JSFiddle - React, Tailwind, and code Playground

HTML

<header></header>
<div id="platformContainer">
    
    <section id="platformContent">
        <div id="platformBody">        
             <ul class="mainList">
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li class="mainStrip">Main Strip</li>
                 <li...

CSS

header{
     background: lavender;
     position: fixed;
     width: 100%;
     height: 70px;
     z-index: 999;
}

#platformContainer{
    position: relative;
    top: 70px;
    width: 100%;   
}

#platformContainer #platformContent{
    float: left; 
    width: 100%;
    display: block;
}

#platformContainer #platformContent #platformBody{
    margin-right: 200px;
}

#platformContainer #platformContent #platformBody ul{
    
    
}

#platformContainer #platformContent #platformBody ul li{
    height: 55px;
    width: 100%;
    background: #49a1e5;
}





#platformContainer #toolBarContainer{
    float: left;
    width: 200px;
    margin-left: -200px;
    margin-bottom: 70px;
    position: relative;
    background: #2266A8;
}

#platformContainer #toolBarContainer #toolBarContent{
    height: 100%;
    position: fixed;
    width: 200px;
}

#platformContainer #toolBarContainer #toolBarContent ul#toolBarList{
    height: 200px;
    overflow-y: auto;
    background: #2266A8;
}

#platformContainer #toolBarContainer #toolBarContent ul#toolBarList li{
    width: 100%;
    height:40px;
    display: block;
    background: #2266A8;
    
}

#platformContainer footer{
    clear: left;
    height: 70px;
    width: 100%;
    
    background: #2266A8;
    
}