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;
    }

}