JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://code.jquery.com/ui/jquery-ui-git.css">
<script src="http://code.jquery.com/ui/jquery-ui-git.js"></script>
<div class="container">
    <div class="block blue" id="blueBlock"></div>
    <div class="block red" id="redBlock">
        <div class="content">
            <button id="showUIPositionBtn">Show UI Position Block</button> 
        </div>     
    </div>
</div>

<div id="uiPosition"></div>

CSS

body {
    margin: 0;
    padding: 0;
}
.container {
    width: 100%;
    overflow: hidden;
}
.blue {
    background: #5AB7DE;
    float: left;
    width: 100px;
}
.red {
    background: #E52121;
    float: left;
    overflow-x: auto;
    overflow-y: hidden;
}
.content {
    width: 2000px;
    height: 400px;
}
#uiPosition {
    width: 100px;
    height: 100px;
    background: #ccc;
    position: absolute;
    top: 0;
    left: 0;
    display: none;
}
#showUIPositionBtn {
    display: block;
    width: 200px;
    margin: 40px 20px 0;
    text-align: center;
    border: 1px solid #000;
}

JavaScript

var _setSize_ = function () {
    $('#redBlock').css({
        width: $(window).width() - $('#blueBlock').width()
    });
    $('.block').css({
        height: $(window).height()
    });
}
_setSize_();
$(window).resize(function (e) {
    _setSize_();
});

$('#showUIPositionBtn').on('click', function (e) {
    $('#uiPosition')
        .show()
        .position({
            my: 'left top',
            at: 'left bottom',
            of: e.target,
            within: $('#redBlock'),
            collision: 'flip'
        });
});