Base path duplicated for sites hosted in sub-directories

Problem/Motivation

When a site is hosted out of a sub-directory. For example, if the root of the site is at example.com/siteroot. This causes the links set in the sort widget to duplicate the base path. So the links set in the sort widget would point to /siteroot/siteroot/search?sort=created&order=desc for example. I think this is due to the way these URLs are being set in WidgetForm.php.

public function submitForm(array &$form, FormStateInterface $form_state) {
    $links = $form_state->get('links');
    [$key, $order] = explode('|', $form_state->getValue('sort_by'));

    foreach ($links['#items'] as $link) {
      $name = $link['#sort_field'];
      if ($name == $key) {
        $url = $link['#url'];

        $url_info = parse_url($url);
        parse_str($url_info['query'], $query);
        $query['order'] = $order;
        $url_info['query'] = UrlHelper::buildQuery($query);
        $url = $url_info['path'] . '?' . $url_info['query'];

        $form_state->setRedirectUrl(Url::fromUserInput($url));
      }
    }
  }

The URLs fetched from $form_state contain the base path. Url::fromUserInput does not seem to expect the base path and since the URL passed here doesn't match a Drupal route it appends it the path which already contains the base path to the base path, resulting in the duplication.

Steps to reproduce

Host a site out of a subdirectory. So it's base path would be something like example.com/siteroot.
Enable the search_api_sorts_widget.
Set up a search page and specify 1 or more sorts.
Set the sort widget settings to be: Active, Autosubmit, Hide submit button.
Click on an option displayed in the sort widget.
The page that will load will have a duplicated base path.

Proposed resolution

Check the URL being passed to Url::fromUserInput for the presence of the base path and remove it if it exists.
Or
Use some other method to set URLs that doesn't include the base path.

Remaining tasks

User interface changes

API changes

Data model changes