https://www.espboards.dev/js/scroll-top.js

1(function () {
2	var btn = document.getElementById('scroll-top');
3	if (!btn) return;
4	var lastY = window.scrollY;
5	var ticking = false;
6	var hideTimer;
7
8	function show() {
9		clearTimeout(hideTimer);
10		if (btn.hasAttribute('hidden')) {
11			btn.removeAttribute('hidden');
12			requestAnimationFrame(function () { btn.classList.add('in'); });
13		}
14	}
15
16	function hide() {
17		if (!btn.hasAttribute('hidden') && btn.classList.contains('in')) {
18			btn.classList.remove('in');
19			clearTimeout(hideTimer);
20			hideTimer = setTimeout(function () { btn.setAttribute('hidden', ''); }, 200);
21		}
22	}
23
24	function update() {
25		ticking = false;
26		var y = window.scrollY;
27		var up = y < lastY - 2;
28		var down = y > lastY + 2;
29		lastY = y;
30		if (up && y > 600) show();
31		else if (down || y <= 600) hide();
32	}
33
34	function onScroll() {
35		if (!ticking) { ticking = true; requestAnimationFrame(update); }
36	}
37
38	window.addEventListener('scroll', onScroll, { passive: true });
39	btn.addEventListener('click', function () {
40		var reduce = window.matchMedia('(prefers-reduced-motion: reduce)').matches;
41		window.scrollTo({ top: 0, behavior: reduce ? 'auto' : 'smooth' });
42	});
43})();