theme_region_wrapper-1.0.x-dev/tests/modules/theme_region_wrapper_region_hook_test/theme_region_wrapper_region_hook_test.module

<?php

/**
 * @file
 * Provides hook implementations for testing purposes.
 */

/**
 * Implements hook_preprocess_HOOK() for region templates.
 */
function theme_region_wrapper_region_hook_test_preprocess_region(array &$variables):void {
  if ($variables['region'] == 'content') {
    $variables['attributes']['class'][] = 'new_class';
  }
}