JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
    <div class="btn-wrapper">
        <button>X</button>
    </div>
    <textarea></textarea>
</div>

CSS

.container
{
    height: 220px;
    width: 220px;
    background-color: pink;

    /* http://css-tricks.com/snippets/css/a-guide-to-flexbox/ */
    display: flex;
    flex-direction: column;
}
.container textarea {
    /* set textarea width 100% width border-box */
    box-sizing: border-box;
    height: 100%;
}