JSFiddle - React, Tailwind, and code Playground

HTML

<div id="a">
    <div id="b">
        b b b b b b b b bb
        b b b b b b b b b b b
        b b b b b b b b bb b 
        b b b b b b b b b b
    </div>
</div>

CSS

* { box-sizing: border-box; }
#a {
    width: 40px;
    max-height: 70vh;
    border: 1px solid blue;
}
#b {
    width:100%;
    max-height: calc(100% - 30px);
    margin-top: 30px;
    overflow-y: scroll;
    border: 1px solid red;
}