Yes, you can leave the "Buy" page as soon as you reach the "Almost Done" page.
When buying Bitcoin or other cryptocurrencies, you'll reach the "Almost Done" screen. It shows that your order has been submitted and the crypto transaction is being processed. Delivery of your coins is not instant. This is why this page will continue to show the delivery progress until it is complete.
You can leave this page at any time. You'll receive all order information by email and can check on the delivery status directly in your internet browser with the link provided in that email.