I've just done some random checks in Firefox, IE8, IE11, and it seems that `body.scrollTop` is not equivalent to `pageYOffset` (at least not always).

MDN suggests[1] to use sth like
`window.pageYOffset  || (document.documentElement || document.body.parentNode || document.body).scrollTop`
for maximum portability so I'd stick to that.

[1] https://developer.mozilla.org/en-US/docs/Web/API/Window.scrollY#Notes