Why can Scene's only be constructed on the UI thread?

[Mike Hearn]

I'm trying an alternative approach. Is there a convenient way to know that
the current set of pixels has hit the screen so it's OK to temporarily
block the UI loop again after reconfiguring the UI? I could add  in manual
delays before doing Platform.runLater but that's pretty hacky.

What I'm seeing is if I do a stage.show() followed by Platform.runLater()
the pixels for the show don't hit the screen for quite a long time, so my
splash appears only when loading is nearly finished.