Base URL option in generate form doesn't work when running Drupal in a sub-directory

I've got a quick hack working with an outbound path processor service:

class SiteRootOutboundPathProcessor implements OutboundPathProcessorInterface {

  /**
   * {@inheritdoc}
   */
  public function processOutbound($path, &$options = [], Request $request = NULL, BubbleableMetadata $bubbleable_metadata = NULL) {
    if (!$request) {
      // WTF? Why is $request sometimes NULL?
      return $path;
    }

    if (!$request->attributes->has(StaticGeneratorInterface::REQUEST_KEY)) {
      return $path;
    }


    // Hardcoded for now - should get this value from the base URL option in the form.
    $options['base_url'] = 'http://localhost:8888/tome-blog/html/tome-blog/web';

    return $path;
  }

}

This also isn't working when I try to generate to a domain.

With my site at http://localhost:8888/joachim-blog/htdocs on my local, and generating to https://joachim-n.github.io/, I get:

- the root of the generated at http://localhost:8888/joachim-blog/html/joachim-blog/htdocs/index.html
- the CSS links in the generated HTML have URLs like this: href="/joachim-blog/htdocs/themes/custom/joachim_blog_theme/fonts/metropolis/Metropolis-Regular.woff2"

I've tried adding the SiteRootOutboundPathProcessor I posted in an earlier comment, and that doesn't actually fix it -- I don't seem to have pages for path aliases exported at all with that in place!

For entity paths, the problem happens deep in core's UrlGenerator.

Tome's EntityPathSubscriber calls it in the

      $event->setPath(parse_url($url->toString(), PHP_URL_PATH));

This has the base URL for the actual site:

    $base_url = $this->context->getBaseUrl();

and so returns "/drupal-contrib/web" (I've switched to a different site to experiment since my last comment!)

UrlGenerator then does this, prepending the base URL:

    if (!$absolute || !$host = $this->context->getHost()) {
      $url = $base_url . $path . $query . $fragment;

Then tome's EntityPathSubscriber get this URL for the entity:

> '/drupal-contrib/web/node/1'