so it seems like what you want:
if pagecontent is not fullheight = place button at bottom of page.
if pagecontent is fullheigt or bigger = place button at bottom of content.
the best way to do this would be with flex boxes, unfortunitly the current atlas layouts are not made with flexbox, so you’d have to rewrite the layouts.
or just accept that the button is under your content, or in a fixxed location.
Hi! Not a CSS-Guru here.
I had a similar issue as you had when using the proposed answer. In cases where it did not work for some reason, I used the following instead. Not quite the same but works for my use case.
.bottomRight {
position: fixed;
top: 90%;
left: 90%;
transform: translate(-49.9%, -49.9%);
z-index: 1; }