Test alternative text for Transition navigation

We want to test whether changing the text on the banner navigation improves
click through rate. We want to keep the test simple, so we've just
chosen a [single, high traffic page](https://www.gov.uk/visit-europe-1-january-2021) as
victim.

The displayed navigation is determined in govuk_publishing_components, however
we know how it works. If a certain taxonomy content_id is found in the taxon links
of a content item, then a step by step banner (colloquially a "super breadcrumb")
will be displayed using the title of the taxon.

If we intercept the content item before it is passed to the component, then
we can tweak what is displayed.

https://trello.com/c/igPWXPvz/482-put-new-transition-taxon-live

app/controllers/content_items_controller.rb

@@ -92,13 +92,32 @@ def load_content_item
       content_item["links"]["ordered_related_items"] = content_item["links"].fetch("suggested_ordered_related_items", [])
     end
 
+    if update_brexit_navigation?(content_item)
+      content_item["links"]["taxons"] = taxons_updated_for_brexit_test(content_item)
+    end
+
     @content_item = PresenterBuilder.new(
       content_item,
       content_item_path,
       view_context,
     ).presenter
   end
 
+  def update_brexit_navigation?(content_item)
+    content_item["content_id"] == "7a616597-c921-47ba-bd50-7e73449e140b" # /visit-europe-1-january-2021
+  end
+
+  def taxons_updated_for_brexit_test(content_item)
+    content_item["links"]["taxons"].map do |taxon|
+      taxon["title"] = "The UK and EU transition: new rules for 2021" if brexit_taxon?(taxon)
+      taxon
+    end
+  end
+
+  def brexit_taxon?(taxon)
+    taxon["content_id"] == "d6c2de5d-ef90-45d1-82d4-5f2438369eea"
+  end
+
   def format_banner_links(links, type)
     links.each.with_index(1).map do |(title, base_path), index|
       view_context.link_to(

test/integration/guide_test.rb

@@ -112,6 +112,35 @@ class GuideTest < ActionDispatch::IntegrationTest
     assert_nil faq_schema
   end
 
+  test "a specific guide has tweaked Brexit navigation" do
+    visit_europe_guide_id = "7a616597-c921-47ba-bd50-7e73449e140b"
+    setup_and_visit_a_guide_with_the_brexit_taxon(visit_europe_guide_id)
+
+    assert page.has_css?(".gem-c-step-nav-header__title", text: "The UK and EU transition: new rules for 2021")
+  end
+
+  test "a normal Brexit guide has normal Brexit navigation" do
+    setup_and_visit_a_guide_with_the_brexit_taxon
+
+    assert page.has_css?(".gem-c-step-nav-header__title", text: "Brexit things")
+  end
+
+  def setup_and_visit_a_guide_with_the_brexit_taxon(content_id = nil)
+    @content_item = get_content_example("guide").tap do |item|
+      item["content_id"] = content_id if content_id.present?
+      item["links"]["taxons"] = [brexit_taxon]
+      stub_content_store_has_item(item["base_path"], item.to_json)
+      visit_with_cachebust(item["base_path"])
+    end
+  end
+
+  def brexit_taxon
+    @brexit_taxon ||= GovukSchemas::Example.find("taxon", example_name: "taxon").tap do |taxon|
+      taxon["title"] = "Brexit things"
+      taxon["content_id"] = "d6c2de5d-ef90-45d1-82d4-5f2438369eea" # the real Brexit taxon ID
+    end
+  end
+
   def setup_and_visit_part_in_guide
     @content_item = get_content_example("guide").tap do |item|
       chapter_path = "#{item['base_path']}/key-stage-1-and-2"