JSFiddle - React, Tailwind, and code Playground

HTML

<div id='mydiv'><div class="inner">vnnv<br>lsn<br>vsdl<br>vnl<br>sdnvlksdnv<br>sdv<br>vnnv<br>lsn<br>vsdl<br>vnl<br>sdnvlksdnv<br>sdv<br>sdvsdvdsv<br>lsn<br>vsdl<br>lsn<br>vsdl<br>vnl<br>sdnvlksdnv<br>sdv<br>vnnv<br>lsn<br>vsdl<br>vnl<br>sdnvlksdnv<br>sdv<br>sdvsdvdsv<br>lsn<br>vsdl<br>lsn<br>vsdl<br>vnl<br>sdnvlksdnv<br>sdv<br>vnnv<br>lsn<br>vsdl<br>vnl<br>sdnvlksdnv<br>sdv<br>sdvsdvdsv<br>lsn<br>vsdl</div></div>
<div class="outer" style="height:1000px;width:100%">
123
</div>

CSS

#mydiv{
  position: fixed;
 width:100px ; 
    height :90%;
        border : 1px solid black ; 
    overflow : auto;
    background:#fff;
}

.inner{
  background: #ccc;
}
.outer {
  background: linear-gradient(135deg, #f0b7a1 0%,#8c3310 50%,#752201 51%,#bf6e4e 100%);
}

JavaScript

//alert( "mydiv: " + $('#mydiv').height() + " ;inner: "+$('.inner').height() );