HTML5 make empty div full viewport size

I’m trying to create an EMPTY div that takes up 100% of the viewport and that moves with the viewport (or position: fixed)

The background reason for this is so I can use the div as a full page tooltip that shows the mouse x and y positions and the tooltip follows the mouse.

How can this full page div be achieved? My many attempts have failed to create a div with any height.

I am away from my pc but can add what I’ve tried already soon.