BODY has become the container (or parent) in the DOM?
I’d have to say “yes” mostly…
fixed not only takes it out of the flow but it positions / and sizes it directly in relation to the viewport.
Clearly it’s severing all connection to it’s original parent.
In fact, insofar as the ‘fixing’ goes we know that it doesn’t matter where the element is in the source..so it seems logical that, in fact, it’s parent is now effectively the viewport…not the body.
As for a solution to your issue, I can’t think of a CSS method.