Provide a way to alter the block plugin configuration form

Totally agree this would still make sense to make this alterable. Or are there any other ways to hook into this form cleanly?

This is really needed in cases where users select from menu_blocks as options only. Instead, they get two sets of blocks defined by both the system and menu_block modules, where I only want menu_block (because this presents the extra features of that module).

To get around this in my case, I extended the field widget until there is another solution.

/**
 * Plugin implementation of the 'block_field' widget.
 *
 * @FieldWidget(
 *   id = "mymodule_block_field",
 *   label = @Translation("Block field (for menus)"),
 *   field_types = {
 *     "block_field"
 *   }
 * )
 */
class BlockFieldWidget extends BaseBlockFieldWidget {

  /**
   * {@inheritdoc}
   */
  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
    $element = parent::formElement($items, $delta, $element, $form, $form_state);

    if (!empty($element["plugin_id"]["#options"]["Menus"])) {
      $options = $element["plugin_id"]["#options"]["Menus"];
      $element["plugin_id"]["#options"]["Menus"] = array_filter($options, fn ($option) => !str_contains($option, 'system_'), ARRAY_FILTER_USE_KEY);
    }

    return $element;
  }

}

vlad.dancer → made their first commit to this issue’s fork.