JSFiddle - React, Tailwind, and code Playground

HTML

<section id="screen1">
    <div class="videoRecipiente">
        <video autoplay loop muted>
            <source src="http://www.w3schools.com/html/mov_bbb.mp4" type='video/mp4; codecs="avc1.42E01E, mp4a.40.2"'>
                <source src="http://www.w3schools.com/html/mov_bbb.mp4" type='video/webm; codecs="vp8, vorbis"' />
            Este video não é suportado pelo seu navegador.
            </video>
            <div class="textoSobreVideo">
            <p>Olá amigos somos a <strong>Tec Mov!</strong></p>
            </div>
    </div>
	<nav>
		<ul>
			<li><a href="#">Início</a></li>
			<li><a href="#">Sobre</a></li>
			<li><a href="#">Games</a></li>
			<li><a href="#">Eventos</a></li>
			<li><a href="#">Contactos</a></li>
		</ul>
	</nav>
</section>
	
<section id="screen2"></section>
<section id="screen3"></section>

CSS

/* Este CSS faz o video preencher a 100% o seu Recipiente */
.videoRecipiente {
    height:100%;
    overflow: hidden;
}
.videoRecipiente video {
    min-width: 100%;
    min-height: 100%;
}
.textoSobreVideo {
    color: #fff;
    position: absolute;
    bottom: 60px;
    text-align: center;
    width: 100%;
}

/* Esta class é adicionada ao fazer - scroll */
.fixed {
    position: fixed; 
    top: 0; 
    height: 70px; 
    z-index: 1;
}
body {
    color: #fff;
    font-family: 'open-sans-bold'; 
    font-size: 18px;
    text-align: center;
}
section {
    height: 88vh;
}

/* Navegação */
nav {
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 70px;
    background: #fff;
}
nav ul {
    margin: 0;
    padding: 0;
}
nav li {
    display: inline-block;
    padding: 24px 10px;
}
nav li a {
    color: #757575;
    text-transform: uppercase;
}

/* Seccões */
#screen1 {	
    background: #43b29d;
}
#screen2 {
    background: #efc94d;
}
#screen3 {
    background: #e1793d;
}

JavaScript

$(document).scroll(function() {
    var navHeight = $( window ).height() - 70;
    if ($(window).scrollTop() > navHeight) {
        $('nav').addClass('fixed');
    }
    else {
        $('nav').removeClass('fixed');
    }
});