diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 3ec95da..05319e9 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -639,7 +639,7 @@ function drupal_valid_test_ua($new_prefix = NULL) {
     // The file properties add more entropy not easily accessible to others.
     $key = $private_key . filectime(__FILE__) . fileinode(__FILE__);
     $test_hmac = Crypt::hmacBase64($check_string, $key);
-    // Check that the HMAC match.
+    // Check that the HMAC matches.
     if ($hmac === $test_hmac) {
       $test_prefix = $prefix;
     }
@@ -671,7 +671,7 @@ function drupal_generate_test_ua($prefix) {
     else {
       // Generate and save a new hash salt for a test run.
       // Consumed by drupal_valid_test_ua() before settings.php is loaded.
-      $private_key = Crypt::randomBytesBase64(55);
+      $private_key = Crypt::randomBytesBase64(110);
       file_put_contents($key_file, $private_key);
     }
     // The file properties add more entropy not easily accessible to others.
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Core/Session/SessionTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Core/Session/SessionTest.php
index 718b6d2..1785aee 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/Core/Session/SessionTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Core/Session/SessionTest.php
@@ -3,7 +3,6 @@
 namespace Drupal\FunctionalJavascriptTests\Core\Session;
 
 use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
-use Drupal\user\Entity\User;
 
 /**
  * Tests that sessions don't expire.
@@ -18,27 +17,25 @@ class SessionTest extends JavascriptTestBase {
   public static $modules = ['user', 'views', 'node'];
 
   /**
-   * @var User
-   * The current logged in user.
-   */
-  protected $user;
-
-  /**
    * {@inheritdoc}
    */
   protected function setUp() {
     parent::setUp();
 
-    $this->user = $this->drupalCreateUser([
+    $user = $this->drupalCreateUser([
       'administer site configuration',
       'access administration pages',
       'access content overview',
     ]);
-    $this->drupalLogin($this->user);
+    $this->drupalLogin($user);
   }
 
   /**
    * Tests that the session doesn't expire.
+   *
+   * Makes sure that drupal_valid_test_ua() works for multiple requests
+   * performed by the Mink browser. The SIMPLETEST_USER_AGENT cookie must always
+   * be valid.
    */
   public function testSessionExpiration() {
     // Visit the admin page and click on Title sort 15 times.