FIX: TOC anchors in a subfolder setup (#21985)

Clicking on TOC heading anchors in a subfolder setup was breaking the current URL for users.

Other than the fix this change introduces the ability to test the subfolder setup in system specs.

spec/system/topic_page_spec.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+describe "Topic page", type: :system do
+  fab!(:topic) { Fabricate(:topic) }
+
+  before { Fabricate(:post, topic: topic, cooked: <<~HTML) }
+    <h2 dir="ltr" id="toc-h2-testing" data-d-toc="toc-h2-testing" class="d-toc-post-heading">
+      <a name="toc-h2-testing" class="anchor" href="#toc-h2-testing">x</a>
+      Testing
+    </h2>
+    HTML
+
+  it "allows TOC anchor navigation" do
+    visit("/t/#{topic.slug}/#{topic.id}")
+
+    find("#toc-h2-testing").hover
+    find("a.anchor").click
+
+    try_until_success(timeout: 5) do
+      expect(current_url).to match("/t/#{topic.slug}/#{topic.id}#toc-h2-testing")
+    end
+  end
+
+  context "with a subfolder setup" do
+    before { set_subfolder "/forum" }
+
+    it "allows TOC anchor navigation" do
+      visit("/forum/t/#{topic.slug}/#{topic.id}")
+
+      find("#toc-h2-testing").hover
+      find("a.anchor").click
+
+      try_until_success(timeout: 5) do
+        expect(current_url).to match("/forum/t/#{topic.slug}/#{topic.id}#toc-h2-testing")
+      end
+    end
+  end
+end