JSFiddle - React, Tailwind, and code Playground

HTML

<div class="sidebar">
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
    <p>Sidebar block</p>
</div>
<div class="main-content">
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
    <p>Main content block</p>
</div>

CSS

.main-content {
    position: relative;
    width: 75%;
    background: #acacac;
}
.sidebar {
    position: fixed;
    top: 0;
    right: 0;
    bottom: 0;
    width: 25%;
    overflow-y: scroll;
    background: #a9e9a9;
}