Yep. With the solution discussed, as I understand it, the e-mail program just needs to be modified to request a focus token and send it along with the URL request to the browser or the OS browser dispatch service to keep the expected behavior.

This could be abstracted by libraries (e.g. a method in Qt to open a URL in the system browser automatically gets the token) so each application doesn't need to be updated separately, or possibly even OS services.