JSFiddle - React, Tailwind, and code Playground

HTML

<div class="CONTAINER">
    <div class="BOX">
        <div class="TOOLBAR">
            Toolbar
        </div>
        <div class="STRETCH">
            <div class="TL">
                abc
            </div>
        </div>
    </div>
</div>

CSS

.CONTAINER {
    border: 1px solid red;
    height: 400px;
    width: 400px;
    position: relative;
}

.BOX {
    -moz-box-orient: vertical;
    display: -moz-box;
    height: 100%;
}

.TOOLBAR {
    -moz-box-flex: 0;
    height: 44px;
    background: blue;
}

.STRETCH {
    -moz-box-flex: 1;
    background: green;
    position: relative;
}

.TL {
    background: red;
    height: 100%;
}