Revert "Narrow declared type of document.body to HTMLBodyElement | HT…

[sandersn]

…MLFrameSetElement"

This reverts commit 1f7b275.

That commit made document.body a union type, which in turns makes methods like addEventListener unions of overloaded signatures, which do not resolve correctly for the following example code, which I believe is very common:

document.body.addEventListener('click', (event) => {
    cx = event.pageX;
    cy = event.pageY;
});

With the union type, event: Event, which does not have pageX or pageY properties. After reverting this commit, event: MouseEvent, which does.

[HolgerJeromin]

Why does a click handler on a HTMLFrameSetElement does not get a MouseEvent?

[sandersn]

It does, and so one on a HTMLBodyElement. But
document.body : HTMLBodyElement | HTMLFrameSetElement,
and (HTMLBodyElement | HTMLFrameSetElement)['addEventListener']
isn't an overloaded signature like HTMLFrameSetElement['addEventListener'] is. It just has one signature with a callback that passes event: Event. So document.body.addEventListener's callback never gets anything but event: Event.