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.

app/assets/javascripts/discourse/app/lib/url.js

@@ -155,6 +155,10 @@ const DiscourseURL = EmberObject.extend({
   },
 
   replaceState(path) {
+    if (path.startsWith("#")) {
+      path = this.router.currentURL.replace(/#.*$/, "") + path;
+    }
+
     if (this.router.currentURL !== path) {
       // Always use replaceState in the next runloop to prevent weird routes changing
       // while URLs are loading. For example, while a topic loads it sets `currentPost`

spec/rails_helper.rb

@@ -388,7 +388,6 @@ RSpec.configure do |config|
   end
 
   # Match the request hostname to the value in `database.yml`
-  config.before(:all, type: %i[request multisite system]) { host! "test.localhost" }
   config.before(:each, type: %i[request multisite system]) { host! "test.localhost" }
 
   last_driven_by = nil

spec/support/helpers.rb

@@ -149,13 +149,24 @@ module Helpers
     capture_output(:stderr, &block)
   end
 
-  def set_subfolder(f)
-    global_setting :relative_url_root, f
+  def set_subfolder(new_root)
+    global_setting :relative_url_root, new_root
+
     old_root = ActionController::Base.config.relative_url_root
-    ActionController::Base.config.relative_url_root = f
-    Rails.application.routes.stubs(:relative_url_root).returns(f)
+    ActionController::Base.config.relative_url_root = new_root
+    Rails.application.routes.stubs(:relative_url_root).returns(new_root)
 
     before_next_spec { ActionController::Base.config.relative_url_root = old_root }
+
+    if RSpec.current_example.metadata[:type] == :system
+      Capybara.app.map("/") { run lambda { |env| [404, {}, [""]] } }
+      Capybara.app.map(new_root) { run Rails.application }
+
+      before_next_spec do
+        Capybara.app.map(new_root) { run lambda { |env| [404, {}, [""]] } }
+        Capybara.app.map("/") { run Rails.application }
+      end
+    end
   end
 
   def setup_git_repo(files)

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