diff --git a/core/misc/collapse.js b/core/misc/collapse.js
index 767325e..49dee6f 100644
--- a/core/misc/collapse.js
+++ b/core/misc/collapse.js
@@ -140,6 +140,34 @@
     }
   };
 
+  /**
+   * Reveal all children of a grouping element when a child gets the focus.
+   *
+   * @type {Drupal~behavior}
+   *
+   * @prop {Drupal~behaviorAttach} attach
+   *   Attaches behavior for the details element.
+   */
+  Drupal.behaviors.revealChildrenOfGroupedElements = {
+    attach: function (context) {
+      $(context).once('revealChildrenOfGroupedElements').on('click', 'a[href^="#"]', function(event) {
+        var $item = $(event.target.hash);
+        if ($item.length) {
+
+          // Keep aria attributes in sync as per detailsAria behavior.
+          $item.parents('details').attr('open', true).find('> summary').attr({
+            'aria-expanded': true,
+            'aria-pressed': true
+          });
+
+          $item.parents('.vertical-tabs__pane').each(function(index, pane) {
+            $(pane).data('verticalTab').focus();
+          });
+        }
+      });
+    }
+  };
+
   // Expose constructor in the public space.
   Drupal.CollapsibleDetails = CollapsibleDetails;