JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://rawgit.com/janpaepke/ScrollMagic/master/js/jquery.scrollmagic.js"></script>
<div>Eu estou aqui!!</div>

CSS

div {
    position: absolute;
    top: 1000px;
    transition: all .5s;
    font-size: 1em;
}
body {
    height: 2000px;
}
div.mostrar{
    font-size: 3em;
    color: #558;
}

JavaScript

var div = document.querySelector('div');
var pos = $(div).position().top;
var alturaTela = window.innerHeight
$(document).scroll(function () {
    var posicaoScroll = $(document).scrollTop();
    var ativar = pos < posicaoScroll + (alturaTela/2);
    
    if (ativar) $(div).addClass('mostrar');
    else $(div).removeClass('mostrar');
});