JSFiddle - React, Tailwind, and code Playground

HTML

<section class="viewbox">
    <section class="menubox open">
        <ul>
            <li>this width can change</li>
            <li>depending on the content</li>
            <li>and some javascript actions</li>
            <li>increasing and reducing it's sice</li>
        </ul>
    </section>

    <section class="contentbox">
        <header>
            xx
        </header>
        <section class="content">
            <p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</p>
            <p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.</p>
            <p>Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing...

CSS

*, *:before, *:after {
    box-sizing: inherit;
}

body {
    padding: 0;
    margin: 0;
}

.viewbox {
    display: flex;
    height: 100vh;
}

.menubox {
    flex: 1 0 content;
    background: gray;
    overflow-y: scroll;
    padding: 20px;
}

.contentbox {
    flex: 1 1 auto;
    width: 100%;
    border: solid 5px red;
    display: flex;
    flex-direction: column;
}
.contentbox header {
    background-color: orange;
    border: 5px solid green;
}
.content {
    padding: 10px;
    overflow: auto;
}