Symfony version(s) affected: 4.4.11

Description
Debug bar won't load anymore for our use case after changes introduces in #37576 because it's blocked by CORS.

We're using a setup, where we have 2 application modules accessible behind a proxy. The proxy is running at proxy.test and the modules are accessible at proxy.test/foo and proxy.test/bar. The modules itself are running at their own internal URLs -> foo.test and bar.test.

It worked before with the relative URL, because the /foo/_wdt/66dc93 was loaded by browser as proxy.test/foo/_wdt/66dc93, but now it's loaded as foo.test/foo/_wdt/66dc93.

It's very much possible this can be solved by some configuration on our part, but IMO this is BC break and as such should not be released in patch version.