diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestSelectForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestSelectForm.php
index 5c1f1d64f5..e4ab781468 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestSelectForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestSelectForm.php
@@ -136,14 +136,17 @@ public function buildForm(array $form, FormStateInterface $form_state) {
     // option groups sorted and some left alone.
     $sortable_options = $this->makeSortableOptions('sso');
     $sortable_options['sso_zzgroup']['#sort_options'] = TRUE;
+    $sortable_options['sso_xxgroup']['#sort_options'] = TRUE;
+    $sortable_options['sso_xxgroup']['#sort_start'] = 1;
+    // Do not use a sort start on this one.
     $form['sorted'] = [
       '#type' => 'select',
       '#options' => $sortable_options,
       '#sort_options' => TRUE,
-      '#sort_start' => 3,
     ];
 
-    // Add a select to test sorting with a -NONE- option included.
+    // Add a select to test sorting with a -NONE- option included,
+    // and a sort start set.
     $sortable_none_options = $this->makeSortableOptions('sno');
     $sortable_none_options['sno_zzgroup']['#sort_options'] = TRUE;
     $form['sorted_none'] = [
@@ -184,6 +187,11 @@ protected function makeSortableOptions($prefix) {
         $prefix . '_gd' => new TranslatableMarkup('group d'),
         $prefix . '_gf' => new TranslatableMarkup('group f'),
       ],
+      $prefix . '_xxgroup' => [
+        $prefix . '_gz' => new TranslatableMarkup('group z'),
+        $prefix . '_gi' => new TranslatableMarkup('group i'),
+        $prefix . '_gh' => new TranslatableMarkup('group h'),
+      ],
       $prefix . '_d' => 'd',
       $prefix . '_c' => new TranslatableMarkup('main c'),
       $prefix . '_b' => new TranslatableMarkup('main b'),
diff --git a/core/modules/system/tests/src/Functional/Form/FormTest.php b/core/modules/system/tests/src/Functional/Form/FormTest.php
index 43b9bf3d9c..3731f06d0a 100644
--- a/core/modules/system/tests/src/Functional/Form/FormTest.php
+++ b/core/modules/system/tests/src/Functional/Form/FormTest.php
@@ -12,6 +12,7 @@
 use Drupal\Tests\BrowserTestBase;
 use Drupal\user\RoleInterface;
 use Drupal\filter\Entity\FilterFormat;
+use Behat\Mink\Element\NodeElement;
 
 /**
  * Tests various form element validation mechanisms.
@@ -470,10 +471,9 @@ public function testSelectSorting() {
     $this->drupalGet('form-test/select');
     $content = $this->getSession()->getPage()->getContent();
 
-    // Verify the expected order of the options in the unsorted and
-    // sorted elements.
-    $expected_order = [
-      'uso_first',
+    // Verify the order of the select options.
+    $this->validateSelectSorting('unsorted', [
+      'uso_first_element',
       'uso_second',
       'uso_zzgroup',
       'uso_gc',
@@ -483,26 +483,40 @@ public function testSelectSorting() {
       'uso_ge',
       'uso_gd',
       'uso_gf',
+      'uso_xxgroup',
+      'uso_gz',
+      'uso_gi',
+      'uso_gh',
       'uso_d',
       'uso_c',
       'uso_b',
       'uso_a',
-      'sso_first',
-      'sso_second',
-      'sso_zzgroup',
-      'sso_ga',
-      'sso_gb',
-      'sso_gc',
+    ]);
+
+    $this->validateSelectSorting('sorted', [
       'sso_a',
       'sso_d',
+      'sso_first_element',
       'sso_b',
       'sso_c',
+      'sso_second',
+      'sso_xxgroup',
+      'sso_gz',
+      'sso_gh',
+      'sso_gi',
       'sso_yygroup',
       'sso_ge',
       'sso_gd',
       'sso_gf',
+      'sso_zzgroup',
+      'sso_ga',
+      'sso_gb',
+      'sso_gc',
+    ]);
+
+    $this->validateSelectSorting('sorted_none', [
       'sno_empty',
-      'sno_first',
+      'sno_first_element',
       'sno_second',
       'sno_zzgroup',
       'sno_ga',
@@ -512,22 +526,41 @@ public function testSelectSorting() {
       'sno_d',
       'sno_b',
       'sno_c',
+      'sno_xxgroup',
+      'sno_gz',
+      'sno_gi',
+      'sno_gh',
       'sno_yygroup',
       'sno_ge',
       'sno_gd',
       'sno_gf',
-    ];
-
-    $prev_position = 0;
-    foreach ($expected_order as $string) {
-      $position = strpos($content, $string);
-      $this->assertTrue($position > $prev_position, 'Item ' . $string . ' is in correct order');
-      $prev_position = $position;
-    }
+    ]);
 
     // Verify that #sort_order and #sort_start are not in the page.
-    $this->assertFalse(strpos($content, '#sort_order'), 'Sort order property has been removed');
-    $this->assertFalse(strpos($content, '#sort_start'), 'Sort start property has been removed');
+    $this->assertSession()->responseNotContains('#sort_order');
+    $this->assertSession()->responseNotContains('#sort_start');
+  }
+
+  /**
+   * Validates that the options are in the right order in a select.
+   *
+   * @param string $select
+   *   Name of the select to verify.
+   * @param string[] $order
+   *   Expected order of its options.
+   */
+  protected function validateSelectSorting($select, $order) {
+    $option_map_function = function (NodeElement $option) {
+      if ($option->getTagName() === 'optgroup') {
+        return $option->getAttribute('label');
+      }
+      return $option->getValue();
+    };
+
+    $field = $this->getSession()->getPage()->findField($select);
+    $options = array_map($option_map_function,
+      $field->findAll('css', 'option, optgroup'));
+    $this->assertIdentical($order, $options);
   }
 
   /**