ADDITIONAL SYSTEM INFORMATION :
Windows 10 x64, Java 15.0.2, JavaFX 16, IntelliJ 2020.3

A DESCRIPTION OF THE PROBLEM :
A few days ago for some reason I don't remember I opened the folder "AppData\Local\Temp" and I found out it was flooded with files names like this "+JXF(a series of numbers).tmp". (There were almost 170,000 files for a total size of about 10 GB)
Later on I learned they are temporary files generated by JavaFX.
The problem is that they are not deleted if the JVM process is force closed. I noticed that in IntelliJ if I close the window normally, by using the window's button, the files are deleted upon exit. However, if the app is closed with IntelliJ's stop button the files are not deleted.
This is a big problem, if you don't know this problem you end up with a large part of the disk occupied by temporary files.

Side note: the same thing happens if the SceneBuilder process is killed

I think JavaFX should check if there are already temporary files leftovers and delete them before generating new ones

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Setup a little example, open the"AppData\Local\Temp" folder and start the app. Force close the app with a process manager or with the IDE stop button (tested only with IntelliJ)

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The old temporary files should always be deleted even if the process is killed
ACTUAL -
The temporary files will flood the temp folder eventually, occupying a lot of disk space

FREQUENCY : always