add back to top button

[llouison]

couldn't get the scroll function to work in javascript

window.onscroll = function() {scrollFunction()};
let didScroll = false;

function scrollFunction() {
const topButton = document.getElementById('topButton');
if (document.body.scrollTop > 120 || document.documentElement.scrollTop > 120) {
topButton.style.display = 'block';
} else {
topButton.style.display = 'none';
}
}