JSFiddle - React, Tailwind, and code Playground

HTML

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

CSS

.container {
    height: 220px;
    width: 220px;
    background-color: pink;
}
.container textarea {
    width: 100%;
    height: 100%;
    background-color: rgba(255, 255, 0, 0.5);
    box-sizing: border-box;
}
.container > div {
    background-color: cyan;
}
.container .button-wrapper {
    background-color: yellow;
    height: 26px;
}
.container .textarea-wrapper {
    background-color: green;
    height: calc(100% - 26px);
}