[ruby/rdoc] Darkfish: Nest sidebar ToC as a tree of headings

This uses `<details><summary>heading</summary><ul>nested</ul></detail>`,
similar to how the classes and pages lists are now nested.

https://github.com/ruby/rdoc/commit/e57beff287

lib/rdoc/generator/template/darkfish/_sidebar_table_of_contents.rhtml

@@ -3,16 +3,37 @@
              else
                current.comment
              end
-   table = current.parse(comment).table_of_contents
+   table = current.parse(comment).table_of_contents.dup
 
    if table.length > 1 then %>
 <div class="nav-section">
   <h3>Table of Contents</h3>
 
+    <%- display_link = proc do |heading| -%>
+      <a href="#<%= heading.label current %>"><%= heading.plain_html %></a>
+    <%- end -%>
+
+    <%- list_siblings = proc do -%>
+      <%- level = table.first&.level -%>
+      <%- while table.first && table.first.level >= level -%>
+        <%- heading = table.shift -%>
+        <%- if table.first.nil? || table.first.level <= heading.level -%>
+          <li><% display_link.call heading -%>
+        <%- else -%>
+          <li>
+            <details open>
+              <summary><%- display_link.call heading -%></summary>
+              <ul class="link-list" role="directory">
+                <% list_siblings.call %>
+              </ul>
+            </details>
+          </li>
+        <%- end -%>
+      <%- end -%>
+    <%- end -%>
+
   <ul class="link-list" role="directory">
-<%-   table.each do |heading| -%>
-    <li><a href="#<%= heading.label current %>"><%= heading.plain_html %></a>
-<%-   end -%>
+    <% list_siblings.call %>
   </ul>
 </div>
 <%- end -%>

test/rdoc/test_rdoc_generator_darkfish.rb

@@ -73,6 +73,22 @@ class TestRDocGeneratorDarkfish < RDoc::TestCase
     top_level = @store.add_file 'file.rb'
     top_level.add_class @klass.class, @klass.name
     @klass.add_class RDoc::NormalClass, 'Inner'
+    @klass.add_comment <<~RDOC, top_level
+    = Heading 1
+    Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod
+    == Heading 1.1
+    tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,
+    === Heading 1.1.1
+    quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo
+    ==== Heading 1.1.1.1
+    consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse
+    == Heading 1.2
+    cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat
+    == Heading 1.3
+    non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.
+    === Heading 1.3.1
+    etc etc...
+    RDOC
 
     @g.generate
 
@@ -97,6 +113,15 @@ class TestRDocGeneratorDarkfish < RDoc::TestCase
     refute_match(/Ignored/, File.read('index.html'))
     summary = File.read('index.html')[%r[<summary.*Klass\.html.*</summary>.*</details>]m]
     assert_match(%r[Klass/Inner\.html".*>Inner<], summary)
+    klassnav = File.read('Klass.html')[%r[<div class="nav-section">.*<div id="class-metadata">]m]
+    assert_match(
+      %r[<li>\s*<details open>\s*<summary>\s*<a href=\S+>Heading 1</a>\s*</summary>\s*<ul]m,
+      klassnav
+    )
+    assert_match(
+      %r[<li>\s*<a href=\S+>Heading 1.1.1.1</a>\s*</ul>\s*</details>\s*</li>]m,
+      klassnav
+    )
   end
 
   def test_generate_page