Issue #565220 by voleger, Yogesh Pawar, gpk, alexpott: Fix Weight form element behavior

      for ($n = (-1 * $element['#delta']); $n <= $element['#delta']; $n++) {

        $weights[$n] = $n;

      }

      $default_value = (int) $element['#default_value'];

      if (!isset($weights[$default_value])) {

        $weights[$default_value] = $default_value;

        ksort($weights);

      }

      $element['#options'] = $weights;

      $element += $element_info_manager->getInfo('select');

    }

<?php

namespace Drupal\Tests\Core\Render\Element;

use Drupal\Core\Form\FormState;

use Drupal\Core\Render\Element\Weight;

use Drupal\KernelTests\KernelTestBase;

/**

 * @coversDefaultClass \Drupal\Core\Render\Element\Weight

 * @group Render

 */

class WeightTest extends KernelTestBase {

  /**

   * {@inheritdoc}

   */

  protected static $modules = ['system'];

  /**

   * {@inheritdoc}

   */

  protected function setUp() {

    parent::setUp();

    $this->installConfig(['system']);

  }

  /**

   * Test existing #default_value value in #options list.

   *

   * @covers ::processWeight

   */

  public function testProcessWeight() {

    $element = [];

    $form_state = new FormState();

    $complete_form = [];

    $element_object = new Weight([], 'weight', []);

    $info = $element_object->getInfo();

    $element += $info;

    $element['#default_value'] = $element['#delta'] + 5;

    Weight::processWeight($element, $form_state, $complete_form);

    $this->assertTrue(

      isset($element['#options'][$element['#default_value']]),

      'Default value exists in the #options list'

    );

  }

}