JSFiddle - React, Tailwind, and code Playground

HTML

<div>Блок</div>

CSS

body {
        height: 3000px;
    }
    div {
        width: 100%;
        height: 100px;
        background-color: #333;
        color: #fff;
        text-align: center;
    }

JavaScript

let block = document.querySelector('div');
    window.addEventListener('scroll', function() {
        if (block.offsetHeight <= window.pageYOffset) {
            block.style.top = '0'
            block.style.position = 'fixed'
        } else {
            block.style.removeProperty('top')
            block.style.removeProperty('position')
        }
    });