::CLOCK_MONOTONIC) >= deadline
+      sleep 0.05
+    end
+  end
 end