Cooperating websites can subvert first-party isolation by redirecting the top level page through multiple first-party domains (with an ID in the URL). And Googl

Cooperating websites can subvert first-party isolation by redirecting the top level page through multiple first-party domains (with an ID in the URL). And Google does exactly that when you login. How to properly prevent it is still an open question: https://bugzilla.mozilla.org/show_bug.cgi?id=1319839 https://bugzilla.mozilla.org/show_bug.cgi?id=1319839