JSFiddle - React, Tailwind, and code Playground

HTML

<div id="ipsum">
    Lorem ipsum <br />
    Lorem ipsum <br />
    Lorem ipsum <br />
    Lorem ipsum <br />
    Lorem ipsum <br />
    Lorem ipsum <br />
    Lorem ipsum <br />
    Lorem ipsum
</div>
<button id="yay" disabled=true>Yay</button>

CSS

#ipsum{
    height:80px;
    width:200px;
    overflow : auto;
}

JavaScript

var i=document.getElementById("ipsum");
i.onscroll=function(){
    var y = document.getElementById("yay");
    var x = i.scrollHeight - i.offsetHeight - 1;
    if(i.scrollTop >= x)
        y.removeAttribute("disabled");
    else if(!y.hasAttribute("disabled"))
        y.setAttribute("disabled",true);
};