TYPO3 Core

We worked around this by using an XCLASS to extend the current SiteResolver implementation with the following. It would be nice if this fix or probably something more robust would find its way into the core!

SiteResolver

<?php
namespace Your\Ext\Xclass\TYPO3\CMS\Backend\Middleware;

/*
 * This file is part of the TYPO3 CMS project.
 *
 * It is free software; you can redistribute it and/or modify it under
 * the terms of the GNU General Public License, either version 2
 * of the License, or any later version.
 *
 * For the full copyright and license information, please read the
 * LICENSE.txt file that was distributed with this source code.
 *
 * The TYPO3 project - inspiring people to share!
 */

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\RequestHandlerInterface;
use TYPO3\CMS\Backend\Utility\BackendUtility;
use TYPO3\CMS\Core\Routing\SiteMatcher;
use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Core\Utility\MathUtility;

/**
 * SiteResolver, see https://forge.typo3.org/issues/90624
 */
class SiteResolver extends \TYPO3\CMS\Backend\Middleware\SiteResolver {
    /**
     * @inheritDoc
     */
    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
        $pageId = ($request->getQueryParams()['id'] ?? $request->getParsedBody()['id'] ?? 0);

        // BEGIN added code
        if ($pageId === 0) {
            $queryString = parse_url($request->getQueryParams()['returnUrl'], PHP_URL_QUERY);

            if ($queryString !== NULL) {
                parse_str($queryString, $parsedQueryString);
                $pageId = (int) $parsedQueryString['id'];
            } else {
                $queryString = parse_url(GeneralUtility::getIndpEnv('HTTP_REFERER'), PHP_URL_QUERY);

                parse_str($queryString, $parsedQueryString);
                parse_str($parsedQueryString['returnUrl'], $parsedString);

                $pageId = (int) $parsedString['id'];
            }
        }
        // END added code

        // Check if we have a numeric _GET/_POST parameter for "id", then a site information can be resolved based.
        if (MathUtility::canBeInterpretedAsInteger($pageId)) {
            $pageId = (int)$pageId;
            $rootLine = null;
            if ($pageId > 0) {
                $rootLine = BackendUtility::BEgetRootLine($pageId);
            }
            $site = GeneralUtility::makeInstance(SiteMatcher::class)->matchByPageId($pageId, $rootLine);
            $request = $request->withAttribute('site', $site);
            $GLOBALS['TYPO3_REQUEST'] = $request;
        }
        return $handler->handle($request);
    }
}

ext_localconf.php

$GLOBALS['TYPO3_CONF_VARS']['SYS']['Objects'][\TYPO3\CMS\Backend\Middleware\SiteResolver::class] = [
    'className' => \Your\Ext\Xclass\TYPO3\CMS\Backend\Middleware\SiteResolver::class
];

Thanks for your response! I was not aware of that possibility, using the Formengine is probably the better way to solve that particular issue. However I am not yet sure whether this is a valid issue nevertheless. I think it would be best if the SiteResolver could resolve the site in order for it to be available in any part of the code (including user functions). But on the other hand the site can always be easily resolved any time we have a data record and it's pid at hand.