Scope
What happens if the user closes the checkout page after having clicked the "Confirm" button but before the successful callback
Could it be possible that I close the popup at a particular time and the transaction succeeds anyway?
Summary
Sometimes when you close the checkout popup/page before the transaction ends, the transaction aborts and the payment email is not received but this cannot be the case always. It is possible for the transaction to succeed even though you close the window before the success message is displayed. In such a case you wouldn't know the instant outcome unless you have enabled the relevant Email Notifications. You can also refer to the Event records under Logs > Events to verify this.
Solution
We cannot assure that the subscription will be created in case the window is closed immediately after confirming due to different factors, such as network, gateways, etc. The call-out is made by Chargebee, however, the call-back depends on other parties.
It is possible for the transaction to succeed even though you close the window before the success message is displayed. In such case you wouldn't know the instant outcome unless you have enabled the relevant Email Notifications (Settings > Configure Chargebee > Email notifications > Onboarding emails / Subscription Activation / Subscription Changes / Payment and Credits / Payment failures).
Additionally, you can refer to the Event records under Logs > Events. All changes related to customers, subscriptions, plans, addons, and coupons are logged here.