Mailer preview now uses `url_for` to fix links to emails for apps running on a...

Mailer preview now uses `url_for` to fix links to emails for apps running on a subdirectory, closes #19092.

actionmailer/CHANGELOG.md

+*   Mailer preview now uses `url_for` to fix links to emails for apps running on
+    a subdirectory.
+
+    *Remo Mueller*
+
 *   Mailer previews no longer crash when the `mail` method wasn't called
     (`NullMail`).
 

railties/lib/rails/templates/rails/mailers/index.html.erb

 <% @previews.each do |preview| %>
-<h3><%= link_to preview.preview_name.titleize, "/rails/mailers/#{preview.preview_name}" %></h3>
+<h3><%= link_to preview.preview_name.titleize, url_for(controller: "rails/mailers", action: "preview", path: preview.preview_name) %></h3>
 <ul>
 <% preview.emails.each do |email| %>
-<li><%= link_to email, "/rails/mailers/#{preview.preview_name}/#{email}" %></li>
+<li><%= link_to email, url_for(controller: "rails/mailers", action: "preview", path: "#{preview.preview_name}/#{email}") %></li>
 <% end %>
 </ul>
 <% end %>

railties/lib/rails/templates/rails/mailers/mailer.html.erb

 <h3><%= @preview.preview_name.titleize %></h3>
 <ul>
 <% @preview.emails.each do |email| %>
-<li><%= link_to email, "/rails/mailers/#{@preview.preview_name}/#{email}" %></li>
+<li><%= link_to email, url_for(controller: "rails/mailers", action: "preview", path: "#{@preview.preview_name}/#{email}") %></li>
 <% end %>
 </ul>

railties/test/application/mailer_previews_test.rb

@@ -454,6 +454,36 @@ def foo
       assert_match '<option selected value="?part=text%2Fplain">View as plain-text email</option>', last_response.body
     end
 
+    test "mailer previews create correct links when loaded on a subdirectory" do
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app('development')
+
+      get "/rails/mailers", {}, 'SCRIPT_NAME' => '/my_app'
+      assert_match '<h3><a href="/my_app/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+      assert_match '<li><a href="/my_app/rails/mailers/notifier/foo">foo</a></li>', last_response.body
+    end
+
     private
       def build_app
         super