The cart is not emptied if the shopper closes the browser or presses the back button. Whenever the shopper is redirected to another site for payment, Magento locks the order and the items.
This can occur during two scenarios:
1. The shopper clicks the Previous button on AdyenHPP, issuer website or anything else that sends a Cancelled result code to Magento. Note that this Previous button is different from the back button on the browser tab.
In this scenario, Magento will cancel the order and the items in the order will be added to a new basket. This basket will be visible to the shopper. The shopper can then proceed to the checkout again without having to add the items to the basket.
2. The shopper clicks the back button on the browser tab, closes the browser, or returns back to Magento without AdyenHPP or the issuer website sending a Cancelled result code.
In this scenario, Magento is unaware that the order is cancelled and so the order is still locked with the items. The shopper will see a new empty cart as the previous one is locked, so they need to add the items in the cart again. The previously locked order will be cancelled only after an OFFER_CLOSED notification reaches Magento or after the merchant cancels the order manually on the database.
This behavior of shopping carts is the same in both Magento 2 and Magento 1.