Long task free getboundingClientRect

getboundingClientRect.js

function getBoundingClientRect(element) {
	return new Promise(resolve => {
		const observer = new IntersectionObserver(([entity]) => {
			observer.unobserve(element);
			observer.disconnect();
			resolve(entity.boundingClientRect);
		});

		observer.observe(element);
	});
}