If interoperable behavior emerges amongst user agents, then that behavior will be standardized here before progressing the specification along the W3C Recommendation Track.

It looks like this is already noted in the CR, but I wanted to open a ticket for this to track status on this.

ApplePayJS (whose behavior I imagine will eventually make it into Safari's PaymentRequest implementation) only allows .show() to be called by user activation (e.g. click). On the other hand, Chrome allows .show() to be called as long as the page is in focus. This makes it hard to build consistent UIs.