Remove config translation textfield maxlength limits

In the COOKiES Drupal 8+ module, we had the issue that config entity string translations were not possible with text > 128 characters length: https://www.drupal.org/project/cookies/issues/3327045

The reason for that is in core. Form API Elements of type Textfield have a general #maxlength of 128 characters:

  /**
   * {@inheritdoc}
   */
  public function getInfo() {
    $class = static::class;
    return [
      '#input' => TRUE,
      '#size' => 60,
      '#maxlength' => 128,
      [...]

This default is "inherited" by Drupal\config_translation\FormElement\Textfield:

  /**
   * {@inheritdoc}
   */
  public function getTranslationElement(LanguageInterface $translation_language, $source_config, $translation_config) {
    return [
      '#type' => 'textfield',
    ] + parent::getTranslationElement($translation_language, $source_config, $translation_config);
  }

This is hard to understand, as on the other hand there's no technical limit documented for the String (StringData) Schema datatype: https://git.drupalcode.org/project/drupal/blob/8.0.x/core/config/schema/core.data_types.schema.yml

So while this limit may (have) be(en) useful for database values with VARCHAR(128) like in former Drupal 7 times, I don't think this limit still makes a lot of sense.

And if it does, why 128 characters? Wouldn't 256 be a better general value than if there has to be a limit or an assumption to be made?

That's why I created a Drupal core issue to discuss a change here:

https://www.drupal.org/project/drupal/issues/3331028

As I saw, we were not the first Drupal users running into this issue, as this forum post shows:

https://www.drupal.org/forum/support/module-development-and-code-questions/2020-09-16/d9-change-max-length-of-slogan-field

So finally there are different ways to work around this, until there's a possible Core fix. The one from the forum post, or ours from the COOKiES module. For us, the hook_form_alter() solution wasn't fitting.

Here's our snippet from the related commit:

/**
 * Implements hook_preprocess_HOOK().
 */
function cookies_preprocess_config_translation_manage_form_element(&$variables) {
  // Temporary fix for: https://www.drupal.org/project/cookies/issues/3327045
  // Drupal limits config translation textfields (string) to a maxlength of 128.
  // In our forms we set #maxlength to a higher limit so we need to remove
  // the limit also for the config translation forms:
  // Only for textfields:
  if (!empty($variables['element']['translation']['#type']) && $variables['element']['translation']['#type'] == 'textfield') {
    // Only for items starting with 'source[config_names][cookies.',
    // so we don't affect other items.
    if (strpos($variables['element']['source']['#name'], 'source[config_names][cookies.') === 0) {
      // Remove the #maxlength limit entirely. There's no technical limit.
      unset($variables['element']['translation']['#maxlength']);
    }
  }
}

I hope this helps some of you.

Please help to improve things and participate in fixing Drupal issues :)