class TextareaWidget

class TextareaWidget extends StringTextareaWidget {
  
  /**
   * {@inheritdoc}
   */
  public function settingsForm(array $form, FormStateInterface $form_state) {
    $element = parent::settingsForm($form, $form_state);
    $element['rows']['#description'] = $this->t('Text editors may override this setting.');
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $main_widget = parent::formElement($items, $delta, $element, $form, $form_state);
    $allowed_formats = $this->getFieldSetting('allowed_formats');
    $element = $main_widget['value'];
    $element['#type'] = 'text_format';
    $element['#format'] = $items[$delta]->format;
    $element['#base_type'] = $main_widget['value']['#type'];
    if ($allowed_formats && !$this->isDefaultValueWidget($form_state)) {
      $element['#allowed_formats'] = $allowed_formats;
    }
    return $element;
  }
  
  /**
   * {@inheritdoc}
   */
  public function errorElement(array $element, ConstraintViolationInterface $violation, array $form, FormStateInterface $form_state) {
    if (isset($element['format']['#access']) && !$element['format']['#access'] && preg_match('/^[0-9]*\\.format$/', $violation->getPropertyPath())) {
      // Ignore validation errors for formats if formats may not be changed,
      // such as when existing formats become invalid.
      // See \Drupal\filter\Element\TextFormat::processFormat().
      return FALSE;
    }
    return $element;
  }

}