JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
  <div class="full_width"></div>
</div>

CSS

body {
    margin: 0;
}

html { box-sizing: border-box; }

*, *::before, *::after {
    box-sizing: inherit;
    position: relative;
}

.parent {
    width: 30rem;
}

.full_width {
    position: absolute;
    top: 0;
    left: 0;
    width: 100vw;
    height: 100px;
    background: rgba(0, 0, 0, .4);
}