How the Full Page Zoom effect works
When a product image is clicked on, it is zoomed in till is fits your entire screen.
Unless the image and the screen have the same dimensions, the image will overflow the screen, as in the following image:
Depending on both your image and screen dimensions, the image can overflow vertically or horizontally.
The whole image can always be viewed by scrolling it. The scrolling is triggered on mouse move on desktop and by dragging your finger on mobile.