This thread is actually about something else.
Any site using Google accounts for cllient-side auth will use cookies on the
google.com domain. The alternative might be to proxy auth request via a server, but in that case you have a server that is authenticated for you on your behalf, even after you close the browser tab. This way the auth never leaves your client.
In addition, with server-side auth, you diagram save/loads would be proxied via the server too, i.e. we would see your data. The current setup is far better for your privacy, though I see the problem with the config here. We could write a troubleshooting doc for admins.