[ruslan_mart]

window.addEventListener('DOMContentLoaded', function() {
	var elem = document.querySelector('.opacTop'),
		body = document.body,
		html = document.documentElement;
		isVisible = true,
        maxHeight = 200;
 
	window.addEventListener('scroll', function() {
		var scrollHeight = (html.scrollHeight || body.scrollHeight) - this.innerHeight,
			scrollTop = html.scrollTop || body.scrollTop || 0,
			value = 1 - 1 / Math.min(scrollHeight, maxHeight) * Math.min(scrollTop, maxHeight);
		elem.style.opacity = value;
		if(isVisible != !!value) {
			elem.style.display = isVisible ? 'none' : '';
			isVisible = !isVisible;
		}
	});
});

Вот так точно работать будет.