A DESCRIPTION OF THE PROBLEM :
Explanation:
The most relevant class for this issue is com.sun.javafx.tk.quantum.StageWindow.
When the bug happens, a StageWindow gets registered in the static HashMap platformWindows but newer get's removed.
Usually, the HashMaps gets cleared, when StageWindow.closed is called.
But when StageWindow.setPlatformWindowClosed gets called, which only happens on a WindowEvent.DESTROY,
then the reference platfromWindow get nulled, but the hashmap platformWindows never gets cleaned up.


Solution:
StageWindow.setPlatformWindowClosed must be changed, to cleanup platformWindows.
This change shouldn't cause any problems, because platformWindows is only used in PaintCollecter.
This usage contains a null-check and not rendering the destroyed window shouldn't have any effect.
I will provide a pull request for it when the bug is in the bug tracker.

REGRESSION : Last worked in version 8u181

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Reproduce:
The bug can be reproduced, by creating a WindowEvent.DESTROY event, but I don't know how to create such an event.

I found the memory leak inside a heap dump from a server running JPro (https://jpro.one).
After 600 opened Stages, the memory leak happened twice.

It's save to assume that this bug also happens on the desktop.
It might happen much more often on a specific OS or applications.


FREQUENCY : rarely