listen to document height changes
// create an Observer instance
const resizeObserver = new ResizeObserver(entries =>
console.log('Body height changed:', entries[0].target.clientHeight)
)
// start observing a DOM node
resizeObserver.observe(document.body)
// click anywhere to rnadomize height
window.addEventListener('click', () =>
document.body.style.height = Math.floor((Math.random() * 5000) + 1) + 'px'
)