Suppose I have:
- two screens (Panes): screenA, screenB
- a parent pane: parentPane

If screenA is never displayed yet, if I do something like:
parentPane.getChildren().setAll(screenA); //line1
parentPane.getChildren().setAll(screenB); //line2

Obviously, on the UI, I see the outcome of line2, i.e. I see screenB displayed.

But the problem is: if later in the code, I do:
parentPane.getChildren().setAll(screenA);

screenA is no loger being displayed on the UI.

This problem doesn't occur if I initially display screenA, then repeat the above steps.