What do you mean it's not possible?

It's not possible to do without violating one of the conditions I listed. Of the solutions people have posted on SO and on blogs they either will be pushed off the screen when there's too much content (violating #2), will be pulled up to under the last rendered element if there's not enough content (#1), or will occlude content if you use absolution positioning (#3). This forces you to use an empty element of the same size as the footer, which has absolution positioning, which ends up being the element which scrolls under the footer (#4). This is what google podcasts did iirc, but it's just a hack. You'd think there'd be a simple way without having to make sacrificial elements on your page.