So what You really want to do is “stick” an element (button, rect, etc.) at a particular place in the window when it is resized or scrolled - is this correct? If so this is different than a responsive environment - but does employ aspects of a flexible layout.
Checkout the red ellipse which I have centered and set 50 px above the bottom of the page no matter the scroll or window resize… is this the type of expression You are looking for?
sticky element - bottomPin_JHSv2.hype.zip (23.6 KB)
Live Demo here.