What you have here is a horrifically outdated methodology with a significant flaw.

Absolute positioned elements are removed from flow. Thus the parent element has no idea how tall your inner DIV is. This means that if the window is shorter than your inner DIV, it will chop off the top and you' won't be able to scroll up to see the top of it!

Complex, fragile, and silly given that these days we can just display:flex the parent and margin:auto; the child.