diff --git a/core/modules/user/src/Plugin/Block/UserLoginBlock.php b/core/modules/user/src/Plugin/Block/UserLoginBlock.php
index 6709454..a9341f9 100644
--- a/core/modules/user/src/Plugin/Block/UserLoginBlock.php
+++ b/core/modules/user/src/Plugin/Block/UserLoginBlock.php
@@ -100,6 +100,9 @@ public function build() {
     // https://www.drupal.org/node/2562341. The placholder uses a fixed string
     // that is
     // Crypt::hashBase64('\Drupal\user\Plugin\Block\UserLoginBlock::build');
+    // This is based on the implementation in
+    // \Drupal\Core\Form\FormBuilder::prepareForm(), but the user login block
+    // requires different behavior for the destination query argument.
     $placeholder = 'form_action_p_4r8ITd22yaUvXM6SzwrSe9rnQWe48hz9k1Sxto3pBvE';
 
     $form['#attached']['placeholders'][$placeholder] = [
@@ -145,6 +148,8 @@ public function build() {
    *
    * @return array
    *   A renderable array representing the form action.
+   *
+   * @see \Drupal\Core\Form\FormBuilder::renderPlaceholderFormAction()
    */
   public static function renderPlaceholderFormAction() {
     return [
diff --git a/core/modules/user/src/Tests/UserBlocksTest.php b/core/modules/user/src/Tests/UserBlocksTest.php
index 26dcf89..e22fbd4 100644
--- a/core/modules/user/src/Tests/UserBlocksTest.php
+++ b/core/modules/user/src/Tests/UserBlocksTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\user\Tests;
 
+use Drupal\dynamic_page_cache\EventSubscriber\DynamicPageCacheSubscriber;
 use Drupal\simpletest\WebTestBase;
 
 /**
@@ -77,20 +78,26 @@ public function testUserLoginBlock() {
 
     // Now, log out and repeat with a non-403 page.
     $this->drupalLogout();
-    $this->drupalPostForm('filter/tips', $edit, t('Log in'));
+    $this->drupalGet('filter/tips');
+    $this->assertEqual('MISS', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER));
+    $this->drupalPostForm(NULL, $edit, t('Log in'));
     $this->assertNoText(t('User login'), 'Logged in.');
     $this->assertPattern('!<title.*?' . t('Compose tips') . '.*?</title>!', 'Still on the same page after login for allowed page');
 
     // Log out again and repeat with a non-403 page including query arguments.
     $this->drupalLogout();
-    $this->drupalPostForm('filter/tips', $edit, t('Log in'), ['query' => ['foo' => 'bar']]);
+    $this->drupalGet('filter/tips', ['query' => ['foo' => 'bar']]);
+    $this->assertEqual('HIT', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER));
+    $this->drupalPostForm(NULL, $edit, t('Log in'));
     $this->assertNoText(t('User login'), 'Logged in.');
     $this->assertPattern('!<title.*?' . t('Compose tips') . '.*?</title>!', 'Still on the same page after login for allowed page');
     $this->assertTrue(strpos($this->getUrl(),'/filter/tips?foo=bar') !== FALSE, 'Correct query arguments are displayed after login');
 
     // Repeat with different query arguments.
     $this->drupalLogout();
-    $this->drupalPostForm('filter/tips', $edit, t('Log in'), ['query' => ['foo' => 'baz']]);
+    $this->drupalGet('filter/tips', ['query' => ['foo' => 'baz']]);
+    $this->assertEqual('HIT', $this->drupalGetHeader(DynamicPageCacheSubscriber::HEADER));
+    $this->drupalPostForm(NULL, $edit, t('Log in'));
     $this->assertNoText(t('User login'), 'Logged in.');
     $this->assertPattern('!<title.*?' . t('Compose tips') . '.*?</title>!', 'Still on the same page after login for allowed page');
     $this->assertTrue(strpos($this->getUrl(),'/filter/tips?foo=baz') !== FALSE, 'Correct query arguments are displayed after login');