title: Stripe "Simple" Checkout - detect if close button is clicked
body: <p>I've integrated <a href="https://stripe.com/docs/checkout" rel="noreferrer">Stripe Checkout</a> into my site and everything works great except for one aspect.  I use the 'Simple' checkout mode where Stripe renders my checkout button for me.  But I see no way in the docs to detect if the user clicks the close button (effectively cancelling the transaction - see image).</p>
  
  <p><img src="https://i.stack.imgur.com/0YHOk.png" alt="enter image description here"></p>
  
  <p>Is there a way to detect this in 'Simple' mode (as opposed to Custom)?</p>