sessionStorage copying into popups

[annevk]

While creating a new auxiliary browsing context, the session storage area is copied over.

and

If browsingContext's creator origin is same origin with browsingContext's active document's origin, then copy the sessionStorage storage area of opener into browsingContext's set of session storage areas. These areas must be considered separate, not affecting each other in any way.

suggest only a partial copying of all session storage areas takes place while in implementations the storage areas for all origins are copied over.

I.e., if A1 embeds B and popups A2 which also embeds B, B inside A2 sees a clone of the session storage area from B inside A1.

Another problem here is that this is supposedly tied to the top-level browsing context where it ought to be tied to the "browsing session" (to be defined) which would remain the same when a top-level browsing context "underneath it" gets replaced due to Cross-Origin-Opener-Policy.

I plan to fix this as part of whatwg/storage#18, but it seemed good to also document the issue for better mutual understanding.

cc @mkruisselbrink