I need to have a vertically centered Box with content inside, and then an optional second Box underneath. When the second box is shown, the position of the first box should not change – to clarify, the position of the first box must always be in the direct center of the viewport.

The ideal solution will solely use flexbox, without relying on things like absolute/fixed positioning, etc.

I’ve created a codesandbox: https://codesandbox.io/p/sandbox/awesome-mopsa-pd38yw