Sticky Add to cart button for product page Using Intersection Observer - This will show add to car button when native button will be disappear.

IntersectionObserver.js

document.addEventListener("DOMContentLoaded", function () {
    var productCartButton = document.querySelector('.product-cart'), stickyAddToCartButton = document.querySelector('.sticky-add-to-cart'), previousTop = 0;

    if ("IntersectionObserver" in window && "IntersectionObserverEntry" in window && "intersectionRatio" in window.IntersectionObserverEntry.prototype) {
        let productCartButtonObserver = new IntersectionObserver(function (entries, observer) {
            entries.forEach(function (entry) {
                let addToCartButton = entry.target,
                    showstickyAddToCartButton = (entry.boundingClientRect.top < previousTop && entry.boundingClientRect.top < 0);

                if (showstickyAddToCartButton) {
                    stickyAddToCartButton.classList.add('show');
                } else {
                    stickyAddToCartButton.classList.remove('show');
                }

                // Set new top
                previousTop = entry.boundingClientRect.top;
            });
        }, { threshold: 0.5});

        productCartButtonObserver.observe(productCartButton);
    }
});

See also https://gist.github.com/gunjanpatel/6398339f60afdf66a6e73d37ce225445