Alt-Svc + Proxy Pac

Howdy Folks,

I'm curious how Alt-Svc is expect to work with Proxy PAC files. Consider
the scenario where http://www.example.com/ has an Alt-Svc that specified
http/2 on mail.example.com:443. When the browser decides to make an http/2
(over TLS) connection to mail.example.com, on behalf of
http://www.example.com, what URL and host should the browser pass to the
PAC file's FindProxyForURL() method?

I can argue both cases.

* It should pass in the requested url (http://www.example.com/) because
that is the URL being requested. There is no other URL.
* It should pass in a pseudo url (https://mail.exmaple.com/) because, for
example, access to mail.example.com may well requires use of a proxy to
access. By passing in the request URL, the PAC file does not have the
opportunity to send the connection to the correct proxy.

Thoughts?

Cheers,

Ryan