PASS The top browsing context can only show one payment sheet at a time.
PASS If an iframe shows a payment request, the top-level browsing context can't also show one.
PASS An iframe cannot show a payment request if the top-level window is already showing one.
PASS Given multiple nested browsing contexts, and window calls show() first, other nested browsing contexts can't show a request.
PASS Given multiple nested browsing contexts, and an iframe calls show() first, other nested browsing contexts can't show a request.
PASS Navigating an iframe as a nested browsing context sets 'payment request is showing boolean' to false.