Do not allow buttons in disabled payment-step to be clickable.

This is a workaround. Note that it's still possible to trigger the
PayPal button iframe by tabbing to it and hitting enter.

The hack works for the Venmo and PayPal buttons. The Apple Pay Button is
also not yet tested as of this commit. It will be tested in
#105.

app/assets/stylesheets/spree/frontend/solidus_braintree.scss

@@ -49,3 +49,14 @@ the installer will append this file to the app vendored assets here: 'vendor/ass
 .venmo-button.visible {
   visibility: visible;
 }
+
+/*
+WORKAROUND: Do not allow buttons in disabled payment-step to be clickable. Note
+that it's still possible to trigger the PayPal button iframe by tabbing to it
+and hitting enter.
+*/
+fieldset.payment-step__details:disabled {
+  #apple-pay-button, #paypal-button iframe, #venmo-button {
+    pointer-events: none;
+  }
+}