Re: [w3c/payment-handler] CanMakePaymentEvent handling algorithm unclear (#330)

A visual indicator would be useful for other reasons, but I doubt that it'll be effective in communicating the nuance: "the current website is querying hasEnrolledInstrument() on payment handler X and can use this as a way to detect that you're in private browsing mode". It also doesn't actually stop the detection.

It seems we're stuck between two privacy requirements:
1) In private browsing mode, don't signal payment handler about user's current presence on a website.
2) The absence of the signal actually allows a website to detect that user's in private browsing mode.

The same-origin short-circuit you suggested may cut off most of the abuse. But I don't have a good mental model of how a website can abuse the knowledge that user is in private browsing mode, so not sure if that is sufficient. It'll be good to get our privacy team's take on it.

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