JSFiddle - React, Tailwind, and code Playground

HTML

<div class="header">
</div>
<div class="site">
    <div class="container">
        <div class="container-head">
        </div>
        <div class="container-body">
            <div class="right-sidebar">
                Just a menu will be right here.
            </div>
            <div class="left-content">
                <ul>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                    <li></li>
                </ul>
            </div>
        </div>
    </div>
</div>

CSS

body {
    font-family: Tahoma
}

.header {
    background-color: #49c98e;
    width: 100%;
    height: 50px;
}

.site {
    padding: 1.5em 0;
}

.container {
    background-color: #c7c7c7;
    width: 95%;
    margin: 0 auto;
    position: relative;
}

.right-sidebar {
    background-color: #a94cd5;
    width: 300px;
    height: 100%;
    position: absolute;
    right: 0;
}
.container-body{
    height: 600px;
}
.left-content {
    background-color: #dbdbdb;
    width: 100%;
    height: 80%;
    overflow-y: scroll;
}

.left-content ul {
    padding: 25px 0;
    float: left;
}

.left-content ul li {
    background-color: #a94cd5;
    width: 150px;
    height: 100px;
    float: left;
    margin: 15px;
    list-style: none;
}

JavaScript

function calculateResponsiveWidth() {
    $realWidth = $(".container").width() - $(".right-sidebar").outerWidth(),
    $containerContent = $(".left-content");

    $containerContent.css({
        "width": $realWidth
    });
}

$(window).on("resize", function () {
    calculateResponsiveWidth();
});

calculateResponsiveWidth();