How to get current user?

Ser5

In extend.php I have a code like:

use Illuminate\Container\Container;
use Flarum\Frontend\Document;
use Flarum\Extend;

$container = Container::getInstance();
/** @var HtmlParts */
$htmlParts = $container->get(HtmlParts::class);

return [
    (new Extend\Frontend('forum'))
        ->content(function (Document $document) use ($htmlParts) {
            // $document doesn't seem to have anything related to the current request or user,
            // cannot pass them to getBootstrapModalHtml()
            $document->foot = [$htmlParts->getBootstrapModalHtml()];
        }),
];

And here is the [shortened] code of HtmlParts:

use Laminas\Diactoros\ServerRequest;
use Flarum\User\User;

class HtmlParts {
	public function __construct (ServerRequest $serverRequest, User $user) {
		// $serverRequest is not a current request, doesn't has access to the current user
		// $user is not the current user, it's empty
	}

	public function getBootstrapModalHtml () {
		// How to access the current user?
		// app('flarum.user') is outdated and doesn't return anything anyway
	}
}

I can't figure out how to access the current user.