Domain Source does not work when active_domain link option is set in processOutbound

Technically it is still not possible, to set a own active_domain. But I can not remember why it was needed.

    $url = Url::fromUserInput("/admin", ["active_domain" => Domain::load("my_domain_2")]);
    $form['link'] = [
      '#type' => 'link',
      '#url' => $url,
      '#title' => $url->toString()
    ];

Not sure to understand what problem the patch is trying to solve.

Could you provide a MR on 2.0.x please?