JSFiddle - React, Tailwind, and code Playground

HTML

<h2>Estranho quadrado branco no canto da Scrollbar - <a href="http://truquesedicascss.blogspot.pt/2015/08/remover-quadrado-branco-canto-scrollbar.html" target="_blank">Truques &amp; Dicas CSS</a></h2>
<br />

<div id="container">
    <div class="conteudo">
body {
    background: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/3/blurry-blue.jpg);
    background-size: cover;
    padding: 30px;
}
        
section {
    background: #eee;
    max-width: 600px;
    margin: 0 auto;
    padding: 20px;
    overflow: hidden;
}

.module {
    width: 48%;
    min-height: 200px;
    background: white;
    position: relative;
    float: left;
    padding: 20px;
    margin-right: 4%;
    margin-bottom: 4%;
    box-shadow: 0 1px 3px rgba(black, 0.2);
}
    </div>
</div>

CSS

#container ::-webkit-scrollbar-corner {
    background-color: red;
}
#container {
    width: 450px;
    height: 250px;
    padding: 8px 6px;
    background-color: #4679BD;
    overflow: auto;
}
.conteudo {
    width: 95%;
    padding: 8px;
    margin: 0 auto;
    background-color: #23241f;
    color: #fff;
    overflow: auto;
    max-height: 230px;
    white-space: pre;
    word-wrap: normal;
}