how do I center javascript css popup div, no matter what the screen resolution?

Question

I have the following code that opens a new popup window while disabling the background, the problem is that I have to position this so that it\'s 100px from the top (already got

Answer 1

This is where flexbox comes rescue now!

.parent {
  display: flex;
  height: 300px; /* Or whatever */
}

.child {
  width: 100px;  /* Or whatever */
  height: 100px; /* Or whatever */
  margin: auto;  /* Magic! */
}