Determine active page by considering parent too

Currently the logic that works out which page to highlight as ‘active’
in the top navigation is quite naïve (only compares URLs directly).

This commit makes it a bit smarter so that a page can define its
`parent`, then the navigation will highlight the matching ‘parent’
navigation item.

Adapted from: alphagov/govuk-developer-docs@7e5ac51

lib/govuk_tech_docs.rb

@@ -13,6 +13,7 @@
 require 'govuk_tech_docs/redirects'
 require 'govuk_tech_docs/table_of_contents/helpers'
 require 'govuk_tech_docs/contribution_banner'
+require 'govuk_tech_docs/helpers'
 require 'govuk_tech_docs/meta_tags'
 require 'govuk_tech_docs/page_review'
 require 'govuk_tech_docs/pages'
@@ -70,6 +71,10 @@ def current_page_review
       def format_date(date)
         date.strftime('%-e %B %Y')
       end
+
+      def active_page
+          @active_page ||= NavigationHelpers.new(current_page)
+      end
     end
 
     context.page '/*.xml', layout: false

lib/govuk_tech_docs/helpers.rb

@@ -0,0 +1,17 @@
+module GovukTechDocs
+  class NavigationHelpers
+
+    def initialize(current_page)
+      @current_page = current_page
+    end
+
+    def from_path?(page_path)
+      ("/" + @current_page.path) == page_path || @current_page.data.parent == page_path
+    end
+
+  private
+
+    attr_reader :config, :current_page
+
+  end
+end

lib/source/layouts/_header.erb

@@ -31,9 +31,9 @@
 
         <nav id="navigation" class="header__navigation js-nav" aria-label="Top Level Navigation" aria-hidden="true">
           <ul>
-            <% config[:tech_docs][:header_links].each do |title, url| %>
-              <li<% if url == current_page.url %> class="active"<% end %>>
-                <a href="<%= url %>">
+            <% config[:tech_docs][:header_links].each do |title, path| %>
+              <li<% if active_page.from_path?(path) %> class="active"<% end %>>
+                <a href="<%= path %>">
                   <%= title %>
                 </a>
               </li>