maintain products in shopping-cart as long as a payment was not completed. (or succes page was not reached).
Currently when a user cannot complete a payment, he returns to the checkout page with the only option to choose a different payment method.
However: if the user thenleaves the checkout the cart is cleared (= is made empty).
I do not believe that a failed payment automatically cancels the intention of the user to buy what-ever they have put in te cart.
This way you loose potential purchases, when some users are only looking to make some adjustments before continuing to eventually complete the purchase (payment).
I believe it is better to always maintain the full content of the cart (even the promotions, if any apply) as long as the user has not clearly decided not to proceed with the puchase.
Better would be to only empty the cart when a user specifically aims for that: for example by using a button to "empty" the cart.

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Michael Garnreiter commented
We fully back that idea. We are facing the same issue, leaving many customers with an empty card after a failed payment or active cancellation. For both cases I do see this as an issue as customers are spending their precious time to thoughtfully select products, add them to cart and are surprised with an empty cart after they want to adjust something or their payment fails etc.
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Christoph Von Heydebrand commented
We see this issue in hotjar pretty clearly. For us it makes no sense, that the cart is cleared then.