diff --git a/core/includes/form.inc b/core/includes/form.inc
index b9180b2..6ea69d3 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -2851,8 +2851,8 @@ function form_get_options($element, $key) {
  * @param $variables
  *   An associative array containing:
  *   - element: An associative array containing the properties of the element.
- *     Properties used: #attributes, #children, #collapsed, #description, #id,
- *     #title, #value.
+ *     Properties used: #attributes, #children, #description, #id, #title,
+ *     #value.
  *
  * @ingroup themeable
  */
@@ -2897,8 +2897,8 @@ function theme_fieldset($variables) {
  * @param $variables
  *   An associative array containing:
  *   - element: An associative array containing the properties of the element.
- *     Properties used: #attributes, #children, #collapsed, #description, #id,
- *     #title, #value.
+ *     Properties used: #attributes, #children, #description, #id, #title,
+ *     #value.
  *
  * @ingroup themeable
  */
@@ -2915,7 +2915,7 @@ function theme_details($variables) {
     if (!empty($element['#attributes']['id'])) {
       $summary_attributes['aria-controls'] = $element['#attributes']['id'];
     }
-    $summary_attributes['aria-expanded'] = empty($element['#attributes']['open']) ? FALSE : TRUE;
+    $summary_attributes['aria-expanded'] = !empty($element['#attributes']['open']);
     $summary_attributes['aria-pressed'] = $summary_attributes['aria-expanded'];
     $output .= '<summary' . $summary_attributes . '>' . $element['#title'] . '</summary>';
   }
@@ -3918,8 +3918,8 @@ function form_pre_render_details($element) {
 
   // Collapsible details.
   $element['#attached']['library'][] = array('system', 'drupal.collapse');
-  if (empty($element['#collapsed'])) {
-    $element['#attributes']['open'] = 'open';
+  if (!empty($element['#open'])) {
+    $element['#attributes']['open'] =  'open';
   }
 
   return $element;
diff --git a/core/lib/Drupal/Core/Database/Install/Tasks.php b/core/lib/Drupal/Core/Database/Install/Tasks.php
index ab1f771..448593c 100644
--- a/core/lib/Drupal/Core/Database/Install/Tasks.php
+++ b/core/lib/Drupal/Core/Database/Install/Tasks.php
@@ -234,7 +234,6 @@ public function getFormOptions($database) {
     $form['advanced_options'] = array(
       '#type' => 'details',
       '#title' => st('Advanced options'),
-      '#collapsed' => TRUE,
       '#description' => st("These options are only necessary for some sites. If you're not sure what you should enter here, leave the default settings or check with your hosting provider."),
       '#weight' => 10,
     );
diff --git a/core/lib/Drupal/Core/FileTransfer/FileTransfer.php b/core/lib/Drupal/Core/FileTransfer/FileTransfer.php
index 823e17e..95f440c 100644
--- a/core/lib/Drupal/Core/FileTransfer/FileTransfer.php
+++ b/core/lib/Drupal/Core/FileTransfer/FileTransfer.php
@@ -408,7 +408,6 @@ public function getSettingsForm() {
     $form['advanced'] = array(
       '#type' => 'details',
       '#title' => t('Advanced settings'),
-      '#collapsed' => TRUE,
     );
     $form['advanced']['hostname'] = array(
       '#type' => 'textfield',
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php b/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php
index 9a5cbde..2e74add 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php
@@ -131,7 +131,7 @@ public function buildForm(array $form, array &$form_state) {
         '#type' => 'details',
         '#title' => t('Basic configuration'),
         '#description' => t('For most aggregation tasks, the default settings are fine.'),
-        '#collapsed' => FALSE,
+        '#open' => TRUE,
       );
       $form['basic_conf'] += $basic_conf;
     }
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/aggregator/processor/DefaultProcessor.php b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/aggregator/processor/DefaultProcessor.php
index 08777d1..71cb6cc 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Plugin/aggregator/processor/DefaultProcessor.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Plugin/aggregator/processor/DefaultProcessor.php
@@ -45,7 +45,7 @@ public function settingsForm(array $form, array &$form_state) {
         '#type' => 'details',
         '#title' => t('Default processor settings'),
         '#description' => $info['description'],
-        '#collapsed' => !in_array($info['id'], $processors),
+        '#open' => in_array($info['id'], $processors),
       );
     }
 
diff --git a/core/modules/aggregator/tests/lib/Drupal/aggregator_test/Plugin/aggregator/processor/TestProcessor.php b/core/modules/aggregator/tests/lib/Drupal/aggregator_test/Plugin/aggregator/processor/TestProcessor.php
index 3ffe213..1b364a2 100644
--- a/core/modules/aggregator/tests/lib/Drupal/aggregator_test/Plugin/aggregator/processor/TestProcessor.php
+++ b/core/modules/aggregator/tests/lib/Drupal/aggregator_test/Plugin/aggregator/processor/TestProcessor.php
@@ -38,7 +38,7 @@ public function settingsForm(array $form, array &$form_state) {
       '#type' => 'details',
       '#title' => t('Test processor settings'),
       '#description' => $info['description'],
-      '#collapsed' => !in_array($info['id'], $processors),
+      '#open' => in_array($info['id'], $processors),
     );
     // Add some dummy settings to verify settingsForm is called.
     $form['processors'][$info['id']]['dummy_length'] = array(
diff --git a/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
index 2c9845e..7e65ffa 100644
--- a/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
+++ b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockFormController.php
@@ -92,7 +92,7 @@ public function form(array $form, array &$form_state, EntityInterface $block) {
       '#title' => t('Revision information'),
       '#collapsible' => TRUE,
       // Collapsed by default when "Create new revision" is unchecked.
-      '#collapsed' => !$block->isNewRevision(),
+      '#open' => $block->isNewRevision(),
       '#group' => 'advanced',
       '#attributes' => array(
         'class' => array('custom-block-form-revision-information'),
diff --git a/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockTypeFormController.php b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockTypeFormController.php
index 77664e9..738df91 100644
--- a/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockTypeFormController.php
+++ b/core/modules/block/custom_block/lib/Drupal/custom_block/CustomBlockTypeFormController.php
@@ -56,8 +56,6 @@ public function form(array $form, array &$form_state, EntityInterface $block_typ
       $form['language'] = array(
         '#type' => 'details',
         '#title' => t('Language settings'),
-        '#collapsible' => TRUE,
-        '#collapsed' => TRUE,
         '#group' => 'additional_settings',
       );
 
diff --git a/core/modules/block/lib/Drupal/block/BlockBase.php b/core/modules/block/lib/Drupal/block/BlockBase.php
index 0909345..0d75242 100644
--- a/core/modules/block/lib/Drupal/block/BlockBase.php
+++ b/core/modules/block/lib/Drupal/block/BlockBase.php
@@ -285,7 +285,6 @@ public function form($form, &$form_state) {
     $form['visibility']['path'] = array(
       '#type' => 'details',
       '#title' => t('Pages'),
-      '#collapsed' => TRUE,
       '#group' => 'visibility',
       '#weight' => 0,
     );
@@ -348,7 +347,6 @@ public function form($form, &$form_state) {
       $form['visibility']['language'] = array(
         '#type' => 'details',
         '#title' => t('Languages'),
-        '#collapsed' => TRUE,
         '#group' => 'visibility',
         '#weight' => 5,
       );
@@ -381,7 +379,6 @@ public function form($form, &$form_state) {
     $form['visibility']['role'] = array(
       '#type' => 'details',
       '#title' => t('Roles'),
-      '#collapsed' => TRUE,
       '#group' => 'visibility',
       '#weight' => 10,
     );
diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index d97dcf1..c28d2fd 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -432,7 +432,6 @@ function _book_add_form_elements(&$form, &$form_state, EntityInterface $node) {
     '#type' => 'details',
     '#title' => t('Book outline'),
     '#weight' => 10,
-    '#collapsed' => TRUE,
     '#group' => 'advanced',
     '#attributes' => array(
       'class' => array('book-outline-form'),
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index 5690c72..799521d 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -1005,7 +1005,6 @@ function comment_form_node_type_form_alter(&$form, $form_state) {
     $form['comment'] = array(
       '#type' => 'details',
       '#title' => t('Comment settings'),
-      '#collapsed' => TRUE,
       '#group' => 'additional_settings',
       '#attributes' => array(
         'class' => array('comment-node-type-settings-form'),
@@ -1111,7 +1110,6 @@ function comment_form_node_form_alter(&$form, $form_state) {
     '#type' => 'details',
     '#access' => user_access('administer comments'),
     '#title' => t('Comment settings'),
-    '#collapsed' => TRUE,
     '#group' => 'advanced',
     '#attributes' => array(
       'class' => array('comment-node-settings-form'),
diff --git a/core/modules/comment/lib/Drupal/comment/CommentFormController.php b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
index a5e5f15..d488f01 100644
--- a/core/modules/comment/lib/Drupal/comment/CommentFormController.php
+++ b/core/modules/comment/lib/Drupal/comment/CommentFormController.php
@@ -53,7 +53,6 @@ public function form(array $form, array &$form_state, EntityInterface $comment)
       $form['author'] += array(
         '#type' => 'details',
         '#title' => t('Administration'),
-        '#collapsed' => TRUE,
       );
     }
 
diff --git a/core/modules/dblog/dblog.admin.inc b/core/modules/dblog/dblog.admin.inc
index d432b5b..e4c5e3d 100644
--- a/core/modules/dblog/dblog.admin.inc
+++ b/core/modules/dblog/dblog.admin.inc
@@ -326,7 +326,7 @@ function dblog_filter_form($form) {
   $form['filters'] = array(
     '#type' => 'details',
     '#title' => t('Filter log messages'),
-    '#collapsed' => empty($_SESSION['dblog_overview_filter']),
+    '#open' => !empty($_SESSION['dblog_overview_filter']),
   );
   foreach ($filters as $key => $filter) {
     $form['filters']['status'][$key] = array(
@@ -403,7 +403,6 @@ function dblog_clear_log_form($form) {
     '#type' => 'details',
     '#title' => t('Clear log messages'),
     '#description' => t('This will permanently remove the log messages from the database.'),
-    '#collapsed' => TRUE,
   );
   $form['dblog_clear']['clear'] = array(
     '#type' => 'submit',
diff --git a/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php b/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php
index c288750..af18a1a 100644
--- a/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php
+++ b/core/modules/field/lib/Drupal/field/Plugin/views/field/Field.php
@@ -464,7 +464,6 @@ function multiple_options_form(&$form, &$form_state) {
     $form['multiple_field_settings'] = array(
       '#type' => 'details',
       '#title' => t('Multiple field settings'),
-      '#collapsed' => TRUE,
       '#weight' => 5,
     );
 
diff --git a/core/modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php b/core/modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php
index 7ee40f4..821824c 100644
--- a/core/modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php
+++ b/core/modules/field_ui/lib/Drupal/field_ui/DisplayOverview.php
@@ -349,7 +349,6 @@ public function buildForm(array $form, array &$form_state) {
         $form['modes'] = array(
           '#type' => 'details',
           '#title' => t('Custom display settings'),
-          '#collapsed' => TRUE,
         );
         // Collect options and default values for the 'Custom display settings'
         // checkboxes.
diff --git a/core/modules/language/language.admin.inc b/core/modules/language/language.admin.inc
index f59fb38..0b726e3 100644
--- a/core/modules/language/language.admin.inc
+++ b/core/modules/language/language.admin.inc
@@ -617,7 +617,6 @@ function language_negotiation_configure_browser_form($form, &$form_state) {
   $form['new_mapping'] = array(
     '#type' => 'details',
     '#title' => t('Add a new mapping'),
-    '#collapsed' => TRUE,
     '#tree' => TRUE,
   );
   $form['new_mapping']['browser_langcode'] = array(
diff --git a/core/modules/locale/locale.bulk.inc b/core/modules/locale/locale.bulk.inc
index 49273d8..14ec0ae 100644
--- a/core/modules/locale/locale.bulk.inc
+++ b/core/modules/locale/locale.bulk.inc
@@ -177,7 +177,6 @@ function locale_translate_export_form($form, &$form_state) {
     $form['content_options'] = array(
       '#type' => 'details',
       '#title' => t('Export options'),
-      '#collapsed' => TRUE,
       '#tree' => TRUE,
       '#states' => array(
         'invisible' => array(
diff --git a/core/modules/locale/locale.pages.inc b/core/modules/locale/locale.pages.inc
index fc68a51..9ac1d94 100644
--- a/core/modules/locale/locale.pages.inc
+++ b/core/modules/locale/locale.pages.inc
@@ -169,7 +169,7 @@ function locale_translate_filter_form($form, &$form_state) {
   $form['filters'] = array(
     '#type' => 'details',
     '#title' => t('Filter translatable strings'),
-    '#collapsed' => FALSE,
+    '#open' => TRUE,
   );
   foreach ($filters as $key => $filter) {
     // Special case for 'string' filter.
diff --git a/core/modules/menu/menu.module b/core/modules/menu/menu.module
index d13655e..adec17d 100644
--- a/core/modules/menu/menu.module
+++ b/core/modules/menu/menu.module
@@ -554,7 +554,7 @@ function menu_form_node_form_alter(&$form, $form_state) {
     '#type' => 'details',
     '#title' => t('Menu settings'),
     '#access' => user_access('administer menu'),
-    '#collapsed' => !$link['link_title'],
+    '#open' => !empty($link['link_title']),
     '#group' => 'advanced',
     '#attached' => array(
       'library' => array(array('menu', 'drupal.menu')),
@@ -654,7 +654,6 @@ function menu_form_node_type_form_alter(&$form, $form_state) {
   $form['menu'] = array(
     '#type' => 'details',
     '#title' => t('Menu settings'),
-    '#collapsed' => TRUE,
     '#attached' => array(
       'library' => array(array('menu', 'drupal.menu.admin')),
     ),
diff --git a/core/modules/node/content_types.inc b/core/modules/node/content_types.inc
index f8b144a..68838b2 100644
--- a/core/modules/node/content_types.inc
+++ b/core/modules/node/content_types.inc
@@ -192,7 +192,6 @@ function node_type_form($form, &$form_state, $type = NULL) {
   $form['workflow'] = array(
     '#type' => 'details',
     '#title' => t('Publishing options'),
-    '#collapsed' => TRUE,
     '#group' => 'additional_settings',
   );
   $form['workflow']['node_options'] = array('#type' => 'checkboxes',
@@ -210,7 +209,6 @@ function node_type_form($form, &$form_state, $type = NULL) {
     $form['language'] = array(
       '#type' => 'details',
       '#title' => t('Language settings'),
-      '#collapsed' => TRUE,
       '#group' => 'additional_settings',
     );
 
@@ -229,7 +227,6 @@ function node_type_form($form, &$form_state, $type = NULL) {
   $form['display'] = array(
     '#type' => 'details',
     '#title' => t('Display settings'),
-    '#collapsed' => TRUE,
     '#group' => 'additional_settings',
   );
   $form['display']['node_submitted'] = array(
diff --git a/core/modules/node/lib/Drupal/node/NodeFormController.php b/core/modules/node/lib/Drupal/node/NodeFormController.php
index 63b4189..436b1de 100644
--- a/core/modules/node/lib/Drupal/node/NodeFormController.php
+++ b/core/modules/node/lib/Drupal/node/NodeFormController.php
@@ -123,7 +123,7 @@ public function form(array $form, array &$form_state, EntityInterface $node) {
       '#group' => 'advanced',
       '#title' => t('Revision information'),
       // Collapsed by default when "Create new revision" is unchecked.
-      '#collapsed' => !$node->isNewRevision(),
+      '#open' => $node->isNewRevision(),
       '#attributes' => array(
         'class' => array('node-form-revision-information'),
       ),
@@ -159,7 +159,6 @@ public function form(array $form, array &$form_state, EntityInterface $node) {
       '#type' => 'details',
       '#access' => user_access('administer nodes'),
       '#title' => t('Authoring information'),
-      '#collapsed' => TRUE,
       '#group' => 'advanced',
       '#attributes' => array(
         'class' => array('node-form-author'),
@@ -198,7 +197,6 @@ public function form(array $form, array &$form_state, EntityInterface $node) {
       '#type' => 'details',
       '#access' => user_access('administer nodes'),
       '#title' => t('Promotion options'),
-      '#collapsed' => TRUE,
       '#group' => 'advanced',
       '#attributes' => array(
         'class' => array('node-form-options'),
diff --git a/core/modules/node/node.module b/core/modules/node/node.module
index f663387..9264afa 100644
--- a/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -2001,7 +2001,6 @@ function node_form_block_form_alter(&$form, &$form_state) {
   $form['visibility']['node_type'] = array(
     '#type' => 'details',
     '#title' => t('Content types'),
-    '#collapsed' => TRUE,
     '#group' => 'visibility',
     '#weight' => 5,
   );
@@ -2277,7 +2276,6 @@ function node_form_search_form_alter(&$form, $form_state) {
     $form['advanced'] = array(
       '#type' => 'details',
       '#title' => t('Advanced search'),
-      '#collapsed' => TRUE,
       '#attributes' => array('class' => array('search-advanced')),
     );
     $form['advanced']['keywords-fieldset'] = array(
@@ -2338,10 +2336,9 @@ function node_form_search_form_alter(&$form, $form_state) {
     }
     if (count($language_options) > 1) {
       $form['advanced']['lang-fieldset'] = array(
-        '#type' => 'fieldset',
+        '#type' => 'details',
         '#title' => t('Languages'),
-        '#collapsible' => FALSE,
-        '#collapsed' => FALSE,
+        '#open' => TRUE,
       );
       $form['advanced']['lang-fieldset']['language'] = array(
         '#type' => 'checkboxes',
diff --git a/core/modules/path/path.module b/core/modules/path/path.module
index cee17ed..b8ec625 100644
--- a/core/modules/path/path.module
+++ b/core/modules/path/path.module
@@ -118,7 +118,7 @@ function path_form_node_form_alter(&$form, $form_state) {
   $form['path'] = array(
     '#type' => 'details',
     '#title' => t('URL path settings'),
-    '#collapsed' => empty($path['alias']),
+    '#open' => !empty($path['alias']),
     '#group' => 'advanced',
     '#attributes' => array(
       'class' => array('path-form'),
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestSettingsForm.php b/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestSettingsForm.php
index cce1a8a..917b935 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestSettingsForm.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/Form/SimpletestSettingsForm.php
@@ -46,7 +46,6 @@ public function buildForm(array $form, array &$form_state) {
       '#type' => 'details',
       '#title' => t('HTTP authentication'),
       '#description' => t('HTTP auth settings to be used by the SimpleTest browser during testing. Useful when the site requires basic HTTP authentication.'),
-      '#collapsed' => TRUE,
     );
     $form['httpauth']['simpletest_httpauth_method'] = array(
       '#type' => 'select',
diff --git a/core/modules/simpletest/simpletest.pages.inc b/core/modules/simpletest/simpletest.pages.inc
index 4fb908d..e5f8590 100644
--- a/core/modules/simpletest/simpletest.pages.inc
+++ b/core/modules/simpletest/simpletest.pages.inc
@@ -25,10 +25,6 @@ function simpletest_test_form($form, &$form_state) {
   $form_state['storage']['PHPUnit'] = $groups['PHPUnit'];
 
   foreach ($groups as $group => $tests) {
-    $form['tests']['table'][$group] = array(
-      '#collapsed' => TRUE,
-    );
-
     foreach ($tests as $class => $info) {
       $form['tests']['table'][$group][$class] = array(
         '#type' => 'checkbox',
@@ -98,8 +94,8 @@ function theme_simpletest_test_table($variables) {
 
     // Select the right "expand"/"collapse" image, depending on whether the
     // category is expanded (at least one test selected) or not.
-    $collapsed = !empty($element['#collapsed']);
-    $image_index = $collapsed ? 0 : 1;
+    $open = !empty($element['#open']);
+    $image_index = (int) $open;
 
     // Place-holder for checkboxes to select group of tests.
     $row[] = array('id' => $test_class, 'class' => array('simpletest-select-all'));
@@ -159,7 +155,7 @@ function theme_simpletest_test_table($variables) {
         'class' => array('simpletest-test-description'),
       );
 
-      $rows[] = array('data' => $row, 'class' => array($test_class . '-test', ($collapsed ? 'js-hide' : '')));
+      $rows[] = array('data' => $row, 'class' => array($test_class . '-test', ($open ? '' : 'js-hide')));
     }
     $js['simpletest-test-group-' . $test_class] = $current_js;
     unset($table[$key]);
@@ -281,7 +277,7 @@ function simpletest_result_form($form, &$form_state, $test_id) {
 
     // Set summary information.
     $group_summary['#ok'] = $group_summary['#fail'] + $group_summary['#exception'] == 0;
-    $form['result']['results'][$group]['#collapsed'] = $group_summary['#ok'];
+    $form['result']['results'][$group]['#open'] = !$group_summary['#ok'];
 
     // Store test group (class) as for use in filter.
     $filter[$group_summary['#ok'] ? 'pass' : 'fail'][] = $group;
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index e0be14f..4327c34df 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -1858,10 +1858,10 @@ function system_image_toolkit_settings($form, &$form_state) {
     foreach ($toolkits_available as $id => $definition) {
       $toolkit = $manager->createInstance($id);
       $form['image_toolkit_settings'][$id] = array(
-        '#type' => 'fieldset',
+        '#type' => 'details',
         '#title' => t('@toolkit settings', array('@toolkit' => $definition['title'])),
         '#collapsible' => TRUE,
-        '#collapsed' => ($id == $current_toolkit) ? FALSE : TRUE,
+        '#open' => ($id == $current_toolkit),
         '#tree' => TRUE,
       );
       $form['image_toolkit_settings'][$id] += $toolkit->settingsForm();
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index eee53b1..5a70217 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -554,7 +554,7 @@ function system_element_info() {
     '#theme_wrappers' => array('fieldset'),
   );
   $types['details'] = array(
-    '#collapsed' => FALSE,
+    '#open' => FALSE,
     '#value' => NULL,
     '#process' => array('form_process_group', 'ajax_process_form'),
     '#pre_render' => array('form_pre_render_details', 'form_pre_render_group'),
diff --git a/core/modules/system/tests/modules/design_test/form/details.inc b/core/modules/system/tests/modules/design_test/form/details.inc
index 60d153a..7925de0 100644
--- a/core/modules/system/tests/modules/design_test/form/details.inc
+++ b/core/modules/system/tests/modules/design_test/form/details.inc
@@ -13,7 +13,7 @@ function design_test_form_details($form, &$form_state) {
     '#type' => 'details',
     '#title' => 'Collapsible details',
     '#description' => 'Details description',
-    '#collapsed' => FALSE,
+    '#open' => TRUE,
     '#states' => array(
       'collapsed' => array(
         ':input[name="states-trigger"]' => array('checked' => TRUE),
@@ -25,7 +25,6 @@ function design_test_form_details($form, &$form_state) {
     '#type' => 'details',
     '#title' => 'Collapsed details',
     '#description' => 'Details description',
-    '#collapsed' => TRUE,
   );
   $form['collapsed']['textfield'] = array(
     '#type' => 'textfield',
@@ -46,19 +45,17 @@ function design_test_form_details($form, &$form_state) {
     '#type' => 'details',
     '#title' => 'Details',
     '#description' => 'Details description',
-    '#collapsed' => TRUE,
   );
   $form['collapsed2']['collapsible'] = array(
     '#type' => 'details',
     '#title' => 'Collapsible details',
     '#description' => 'Details description',
-    '#collapsed' => FALSE,
+    '#open' => TRUE,
   );
   $form['collapsed2']['collapsed'] = array(
     '#type' => 'details',
     '#title' => 'Collapsed details',
     '#description' => 'Details description',
-    '#collapsed' => TRUE,
   );
   $form['collapsed2']['regular'] = array(
     '#type' => 'details',
diff --git a/core/modules/system/tests/modules/design_test/form/fieldset.inc b/core/modules/system/tests/modules/design_test/form/fieldset.inc
index f55bfe5..8b57dde 100644
--- a/core/modules/system/tests/modules/design_test/form/fieldset.inc
+++ b/core/modules/system/tests/modules/design_test/form/fieldset.inc
@@ -19,7 +19,7 @@ function design_test_form_fieldset($form, &$form_state) {
     '#type' => 'details',
     '#title' => 'Collapsible details',
     '#description' => 'Details description',
-    '#collapsed' => FALSE,
+    '#open' => TRUE,
   );
   $form['collapsible']['fieldset'] = array(
     '#type' => 'fieldset',
@@ -40,7 +40,7 @@ function design_test_form_fieldset($form, &$form_state) {
 
   $form['collapsed'] = $form['collapsible'];
   $form['collapsed']['#title'] = 'Collapsed details';
-  $form['collapsed']['#collapsed'] = TRUE;
+  $form['collapsed']['#open'] = FALSE;
 
   $form['nested'] = $form['regular'];
   $form['nested']['#title'] = 'Parent fieldset';
diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/TermFormController.php b/core/modules/taxonomy/lib/Drupal/taxonomy/TermFormController.php
index 3735ad6..7a780f7 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/TermFormController.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/TermFormController.php
@@ -53,7 +53,7 @@ public function form(array $form, array &$form_state, EntityInterface $term) {
     $form['relations'] = array(
       '#type' => 'details',
       '#title' => t('Relations'),
-      '#collapsed' => ($vocabulary->hierarchy != TAXONOMY_HIERARCHY_MULTIPLE),
+      '#open' => ($vocabulary->hierarchy == TAXONOMY_HIERARCHY_MULTIPLE),
       '#weight' => 10,
     );
 
diff --git a/core/modules/translation/translation.module b/core/modules/translation/translation.module
index 41d8e83..55e0937 100644
--- a/core/modules/translation/translation.module
+++ b/core/modules/translation/translation.module
@@ -215,7 +215,7 @@ function translation_form_node_form_alter(&$form, &$form_state) {
         '#type' => 'details',
         '#title' => t('Translation settings'),
         '#access' => translation_user_can_translate_node($node),
-        '#collapsed' => !$node->translate,
+        '#open' => $node->translate,
         '#tree' => TRUE,
         '#weight' => 30,
       );
diff --git a/core/modules/translation_entity/lib/Drupal/translation_entity/EntityTranslationController.php b/core/modules/translation_entity/lib/Drupal/translation_entity/EntityTranslationController.php
index ce9d3a0..5eb42a1 100644
--- a/core/modules/translation_entity/lib/Drupal/translation_entity/EntityTranslationController.php
+++ b/core/modules/translation_entity/lib/Drupal/translation_entity/EntityTranslationController.php
@@ -156,7 +156,6 @@ public function entityFormAlter(array &$form, array &$form_state, EntityInterfac
       $form['source_langcode'] = array(
         '#type' => 'details',
         '#title' => t('Source language: @language', array('@language' => $languages[$source_langcode]->name)),
-        '#collapsed' => TRUE,
         '#tree' => TRUE,
         '#weight' => -100,
         '#multilingual' => TRUE,
@@ -224,7 +223,6 @@ public function entityFormAlter(array &$form, array &$form_state, EntityInterfac
       $form['translation_entity'] = array(
         '#type' => 'details',
         '#title' => t('Translation'),
-        '#collapsed' => TRUE,
         '#tree' => TRUE,
         '#weight' => 10,
         '#access' => $this->getTranslationAccess($entity, $source_langcode ? 'create' : 'update'),
diff --git a/core/modules/user/lib/Drupal/user/AccountSettingsForm.php b/core/modules/user/lib/Drupal/user/AccountSettingsForm.php
index b3d0d17..0cabd0b 100644
--- a/core/modules/user/lib/Drupal/user/AccountSettingsForm.php
+++ b/core/modules/user/lib/Drupal/user/AccountSettingsForm.php
@@ -182,7 +182,7 @@ public function buildForm(array $form, array &$form_state) {
     $form['email_admin_created'] = array(
       '#type' => 'details',
       '#title' => t('Welcome (new user created by administrator)'),
-      '#collapsed' => ($config->get('register') != USER_REGISTER_ADMINISTRATORS_ONLY),
+      '#open' => ($config->get('register') == USER_REGISTER_ADMINISTRATORS_ONLY),
       '#description' => t('Edit the welcome e-mail messages sent to new member accounts created by an administrator.') . ' ' . $email_token_help,
       '#group' => 'email',
     );
@@ -202,7 +202,7 @@ public function buildForm(array $form, array &$form_state) {
     $form['email_pending_approval'] = array(
       '#type' => 'details',
       '#title' => t('Welcome (awaiting approval)'),
-      '#collapsed' => ($config->get('register') != USER_REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL),
+      '#open' => ($config->get('register') == USER_REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL),
       '#description' => t('Edit the welcome e-mail messages sent to new members upon registering, when administrative approval is required.') . ' ' . $email_token_help,
       '#group' => 'email',
     );
@@ -222,7 +222,7 @@ public function buildForm(array $form, array &$form_state) {
     $form['email_pending_approval_admin'] = array(
       '#type' => 'details',
       '#title' => t('Admin (user awaiting approval)'),
-      '#collapsed' => ($config->get('register') != USER_REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL),
+      '#open' => ($config->get('register') == USER_REGISTER_VISITORS_ADMINISTRATIVE_APPROVAL),
       '#description' => t('Edit the e-mail notifying the site administrator that there are new members awaiting administrative approval.') . ' ' . $email_token_help,
       '#group' => 'email',
     );
@@ -242,7 +242,7 @@ public function buildForm(array $form, array &$form_state) {
     $form['email_no_approval_required'] = array(
       '#type' => 'details',
       '#title' => t('Welcome (no approval required)'),
-      '#collapsed' => ($config->get('register') != USER_REGISTER_VISITORS),
+      '#open' => ($config->get('register') == USER_REGISTER_VISITORS),
       '#description' => t('Edit the welcome e-mail messages sent to new members upon registering, when no administrator approval is required.') . ' ' . $email_token_help,
       '#group' => 'email',
     );
@@ -262,7 +262,6 @@ public function buildForm(array $form, array &$form_state) {
     $form['email_password_reset'] = array(
       '#type' => 'details',
       '#title' => t('Password recovery'),
-      '#collapsed' => TRUE,
       '#description' => t('Edit the e-mail messages sent to users who request a new password.') . ' ' . $email_token_help,
       '#group' => 'email',
       '#weight' => 10,
@@ -283,7 +282,6 @@ public function buildForm(array $form, array &$form_state) {
     $form['email_activated'] = array(
       '#type' => 'details',
       '#title' => t('Account activation'),
-      '#collapsed' => TRUE,
       '#description' => t('Enable and edit e-mail messages sent to users upon account activation (when an administrator activates an account of a user who has already registered, on a site where administrative approval is required).') . ' ' . $email_token_help,
       '#group' => 'email',
     );
@@ -317,7 +315,6 @@ public function buildForm(array $form, array &$form_state) {
     $form['email_blocked'] = array(
       '#type' => 'details',
       '#title' => t('Account blocked'),
-      '#collapsed' => TRUE,
       '#description' => t('Enable and edit e-mail messages sent to users when their accounts are blocked.') . ' ' . $email_token_help,
       '#group' => 'email',
     );
@@ -351,7 +348,6 @@ public function buildForm(array $form, array &$form_state) {
     $form['email_cancel_confirm'] = array(
       '#type' => 'details',
       '#title' => t('Account cancellation confirmation'),
-      '#collapsed' => TRUE,
       '#description' => t('Edit the e-mail messages sent to users when they attempt to cancel their accounts.') . ' ' . $email_token_help,
       '#group' => 'email',
     );
@@ -371,7 +367,6 @@ public function buildForm(array $form, array &$form_state) {
     $form['email_canceled'] = array(
       '#type' => 'details',
       '#title' => t('Account canceled'),
-      '#collapsed' => TRUE,
       '#description' => t('Enable and edit e-mail messages sent to users when their accounts are canceled.') . ' ' . $email_token_help,
       '#group' => 'email',
     );
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php
index 3b88f06..5d79431 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/HandlerBase.php
@@ -291,7 +291,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['more'] = array(
       '#type' => 'details',
       '#title' => t('More'),
-      '#collapsed' => TRUE,
       '#weight' => 150,
     );
     // Allow to alter the default values brought into the form.
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
index 2c36002..248cd2d 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/PluginBase.php
@@ -320,7 +320,6 @@ public function globalTokenForm(&$form, &$form_state) {
     $form['global_tokens'] = array(
       '#type' => 'fieldset',
       '#title' => t('Available global token replacements'),
-      '#collapsed' => TRUE,
     );
     $form['global_tokens']['list'] = array(
       '#theme' => 'item_list',
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/area/AreaPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/area/AreaPluginBase.php
index a8cfb8a..e2e1cd5 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/area/AreaPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/area/AreaPluginBase.php
@@ -107,7 +107,6 @@ public function tokenForm(&$form, &$form_state) {
       $form['tokens'] = array(
         '#type' => 'details',
         '#title' => t('Replacement patterns'),
-        '#collapsed' => TRUE,
         '#id' => 'edit-options-token-help',
         '#states' => array(
           'visible' => array(
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php
index 117ade9..83c9347 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/argument/ArgumentPluginBase.php
@@ -193,7 +193,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['exception'] = array(
       '#type' => 'details',
       '#title' => t('Exceptions'),
-      '#collapsed' => TRUE,
       '#fieldset' => 'no_argument',
     );
     $form['exception']['value'] = array(
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php
index bc8dfff..f59aacc 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/field/FieldPluginBase.php
@@ -535,7 +535,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['style_settings'] = array(
       '#type' => 'details',
       '#title' => t('Style settings'),
-      '#collapsed' => TRUE,
       '#weight' => 99,
     );
 
@@ -684,7 +683,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['alter'] = array(
       '#title' => t('Rewrite results'),
       '#type' => 'details',
-      '#collapsed' => TRUE,
       '#weight' => 100,
     );
 
@@ -885,7 +883,6 @@ public function buildOptionsForm(&$form, &$form_state) {
       $form['alter']['help'] = array(
         '#type' => 'details',
         '#title' => t('Replacement patterns'),
-        '#collapsed' => TRUE,
         '#value' => $output,
         '#states' => array(
           'visible' => array(
@@ -1024,7 +1021,6 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['empty_field_behavior'] = array(
       '#type' => 'details',
       '#title' => t('No results behavior'),
-      '#collapsed' => TRUE,
       '#weight' => 100,
     );
 
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/pager/SqlBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/pager/SqlBase.php
index f96a938..2026be8 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/pager/SqlBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/pager/SqlBase.php
@@ -75,7 +75,7 @@ public function buildOptionsForm(&$form, &$form_state) {
 
     $form['tags'] = array(
       '#type' => 'details',
-      '#collapsed' => FALSE,
+      '#open' => TRUE,
       '#tree' => TRUE,
       '#title' => t('Pager link labels'),
       '#input' => TRUE,
@@ -95,7 +95,7 @@ public function buildOptionsForm(&$form, &$form_state) {
 
     $form['expose'] = array(
       '#type' => 'details',
-      '#collapsed' => FALSE,
+      '#open' => TRUE,
       '#tree' => TRUE,
       '#title' => t('Exposed options'),
       '#input' => TRUE,
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/row/RssFields.php b/core/modules/views/lib/Drupal/views/Plugin/views/row/RssFields.php
index 67b1004..a5bd242 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/row/RssFields.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/row/RssFields.php
@@ -92,7 +92,7 @@ public function buildOptionsForm(&$form, &$form_state) {
     $form['guid_field_options'] = array(
       '#type' => 'details',
       '#title' => t('GUID settings'),
-      '#collapsed' => FALSE,
+      '#open' => TRUE,
     );
     $form['guid_field_options']['guid_field'] = array(
       '#type' => 'select',
diff --git a/core/modules/views/views_ui/lib/Drupal/views_ui/ViewEditFormController.php b/core/modules/views/views_ui/lib/Drupal/views_ui/ViewEditFormController.php
index ade339a..461078a 100644
--- a/core/modules/views/views_ui/lib/Drupal/views_ui/ViewEditFormController.php
+++ b/core/modules/views/views_ui/lib/Drupal/views_ui/ViewEditFormController.php
@@ -472,7 +472,6 @@ public function getDisplayDetails($view, $display) {
     $build['columns']['third'] = array(
       '#type' => 'details',
       '#title' => t('Advanced'),
-      '#collapsed' => TRUE,
       '#theme_wrappers' => array('details', 'container'),
       '#attributes' => array(
         'class' => array(
@@ -483,9 +482,7 @@ public function getDisplayDetails($view, $display) {
     );
 
     // Collapse the details by default.
-    if (config('views.settings')->get('ui.show.advanced_column')) {
-      $build['columns']['third']['#collapsed'] = FALSE;
-    }
+    $build['columns']['third']['#open'] = config('views.settings')->get('ui.show.advanced_column');
 
     // Each option (e.g. title, access, display as grid/table/list) fits into one
     // of several "buckets," or boxes (Format, Fields, Sort, and so on).
diff --git a/core/update.php b/core/update.php
index 4947d09..3144c1b 100644
--- a/core/update.php
+++ b/core/update.php
@@ -67,7 +67,6 @@ function update_script_selection_form($form, &$form_state) {
   $form['start'] = array(
     '#tree' => TRUE,
     '#type' => 'details',
-    '#collapsed' => TRUE,
   );
 
   // Ensure system.module's updates appear first.