commit b760c74f6b425e5c46e2a0f4cae2df6f3cc68d26
Author: Erik Stielstra <info@erikstielstra.nl>
Date:   Sun Aug 31 17:41:56 2014 +0200

    #6

diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 9d0b654..8c7a664 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -1556,6 +1556,7 @@ function theme_indentation($variables) {
  *     Properties used: #id, #attributes, #children.
  */
 function template_preprocess_container(&$variables) {
+  $variables['has_parent'] = FALSE;
   $element = $variables['element'];
   // Ensure #attributes is set.
   $element += array('#attributes' => array());
@@ -1566,6 +1567,7 @@ function template_preprocess_container(&$variables) {
     if (!isset($element['#attributes']['id'])) {
       $element['#attributes']['id'] = $element['#id'];
     }
+    $variables['has_parent'] = TRUE;
   }
 
   $variables['children'] = $element['#children'];
diff --git a/core/modules/system/templates/container.html.twig b/core/modules/system/templates/container.html.twig
index 5502c87..9ec8497 100644
--- a/core/modules/system/templates/container.html.twig
+++ b/core/modules/system/templates/container.html.twig
@@ -9,10 +9,16 @@
  * Available variables:
  * - attributes: HTML attributes for the containing element.
  * - children: The rendered child elements of the container.
+ * - has_parent: A flag to indicate that the container has one or more parent
+     containers.
  *
  * @see template_preprocess_container()
  *
  * @ingroup themeable
  */
 #}
-<div{{ attributes.addClass('form-wrapper') }}>{{ children }}</div>
+{% if has_parent %}
+  <div{{ attributes.addClass('form-wrapper') }}>{{ children }}</div>
+{% else %}
+  <div{{ attributes }}>{{ children }}</div>
+{% endif %}