Move after mouse wheel

[Cristisor]

Hi. I have a div with the z-index 9999 that should move up or down if
I use the scroll. On Safari it leaves a trail on the screen, I don't
understand why, because I hide the div before setting the new
coordinates and I show it after.
    box.setStyle('display', 'none');
    switch (delta) {
        case -1:
            box.style.top = (document.body.clientHeight / 2) -
(box.clientHeight / 2) + document.body.scrollTop + 75;
            break;
        case 1:
            box.style.top = (document.body.clientHeight / 2) -
(box.clientHeight / 2) + document.body.scrollTop - 75;
            break;
    }
        box.setStyle('display', 'block');