only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/system/tests/src/Functional/System/MaintenancePageOfflineTest.php
@@ -0,0 +1,68 @@
+<?php
+
+namespace Drupal\Tests\system\Functional\System;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Tests if the Maintenance page is served when the site is offline.
+ *
+ * @group system
+ */
+class MaintenancePageOfflineTest extends BrowserTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'bartik';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
+    parent::setUp();
+
+    $settings_filename = $this->siteDirectory . '/settings.php';
+    chmod($settings_filename, 0777);
+    $settings_php = file_get_contents($settings_filename);
+    // Ensure we can test errors rather than being caught in
+    // \Drupal\Core\Test\HttpClientMiddleware\TestHttpClientMiddleware.
+    $settings_php .= "\ndefine('SIMPLETEST_COLLECT_ERRORS', FALSE);\n";
+    file_put_contents($settings_filename, $settings_php);
+
+    $settings = [];
+
+    // Set $config['system.logging']['error_level'] = 'hide' in settings.php
+    $settings['config']['system.logging']['error_level'] = (object) [
+      'value' => ERROR_REPORTING_DISPLAY_ALL,
+      'required' => TRUE,
+    ];
+    $this->writeSettings($settings);
+  }
+
+  /**
+   * Tests if the maintenance offline page is served when settings.php contains
+   * invalid database settings.
+   */
+  public function testInvalidDatabaseSettings() {
+    $this->drupalGet('');
+    $this->assertSession()->statusCodeEquals(200);
+    $this->assertSession()->pageTextContains('Log in');
+
+    $settings = [];
+
+    // Set $databases['default']['default']['username'] = 'invalid' in
+    // settings.php
+    $settings['databases']['default']['default']['username'] = (object) [
+      'value' => 'invalid',
+      'required' => TRUE,
+    ];
+    $this->writeSettings($settings);
+    $this->getSession()->restart();
+
+    $this->drupalGet('');
+    $this->assertSession()->statusCodeEquals(500);
+    $this->assertSession()->pageTextContains('Service unavailable');
+  }
+
+}