JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
    <div id="left">
        <div id="floater"></div>
    </div>
    <div id="right">
    </div>
</div>

CSS

#container {
    height: 200px; width: 200px;
    position: relative;
    background-color: #ccc; border: solid 5px black;
}
#left {
    position: absolute;
    top: 0; left: 0; bottom: 0; width: 100px;
    overflow-x: visible;
    overflow-y: auto;    /** REMOVING THIS CHANGES THE RESULT **/
    background-color: blue;
    z-index: 2;
}
#right {
    position: absolute;
    top: 0; right: 0; bottom: 0; width: 100px;
    z-index: 1;
    background-color: green;
}
#floater {
    position: absolute;
    right: -20px; top: 30px; height: 40px; width: 40px;
    background-color: red;
}

JavaScript

http://jsfiddle.net/#save