JavaFX: memoryleak for initial nodes of Window

A DESCRIPTION OF THE PROBLEM :
While working on an update for JPro, I've found a memory-leak in JavaFX.
It's a regression and doesn't happen on JavaFX8

The initial nodes of a Scene are not collected by the GC when they are removed from the Scene.
They get collected when the initial Window is closed, but this is not the expected behavior.

Affected JavaFX versions: 9 10 11
How to reproduce: My pull request will contain a simple UnitTest.

I've got a fix ready, I'm now creating a pull request.

REGRESSION : Last worked in version 8u181

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
How to reproduce: My pull request will contain a simple UnitTest.


FREQUENCY : always