JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css">
<script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>
<div class="container">
    <div class="row">
        <div class="col-md-7">
            <div class="row">
                <div class="col-md-12">
                    <div class="myTarget">
                        <p>This is element 1 of panel left</p>
                        <p>This is element 1 of panel left</p>
                        <p>This is element 1 of panel left</p>
                        <p>This is element 1 of panel left</p>
                        <p>This is element 1 of panel left</p>
                        <p>This is element 1 of panel left</p>
                        <p>This is element 1 of panel left</p>
                        <p>This is element 1 of panel left</p>
                        <p>This is element 1 of panel left</p>
                        <div class="box" class="col-md-8 hidden-xs">
                            <p>This is the third panel of element 1 that need to be re-positioned.</p>
                        </div>
                    </div>
                    <div class="myTarget">
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                        <p>This is element 2 of panel left</p>
                    ...

CSS

.col-md-7 {
    background-color: blue;
}
.col-md-12 {
    padding-right: 0;
}
#rightPanel {
    background-color:green;
    padding-left: 0;
}
.myTarget {
    background-color: white;
    position:relative;
}
.box {
    display:none;
    z-index: 1000;
    background-color: red;
    border-color:blue;
}
.myTarget p {
}
.myTarget:hover .box {
    display: block;
    position: absolute;
    left: 100%;
    top: 0;
}

JavaScript

$('.box').each(function() {
   var newWidth = $('#rightPanel').outerWidth();
   $(this).outerWidth(newWidth); 
});