In the Shopify theme "Dawn," if the product detail page is long, the "Add to Cart" button disappears when you scroll down, which is a bit inconvenient. Here is an example of how to set a sticky button. (It reflects the button label when the item is sold out, but I think it would be fine to just fix it to "Add.")
How to display a fixed Add to Cart button on the product detail page of the theme "Dawn" according to the scroll position of the page
Add a customized Liquid section to your product page template and insert the code
