JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main">
    <div id="conversation">
        <div class="header">
             <h3>TĂ­tulo</h3>
        </div>
        <div class="content">
            <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>
</div>

CSS

body, html {
    height: 150%
}

#conversation {
    width: 400px;
    border: 1px solid #eee;
    height: 400px;
    overflow-x: hidden;
    margin-top: 200px;
}
.header {
    background: #4ECDC4;
    color: #fff;
    text-align: center;
    height: 40px;
    position: fixed;
    width: 385px;
}
.content {
    margin-top:35px;
}
ul {
    margin: 0;
    padding: 0;
}
li {
    list-style: none;
    padding: 20px 10px;
    border-bottom: 1px solid #eee;
}

JavaScript

var header = $('#conversation .header');
var mainPosition = $('.main').position();
$(document).on('scroll', function () {
    header.css('top', -$(document).scrollTop() + mainPosition.top + 'px');
});