Photo mosaic and a sticky Add to cart on product pages

Your product page gets three new things. In Products › Product page, "Gallery on desktop" now also offers "Mosaic": on a computer every photo shows at once - the first large, the rest in pairs - and the column with the price and order button stays on screen while shoppers look. A section such as "Size guide" can also open from the size picker: set "Also opens from the picker of: Size" on the section and a link beside the picker shows it in a pop-up. And on phones, once a shopper scrolls past the order button, a bar with "Add to cart" stays at the foot of the screen - on for every shop.