Show a button after scrolling down and scroll smoothly back to the top when it is clicked.

HTML

<button class="lotgrafix-back-to-top" aria-label="Back to top" hidden>&uarr;</button>

CSS

.lotgrafix-back-to-top {
    position: fixed;
    right: 20px;
    bottom: 20px;
    z-index: 999;
}

main.js

var btn = document.querySelector( '.lotgrafix-back-to-top' );

if ( btn ) {
    window.addEventListener( 'scroll', function () {
        btn.hidden = window.scrollY < 400;
    }, { passive: true } );

    btn.addEventListener( 'click', function () {
        window.scrollTo( { top: 0, behavior: 'smooth' } );
    } );
}