Header Menu appears when scrolling up

by davidxmartins

HTML

<header id="dinamic-header">
    
        <a href="index.html"><img src="images/logo.svg" alt="LOGO" class="wide-screen-logo"></a>
    
        <nav id="top-nav">
            <ul>
                <li><a href="#">Home</a></li>
                <li><a href="#">Services</a></li>
                <li><a href="#">Products</a></li>
                <li><a href="#">About</a></li>
                <li><a href="#">Contacts</a></li>
            </ul>
        </nav>
   
    
    </header>

CSS

#dinamic-header {
    background-color: #ffffff;
    width: 100%;
    height: auto;
    margin: 0;
    padding: 0 0 0 100px;
    position: fixed;
    top: 0;
    transition: top 0.5s;
    z-index: 5;
    border-bottom: 1px solid #dcdcdc;
}

/* Logo */

.wide-screen-logo {
    position: absolute;
    top: 20px;
    left: 30px;
    height: 25px;
    width: auto;
    z-index: 3;
}

/* Navegation */

#top-nav ul {
    list-style-type: none;
    display: table;
    margin: 0 auto;
    padding: 0;
    width: auto;
    height: 65px;
}

#top-nav li {
    width: auto;
    display: table-cell;
}

#top-nav li a {
    display: block;
    white-space: nowrap;
    text-align: center;
    padding: 19px 16px 16px 16px;
    margin: 0;
    text-decoration: none;
    color: black;
    font-family: 'Poppins', sans-serif;
    font-size: 18px;
    box-sizing: border-box;
    border-bottom: 3px solid white;
    -webkit-transition: all 500ms linear;
    -ms-transition: all 500ms linear;
    transition: all 500ms linear;
}

#top-nav li a:hover {
    border-bottom: 3px solid black;
    box-sizing: border-box;
}
body {
  height: 2000px;
  margin: 0;
  opacity: 0.8;
  background-image:  linear-gradient(#ddd 1px, transparent 1px), linear-gradient(to right, #ddd 1px, #fff 1px);
background-size: 40px 40px;
}

JavaScript

var prevScrollpos = window.pageYOffset;
window.onscroll = function () {
    var currentScrollPos = window.pageYOffset;
    if (prevScrollpos > currentScrollPos) {
        document.getElementById("dinamic-header").style.top = "0";
    } else {
        document.getElementById("dinamic-header").style.top = "-650px";
    }
    prevScrollpos = currentScrollPos;
}