CiviEvent: selecting "Pay later" option causes the browser to show "are you sure you want to leave this site"? message
I believe you are you using Stripe payment processor extension as one of the payment gateway. There is a known issue reported by a user facing similar problem and have been issues for other users also. As mentioned in info.xml Stripe extension supports till 5.0 and might have not tested fully for 5.3. The only solution I would give is to upgrade stripe if you haven't to 5.0 or from https://lab.civicrm.org/extensions/stripe or wait until some one from Maintainer fixes the problem.


  You can try if its a stripe problem by disabling the extension and
  submitting the form.


Pradeep