Issue #2172717 by andypost, tim.plunkett: Missing local tasks on the 'Manage form display' page.

core/modules/field_ui/lib/Drupal/field_ui/Routing/RouteSubscriber.php

@@ -101,7 +101,6 @@ protected function alterRoutes(RouteCollection $collection, $provider) {
           "$path/form-display/{form_mode_name}",
           array(
             '_form' => '\Drupal\field_ui\FormDisplayOverview',
-            'form_mode_name' => NULL,
           ) + $defaults,
           array('_field_ui_form_mode_access' => 'administer ' . $entity_type . ' form display')
         );

core/modules/field_ui/lib/Drupal/field_ui/Tests/FieldUIRouteTest.php

@@ -52,8 +52,24 @@ public function testFieldUIRoutes() {
 
     $this->drupalGet('admin/structure/types/manage/article/fields');
     $this->assertTitle('Manage fields | Drupal');
+    $this->assertLocalTasks();
 
     $this->drupalGet('admin/structure/types/manage/article');
+    $this->assertLocalTasks();
+
+    $this->drupalGet('admin/structure/types/manage/article/form-display');
+    $this->assertLocalTasks();
+
+    $this->drupalGet('admin/structure/types/manage/article/display');
+    $this->assertLocalTasks();
+  }
+
+  /**
+   * Asserts that local tasks exists.
+   */
+  public function assertLocalTasks() {
+    $this->assertLink('Edit');
+    $this->assertLink('Manage fields');
     $this->assertLink('Manage display');
     $this->assertLink('Manage form display');
   }