EDIT: didn't expect this to be so controversial (6 downvotes!)
Good chance I'm missing something, or there's some kind of protections in place around this.
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/if...
Main app
My sandboxed iframe
Stripe's iframe
So managing all the bubbling up and down of messages felt like it was going to be complicated. Limiting Stripe to a single page and forcing the new HTTP requests to unload it is a bit hackier, but is really simple to implement. You can see it in the blog post - I only had to add ~5 lines of extra code to my app to make it work.