diff -u b/core/tests/Drupal/FunctionalJavascriptTests/TableDrag/TableDragTest.php b/core/tests/Drupal/FunctionalJavascriptTests/TableDrag/TableDragTest.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/TableDrag/TableDragTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/TableDrag/TableDragTest.php
@@ -206,7 +206,7 @@
     $this->assertDraggableTable([
       ['id' => 1, 'weight' => -10, 'parent' => '', 'indentation' => 0, 'changed' => FALSE],
       ['id' => 3, 'weight' => -10, 'parent' => 1, 'indentation' => 1, 'changed' => FALSE],
-      ['id' => 2, 'weight' => -9, 'parent' => '', 'indentation' => 0, 'changed' => TRUE  ],
+      ['id' => 2, 'weight' => -9, 'parent' => '', 'indentation' => 0, 'changed' => TRUE],
       ['id' => 4, 'weight' => -8, 'parent' => '', 'indentation' => 0, 'changed' => FALSE],
       ['id' => 5, 'weight' => -7, 'parent' => '', 'indentation' => 0, 'changed' => FALSE],
     ]);
@@ -236,7 +236,7 @@
    */
   protected function assertDraggableTable(array $structure) {
     $rows = $this->getSession()->getPage()->findAll('xpath', '//table[@id="tabledrag-test-table"]/tbody/tr');
-    $this->assertSameSize($structure, $rows);
+    $this->assertSession()->elementsCount('xpath', '//table[@id="tabledrag-test-table"]/tbody/tr', count($structure));
 
     foreach ($structure as $delta => $expected) {
       $this->assertTableRow($rows[$delta], $expected['id'], $expected['weight'], $expected['parent'], $expected['indentation'], $expected['changed']);
@@ -260,12 +260,19 @@
    *   Whether or not the row should have been marked as changed.
    */
   protected function assertTableRow(NodeElement $row, $id, $weight, $parent = '', $indentation = 0, $changed = FALSE) {
+    // Assert that the row position is correct by checking that the id
+    // corresponds.
     $this->assertSession()->hiddenFieldValueEquals("table[$id][id]", $id, $row);
+    // To wait until all the JS DOM manipulations are done, wait for the parent
+    // input field to be updated.
+    $row->waitFor(1000, function () use ($row, $id, $parent) {
+      return $row->find('xpath', "//input[@type='hidden'][@name='table[$id][parent]'][@value='$parent']");
+    });
     $this->assertSession()->hiddenFieldValueEquals("table[$id][parent]", $parent, $row);
     $this->assertSession()->fieldValueEquals("table[$id][weight]", $weight, $row);
     $this->assertSession()->elementsCount('css', '.js-indentation.indentation', $indentation, $row);
     // A row is marked as changed when the related markup is present.
-    $this->assertEquals($changed, !empty($row->find('css', 'abbr.tabledrag-changed')));
+    $this->assertSession()->elementsCount('css', 'abbr.tabledrag-changed', (int) $changed, $row);
   }
 
   /**