JSFiddle - React, Tailwind, and code Playground

HTML

<div class="toolkit">
    <div class="toolkit-head">
        HEADER ...
    </div>
    <div class="split-view">
        <div id="panel1" class="split-panel resize panel-25">...</div>
        <div id="panel2" class="split-panel">...</div>
    </div>
    <div class="toolkit-footer">
        FOOTER ...
    </div>
</div>

CSS

html, body {
    height: 100%;
    margin: 0;
    padding: 0;
}

#panel1 {
    padding: 10px;
    background-color: gray;
}

#panel2 {
    padding: 10px;
    background-color: lightgray;
}

.toolkit {
    height: 100%; /* ??? */
    position: relative;
}

.toolkit-head {
    width: 100%;
    height: 60px; /* Nicht zwingend fest! */
    background-color: blue;
}

.toolkit-footer {
    width: 100%;
    height: 80px;
    background-color: blue;
}

.split-view {
    display: table;
    width: 100%;
    height: 100%; /* ??? */
    clear: both;
}

.split-view > .split-panel { 
     margin: 0; padding: 0;
     display: table-cell;
}

.panel-25{ width: 15%; }

.resize { 
    resize: horizontal;
    overflow: auto;
}