Sure, but the separate window can be on a different domain. Now you have a way to share cookies across multiple websites on different domains if all of them include an iframe to this external domain. And you can use in-browser messages (see window.postMessage()) to communicate between iframes and main window.

Yeah i don’t know why, probably exactly because is such a neglected feature that it offers workarounds for some limitations, like in the case of cookie-related patterns.

That’s a good.way to burden a competitor with a huge project.

Does this stop me from adding to my website an iframe to facebook where facebook can keep its cookies for my user? That would be great but I doubt it.

Same for cleopatra and the pyramids