Chrome Version       : 64.0.3253.3
OS Version: 
URLs (if applicable) :
Other browsers tested:
  Add OK or FAIL after other browsers where you have tested this issue:
     Safari 5:
  Firefox 4.x:
     IE 7/8/9:

What steps will reproduce the problem?
1. Install a payment handler from https://rsolomakhin.github.io/pr/apps/basic-card/
2. Pay via this payment handler on https://rsolomakhin.github.io/pr/cc/

What is the expected result?
Clicking "Pay" in the Chrome payment sheet should activate the "Loading..." spinner and prevent user interaction with the payment sheet.

What happens instead of that?
After clicking "Pay" in the Chrome payment sheet, the UI does not change for ~2 seconds and then hides when the payment completes. During these ~2 seconds, it's possible to further interact with the payment sheet. Clicking "Pay" the second time will crash the browser, for example.


Please provide any additional information below. Attach a screenshot if
possible.

UserAgentString: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/64.0.3253.3 Safari/537.36