'#group' Form API property works on details, fieldset and container and can be used on any custom element by module developers.

Freedom For Fieldsets! Long Live The DETAILS removed the ability to use the '#group' property on fieldsets and limit it only on #type 'details'. This patch restores that option and also makes it possible to use it on #type 'container'.

Module developers wanting to use the '#group' property on custom elements can simply use [className, 'processGroup' and [className, 'preRenderGroup'] in the #process and #pre_render key to add grouping support for the element.

  // In a class extending \Drupal\Core\Render\Element\RenderElement...
  public function getInfo() {
    $class = get_class($this);
    return [
      '#process' => [
        [$class, 'processGroup'],
      ],
      '#pre_render' => [
        [$class, 'preRenderGroup'],
      ],
    ];
  }