Signing into a website through an iframe redirects you back to a sign in page inexplicably if the post-signin page requires a cookie. Another example is you're

Signing into a website through an iframe redirects you back to a sign in page inexplicably if the post-signin page requires a cookie. Another example is you're signed into website A, and while on website B, iframes to website A behave in such a way that you're not signed in, and you cannot sign in.