top: 50% (absolute)
top: 50vh (absolute)
top: 50% (fixed)
Mobile addressbar causes body.height=100% to be less than body.height=100vh.
A page with body.height=100% would fit on the screen including the addressbar.
A page with body.height=100vh would fit on the screen when the addressbar is hidden.
This behaviour is not exactly reproducable on a desktop browser.
This page fakes the behaviour by reducing body.height to 90vh. So any element with height=100% is smaller than an element with height=100vh.
Also an fixed element with top: 50% will be below an absolute element with top: 50% since fixed elements are related to the visable height.