The point of this task is to use a simple page we know exists (the payment page with its off-site redirect pane) to understand exactly how the JavaScript is supposed to work to open the modal. This will mean implementing the payment gateway as offsite for now and disabling the auto-redirection JavaScript. From there I don't know if we can just embed the necessary JavaScript in the page and simulate a click on load or what.
Once we know how to open the modal, we can then understand what to do with the various forms of response. We can also research options for moving it earlier in the checkout flow, as with APIs like the Stripe Payment Element, which overrides a specific checkout button in order to facilitate payment via iframe before redirecting through checkout completion after payment has been submitted.
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We pivoted to using the modal dialog from the review page itself instead of relying on the redirect page. This is currently opening, though there is a JavaScript error preventing it from being closed. Details, details...
Automatically closed - issue fixed for 2 weeks with no activity.