JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="sidenav-menu">
        <app-sidenav-menu>123</app-sidenav-menu>
    </div>
    <div id="content">
        <router-outlet></router-outlet>
    </div>
</div>

CSS

#container {
    width: 100%;
    height: 100%;
}

#sidenav-menu {
    position: fixed;
    left: 0%;
    width: 25%;
    top:0;
    bottom:0;
    background: #C5420A; /* для наглядности*/
    
}

#content {
    width: 75%;
    position: fixed;
    top:0;
    right: 0%;
    height: 100%;
    background: #000; /* для наглядности*/

}