A split pane inside any other pane seems not to work?

[Daniel Ziltener]

I'm trying to put a SplitPane inside a BoxPane, but this doesn't work. I
tried literally everything, from wrapping it inside another container before
putting it inside the BoxPane over manually setting the size but nothing
works. It seems like it gets a height of 0, even when manually setting it to
another value. It only seems to work when put in the Window tag as the only
component.
What am I doing wrong?