JSFiddle - React, Tailwind, and code Playground

HTML

<div class="main">
    <div class="left">
        <div class="box">
            <div class="popup"></div>
        </div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
    </div>
    <div class="right">
        <div class="box">
            <div class="popup"></div>
        </div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
        <div class="box"></div>
    </div>
</div>

CSS

.main, html, body {
    height: 98%;
}
.left {
    width: 40%;
    height: 100%;
    background: #f5f5f5;
    position: relative;
    overflow: scroll;    
    float: left;
}
.right {
    width: 40%;
    height: 100%;
    background: #c0c0c0;
    position: relative;
    overflow: scroll;
    float: left;
}
.box {
    position: relative;
    width: 100px;
    height: 50px;
    margin: 20px;
    background: red;
    display: inline-block;
}
.popup {
    position: absolute;
    height: 100px;
    width: 300px;
    z-index: 9999;
}

.left .popup {
    left: 80px;
    top: 20px;
    background: blue;
    z-index: 1;
}
.right .popup {
    left: -140px;
    top: 100px;
    background: orange;
}