diff --git a/core/includes/common.inc b/core/includes/common.inc
index 0fb232e..b9e3ca4 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -2216,7 +2216,8 @@ function drupal_add_js($data = NULL, $options = NULL) {
           url('', array('script' => &$scriptPath, 'prefix' => &$pathPrefix));
           $current_path = current_path();
           $current_path_is_admin = FALSE;
-          if (!defined('MAINTENANCE_MODE') || MAINTENANCE_MODE !== 'update') {
+          // The function path_is_admin() is not available on update.php pages.
+          if (!(defined('MAINTENANCE_MODE') && MAINTENANCE_MODE === 'update')) {
             $current_path_is_admin = path_is_admin($current_path);
           }
           $javascript['settings']['data'][] = array(
diff --git a/core/modules/block/lib/Drupal/block/Controller/BlockController.php b/core/modules/block/lib/Drupal/block/Controller/BlockController.php
index 0536022..f2ebb41 100644
--- a/core/modules/block/lib/Drupal/block/Controller/BlockController.php
+++ b/core/modules/block/lib/Drupal/block/Controller/BlockController.php
@@ -32,7 +32,10 @@ public function demo($theme) {
       '#attached' => array(
         'js' => array(
           array(
-            // Let JavaScript know this really is an admin page.
+            // The block demonstration page is not marked as an administrative
+            // page by path_is_admin() function in order to use the frontend
+            // theme. Since JavaScript relies on a proper separation of admin
+            // pages, it needs to know this is an actual administrative page.
             'data' => array('currentPathIsAdmin' => TRUE),
             'type' => 'setting',
           )
diff --git a/core/modules/toolbar/css/toolbar.icons.css b/core/modules/toolbar/css/toolbar.icons.css
index a0cfce2..0d265d6 100644
--- a/core/modules/toolbar/css/toolbar.icons.css
+++ b/core/modules/toolbar/css/toolbar.icons.css
@@ -319,16 +319,16 @@
   background-image: url("../../../misc/icons/787878/twistie-up.png");
   background-size: auto auto;
 }
-.toolbar .toolbar-icon-escape:before {
+.toolbar .toolbar-icon-escape-admin:before {
   background-image: url("../../../misc/icons/bebebe/chevron-disc-left.svg");
 }
-.no-svg .toolbar .toolbar-icon-escape:before {
+.no-svg .toolbar .toolbar-icon-escape-admin:before {
   background-image: url("../../../misc/icons/bebebe/chevron-disc-left.png");
 }
-[dir="rtl"] .toolbar .toolbar-icon-escape:before {
+[dir="rtl"] .toolbar .toolbar-icon-escape-admin:before {
   background-image: url("../../../misc/icons/bebebe/chevron-disc-right.svg");
 }
-[dir="rtl"] .no-svg .toolbar .toolbar-icon-escape:before {
+[dir="rtl"] .no-svg .toolbar .toolbar-icon-escape-admin:before {
   background-image: url("../../../misc/icons/bebebe/chevron-disc-right.png");
 }
 /**
diff --git a/core/modules/toolbar/js/escapeAdmin.js b/core/modules/toolbar/js/escapeAdmin.js
index 4343ce1..eb580fe 100644
--- a/core/modules/toolbar/js/escapeAdmin.js
+++ b/core/modules/toolbar/js/escapeAdmin.js
@@ -1,36 +1,49 @@
+/**
+ * @file
+ *
+ * Replaces the home link in toolbar with a back to site link.
+ */
 (function ($, Drupal, drupalSettings) {
 
-  "use strict";
+"use strict";
 
-  var escapeAdminPage = sessionStorage.getItem('escapeAdminPage');
+var escapeAdminPath = sessionStorage.getItem('escapeAdminPath');
 
-  if (!drupalSettings.currentPathIsAdmin) {
-    sessionStorage.setItem('escapeAdminPage', drupalSettings.currentPath);
-  }
+// Saves the last non-administrative page in the browser to be able to link back
+// to it when browsing administrative pages.
+if (!drupalSettings.currentPathIsAdmin) {
+  sessionStorage.setItem('escapeAdminPath', drupalSettings.currentPath);
+}
 
-  Drupal.behaviors.escapeAdmin = {
-    attach: function () {
-      var $toolbarEscape = $('[data-toolbar-escape]').once('escapeAdmin');
-      if ($toolbarEscape.length) {
-        if (drupalSettings.currentPathIsAdmin && escapeAdminPage) {
-          $toolbarEscape
-            .removeClass('toolbar-icon-home')
-            .addClass('toolbar-icon-escape')
-            .attr({
-              'href': Drupal.url(escapeAdminPage),
-              'title': Drupal.t('Return to site content')
-            })
-            .text(Drupal.t('Back to site'));
+/**
+ * Replaces the Home link with Back to site link.
+ *
+ * Back to site link points to the last non-administrative page the user visited
+ * withing the same browser tab.
+ */
+Drupal.behaviors.escapeAdmin = {
+  attach: function () {
+    var $toolbarEscape = $('[data-toolbar-escape-admin]').once('escapeAdmin');
+    if ($toolbarEscape.length) {
+      if (drupalSettings.currentPathIsAdmin && escapeAdminPath) {
+        $toolbarEscape
+          .removeClass('toolbar-icon-home')
+          .addClass('toolbar-icon-escape-admin')
+          .attr({
+            'href': Drupal.url(escapeAdminPath),
+            'title': Drupal.t('Return to site content')
+          })
+          .text(Drupal.t('Back to site'));
 
-          // Escape admin when hitting Esc key.
-          $(window).on('keydown', function (event) {
-            if (event.keyCode === 27) {
-              window.location = Drupal.url(escapeAdminPage);
-            }
-          });
-        }
+        // Escape admin when hitting Esc key.
+        $(window).on('keydown', function (event) {
+          if (event.keyCode === 27) {
+            window.location = Drupal.url(escapeAdminPath);
+          }
+        });
       }
     }
-  };
+  }
+};
 
 })(jQuery, Drupal, drupalSettings);
diff --git a/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
index 56f0012..bafda79 100644
--- a/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -416,7 +416,7 @@ function toolbar_toolbar() {
         'attributes' => array(
           'title' => t('Home page'),
           'class' => array('toolbar-icon', 'toolbar-icon-home'),
-          'data-toolbar-escape' => TRUE,
+          'data-toolbar-escape-admin' => TRUE,
         ),
       ),
     ),