Using IntersectionObserver to detect element with 100vh is fully visible

I need to immediately block scroll as soon as a div is fully visible.
The tricky part is that .stopDiv has 100vh height.
I tried using IntersectionObserver to detect it.

    const stopDiv = document.querySelector('.stopDiv');

    const observer = new IntersectionObserver(([e]) => {
        window.addEventListener('mousewheel', defaultScrollHandler, { passive: false });
        window.addEventListener('wheel', defaultScrollHandler, { passive: false });
        window.addEventListener('DOMMouseScroll', defaultScrollHandler, { passive: false });
    },
    {threshold: 1}
    );

    observer.observe(stopDiv);

    function defaultScrollHandler(e) {
        e.preventDefault();

        return false;
    }

The problem is the event is never called, unless the scroll perfectly stop when the div top is exactly on the beginning of the viewport (very low chance). How can I manage it?