JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="outer">
        <div class="inner"></div>
    </div>
    <div class="show-up">this needs to show up ALL 300 pixels high of it</div>
</div>

CSS

.container {
    position:fixed;
    top:30px;
    left:50px;
    position: fixed;
    top: 30px;
    left: 50px;
    height: 30px;
    width: 300px;
    background: red;
}

.outer {
    overflow:hidden;
}

.inner {
    position: absolute;
}

.show-up {
    width: 100px;
    height: 300px;
    background: green;
    position: relative;
    margin-left:20px;
}