- add logic to fix active tab bug

- split the logic out more clearly so we can understand the rules

Author: JC-DSIT

lib/govuk_tech_docs.rb

@@ -101,11 +101,35 @@ def format_date(date)
       end
 
       def active_page(page_path)
-        [
-          page_path == "/" && current_page.path == "index.html",
-          "/#{current_page.path}" == page_path,
-          !current_page.data.parent.nil? && current_page.data.parent.to_s == page_path,
-        ].any?
+        # if we are on the homepage e.g  Documentation: /
+        if page_path == "/" && current_page.url == "/"
+          return true
+        end
+
+        # if we are on a single page the header links point to e.g Expired page: /expired-page.html
+        if "/#{current_page.path}" == page_path
+          return true
+        end
+
+        # If the doc maintainer has set the parent data in the frontmatter for the current page,
+        # and it matches the value in the config e.g
+        # Expired page: /expired-page.html (config) and parent: /expired-page.html (child-of-expired-page.html)
+        if !current_page.data.parent.nil? && current_page.data.parent.to_s == page_path
+          return true
+        end
+
+        # If we have a nested directory structure that matches a navigation link e.g  Active pages: /active-pages/
+        if current_page.url.start_with?(page_path)
+          # feels like a weird check, but stops fals positive where "/" for root is also the start of "/active-pages"
+          if page_path == "/"
+            return current_page.url == "/"
+          end
+
+          # safe to return true, since we aren't in the situation above
+          return true
+        end
+        # no other reason to believe we're active
+        false
       end
     end