DEV: allow themes to render their own custom homepage (#26291)

This PR adds a theme modifier and route so that custom themes can opt to show their own homepage. See PR description for example usage.

spec/system/homepage_spec.rb

@@ -0,0 +1,152 @@
+# frozen_string_literal: true
+
+describe "Homepage", type: :system do
+  fab!(:admin)
+  fab!(:user)
+  fab!(:topics) { Fabricate.times(5, :post).map(&:topic) }
+  let(:discovery) { PageObjects::Pages::Discovery.new }
+  let!(:theme) { Fabricate(:theme) }
+
+  before do
+    # A workaround to avoid the global notice from interfering with the tests
+    # It is coming from the ensure_login_hint.rb initializer and it gets
+    # evaluated before the tests run (and it wrongly counts 0 admins defined)
+    SiteSetting.global_notice = nil
+  end
+
+  it "shows a list of topics by default" do
+    visit "/"
+    expect(discovery.topic_list).to have_topics(count: 5)
+  end
+
+  it "allows users to pick their homepage" do
+    sign_in user
+    visit "/"
+
+    expect(page).to have_css(".navigation-container .latest.active", text: "Latest")
+
+    visit "u/#{user.username}/preferences/interface"
+
+    homepage_picker = PageObjects::Components::SelectKit.new("#home-selector")
+    homepage_picker.expand
+    homepage_picker.select_row_by_name("Top")
+    page.find(".btn-primary.save-changes").click
+
+    # Wait for the save to complete
+    find(".btn-primary.save-changes:not([disabled])", wait: 5)
+
+    visit "/"
+
+    expect(page).to have_css(".navigation-container .top.active", text: "Top")
+    expect(page).to have_css(".top-lists")
+  end
+
+  it "defaults to first top_menu item as anonymous homepage" do
+    SiteSetting.top_menu = "categories|latest|new|unread"
+    visit "/"
+
+    expect(page).to have_css(".navigation-container .categories.active", text: "Categories")
+
+    sign_in user
+    visit "/"
+
+    expect(page).to have_css(".navigation-container .categories.active", text: "Categories")
+  end
+
+  context "when default theme uses a custom_homepage modifier" do
+    before do
+      theme.theme_modifier_set.custom_homepage = true
+      theme.theme_modifier_set.save!
+      theme.set_default!
+    end
+
+    it "shows empty state to regular users" do
+      sign_in user
+      visit "/"
+
+      expect(page).to have_no_css(".list-container")
+      expect(page).to have_no_css(".alert-info")
+    end
+
+    it "shows empty state and notice to admins" do
+      sign_in admin
+      visit "/"
+
+      expect(page).to have_no_css(".list-container")
+      expect(page).to have_css(".alert-info")
+    end
+
+    context "when the theme adds content to the [custom-homepage] connector" do
+      let!(:basic_html_field) do
+        Fabricate(
+          :theme_field,
+          theme: theme,
+          type_id: ThemeField.types[:html],
+          target_id: Theme.targets[:common],
+          name: "head_tag",
+          value: <<~HTML,
+            <script type="text/x-handlebars" data-template-name="/connectors/custom-homepage/new-home">
+              <div class="new-home">Hi friends!</div>
+            </script>
+          HTML
+        )
+      end
+
+      it "shows the custom homepage from the theme on the homepage" do
+        visit "/"
+
+        expect(page).to have_css(".new-home", text: "Hi friends!")
+        expect(page).to have_no_css(".list-container")
+
+        find("#sidebar-section-content-community .sidebar-section-link:first-child").click
+        expect(page).to have_css(".list-container")
+
+        find("#site-logo").click
+
+        expect(page).to have_no_css(".list-container")
+        # ensure clicking on logo brings user back to the custom homepage
+        expect(page).to have_css(".new-home", text: "Hi friends!")
+      end
+
+      it "respects the user's homepage choice" do
+        visit "/"
+
+        expect(page).not_to have_css(".list-container")
+        expect(page).to have_css(".new-home", text: "Hi friends!")
+
+        sign_in user
+
+        visit "/u/#{user.username}/preferences/interface"
+
+        homepage_picker = PageObjects::Components::SelectKit.new("#home-selector")
+        homepage_picker.expand
+        # user overrides theme custom homepage
+        homepage_picker.select_row_by_name("Top")
+        page.find(".btn-primary.save-changes").click
+
+        # Wait for the save to complete
+        find(".btn-primary.save-changes:not([disabled])", wait: 5)
+
+        find("#site-logo").click
+
+        expect(page).to have_css(".navigation-container .top.active", text: "Top")
+        expect(page).to have_css(".top-lists")
+
+        visit "/u/#{user.username}/preferences/interface"
+
+        homepage_picker = PageObjects::Components::SelectKit.new("#home-selector")
+        homepage_picker.expand
+        # user selects theme custom homepage again
+        homepage_picker.select_row_by_name("(default)")
+        page.find(".btn-primary.save-changes").click
+
+        # Wait for the save to complete
+        find(".btn-primary.save-changes:not([disabled])", wait: 5)
+        find("#site-logo").click
+
+        expect(page).not_to have_css(".list-container")
+        expect(page).to have_css(".new-home", text: "Hi friends!")
+      end
+    end
+  end
+end