JSFiddle - React, Tailwind, and code Playground

HTML

<div class="divContainer">
    <div class="conteudoFixo">
        <h3>Objeto Fixo</h3>
    </div>
    <div class="conteudoNormal">
        <ul>
            <li>Primeira conversa</li>
            <li>Segunda conversa</li>
            <li>Terceira conversa</li>
            <li>Terceira conversa</li>
            <li>Terceira conversa</li>
            <li>Terceira conversa</li>
            <li>Terceira conversa</li>
            <li>Terceira conversa</li>
            <li>Terceira conversa</li>
            <li>Terceira conversa</li>
            <li>Terceira conversa</li>
            <li>Terceira conversa</li>
            
        </ul>
    </div>
</div>

CSS

.divContainer {
    height:200px;
    width:300px;
    position:left;
    overflow:auto;
    border: 1px solid #FF0000;
    position: relative;
}

.conteudoFixo {
    position: absolute;
    top:0px;
    left:0px;
    background-color:#FF00FF;
    width:100%;
}