Also I thought that the way Google solves the shared login problem is that completing the login process on accounts.google.com redirects you to YouTube briefly (as a top-level navigation), which sets a first-party cookie for itself and sends you back to Google.
Interactions with other parties are considered third-party, even if the user is transiently informed in context (for example, in the form of a redirect). Merely hovering over, muting, pausing, or closing a given piece of content does not constitute an intention to interact.
and
Navigational tracking is tracking through information controlled by the source of a top-level navigation or a subresource load, transferred to the destination.
It sure sounds to me like Apple intends to block YouTube’s trick. Transiently redirecting to a domain to install a first-party cookie seems like an unambiguous attempt to circumvent third-party cookie restrictions.
