Hi guys,
Hello,
Thanks for asking your question. Without knowing which OS that you are developing for will make it hard to make a diagnosis. A suggestion would be that because the Checkout lightbox normally opens in a frame, it could be that on some mobile browsers and some mobile devices it might be interpreted as a new browser window.
Please see the Visa Checkout documentation here - https://developer.visa.com/capabilities/visa_checkout/docs.
Thank you,
Diana