A browsing context can be associated with a sequence of documents/Window objects, which can each be from a distinct origin. Therefore the whole concept of a browsing context group with similar origins doesn't make much sense. A same-origin iframe can become cross-origin at any time, and the browsing contexts involved will still be the same.

As this is part of HTML's fundamental lifetime problem I don't think we should block adding shared memory on it, since all implementers understand what this is supposed to mean, but we should definitely fix it.

Realized this while thinking through #2521 some more.