JSFiddle - React, Tailwind, and code Playground

HTML

<div class="sidebar">
    <div class="content-a">
        content
    </div>
    bottom
</div>
<div class="main">
    <div class="content-b">
        content
    </div>
    bottom
</div>

CSS

*, *:before, *:after {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;
}

html {
    height: 100%;
}

body {
    min-height: 100%;
    position: relative;
    margin: 0;
    padding: 0;
}

.sidebar {
    position: fixed;
    max-height: 100vh;
    min-height: 100vh;
    background: lightblue;
    width: 200px;
    top: 0;
    right: 0;
    padding: 20px;
    overflow-y: scroll;
}

.content-a {
    background: yellow;
    height: 900px;
}

.main {
    background: grey;
    min-height: 100vh;
    padding: 10px;
}

.content-b {
    height: 600px;
}

body, .main {
    overflow: hidden;
}