diff --git a/core/modules/toolbar/js/toolbar.js b/core/modules/toolbar/js/toolbar.js
index 47d0899..51b1b96 100644
--- a/core/modules/toolbar/js/toolbar.js
+++ b/core/modules/toolbar/js/toolbar.js
@@ -73,6 +73,8 @@ Drupal.behaviors.toolbar = {
       Drupal.toolbar.setSubtrees.done(function (subtrees) {
         menuModel.set('subtrees', subtrees);
         localStorage.setItem('Drupal.toolbar.subtrees', JSON.stringify(subtrees));
+        // Indicate on the toolbarModel that subtrees are now loaded.
+        model.set('areSubtreesLoaded', true);
       });
 
       // Attach a listener to the configured media query breakpoints.
@@ -333,7 +335,18 @@ Drupal.toolbar = {
       this.updateTabs();
       this.updateTrayOrientation();
       this.updateBarAttributes();
-      // Load the subtrees if the toolbar tray is in a vertical orientation.
+      // Load the subtrees if the orientation of the toolbar is changed to
+      // vertical. This condition responds to the case that the toolbar switches
+      // from horizontal to vertical orientation. The toolbar starts in a
+      // vertical orientation by default and then switches to horizontal during
+      // initialization if the media query conditions are met. Simply checking
+      // that the orientation is vertical here would result in the subtrees
+      // always being loaded, even when the toolbar initialization ultimately
+      // results in a horizontal orientation.
+      //
+      // @see Drupal.behaviors.toolbar.attach() where admin menu subtrees
+      // loading is invoked during initialization after media query conditions
+      // have been processed.
       if (this.model.changed.orientation === 'vertical' || this.model.changed.activeTab) {
         this.loadSubtrees();
       }
@@ -506,15 +519,22 @@ Drupal.toolbar = {
 
     /**
      * Calls the endpoint URI that will return rendered subtrees with JSONP.
+     *
+     * The rendered admin menu subtrees HTML is cached on the client in
+     * localStorage until the cache of the admin menu subtrees on the server-
+     * side is invalidated. The subtreesHash is stored in localStorage as well
+     * and compared to the subtreesHash in drupalSettings to determine when the
+     * admin menu subtrees cache has been invalidated.
      */
     loadSubtrees: function () {
       var $activeTab = $(this.model.get('activeTab'));
       var orientation = this.model.get('orientation');
       // Only load and render the admin menu subtrees if:
       //   (1) They have not been loaded yet.
-      //   (2) The active tab is the administration menu tab.
+      //   (2) The active tab is the administration menu tab, indicated by the
+      //       presence of the data-drupal-subtrees attribute.
       //   (3) The orientation of the tray is vertical.
-      if (!this.model.get('areSubtreesLoaded') && $activeTab.data('drupal-defer-subtrees') !== undefined && orientation === 'vertical') {
+      if (!this.model.get('areSubtreesLoaded') && $activeTab.data('drupal-subtrees') !== undefined && orientation === 'vertical') {
         var subtreesHash = drupalSettings.toolbar.subtreesHash;
         var endpoint = Drupal.url('toolbar/subtrees/' + subtreesHash);
         var cachedSubtreesHash = localStorage.getItem('Drupal.toolbar.subtreesHash');
@@ -524,7 +544,6 @@ Drupal.toolbar = {
         // changed, then use the cached data.
         if (isVertical && subtreesHash === cachedSubtreesHash && cachedSubtrees) {
           Drupal.toolbar.setSubtrees.resolve(cachedSubtrees);
-          this.model.set('areSubtreesLoaded', true);
         }
         // Only make the call to get the subtrees if the orientation of the
         // toolbar is vertical.
@@ -537,7 +556,6 @@ Drupal.toolbar = {
           $.ajax(endpoint);
           // Cache the hash for the subtrees locally.
           localStorage.setItem('Drupal.toolbar.subtreesHash', subtreesHash);
-          this.model.set('areSubtreesLoaded', true);
         }
       }
     }
diff --git a/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
index 8a34936..9ef5d54 100644
--- a/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -463,9 +463,12 @@ function toolbar_toolbar() {
         'attributes' => array(
           'title' => t('Admin menu'),
           'class' => array('toolbar-icon', 'toolbar-icon-menu'),
-          // A marker that indicates to the client to defer loading of the admin
-          // menu subtrees until this tab is activated.
-          'data-drupal-defer-subtrees' => '',
+          // A data marker that indicates to the client to defer loading of the
+          // admin menu subtrees until this tab is activated. Admin menu
+          // subtrees will not render to the DOM if this attribute is removed.
+          // The value of the attribute is intentionally left blank. Only the
+          // presence of the attribute is necessary.
+          'data-drupal-subtrees' => '',
         ),
       ),
     ),
@@ -628,7 +631,7 @@ function _toolbar_get_subtrees_hash() {
     // Cache using a tag 'user' so that we can invalidate all user-specific
     // caches later, based on the user's ID regardless of language.
     // Clear the cache when the 'locale' tag is deleted. This ensures a fresh
-    // subtrees rendering if when string translations are made.
+    // subtrees rendering when string translations are made.
     cache('toolbar')->set($cid, $hash, CacheBackendInterface::CACHE_PERMANENT, array('user' => array($uid), 'locale' => TRUE,));
   }
   return $hash;