Tooltip of tab cannot be set of FXApplicationThread for tabs not belonging to the scene graph. Effective stack is:

java.lang.IllegalStateException: Not on FX application thread; currentThread = Default RequestProcessor
at com.sun.javafx.tk.Toolkit.checkFxUserThread(Toolkit.java:237)
at com.sun.javafx.tk.quantum.QuantumToolkit.checkFxUserThread(QuantumToolkit.java:397)
at javafx.scene.Scene.<init>(Scene.java:287)
at javafx.scene.Scene.<init>(Scene.java:195)
at javafx.stage.PopupWindow.<init>(PopupWindow.java:109)
at javafx.scene.control.PopupControl.<init>(PopupControl.java:99)
at javafx.scene.control.Tooltip.<init>(Tooltip.java:143)
at org.vmfx.vm.ui.spi.Utilities$1.run(Utilities.java:48)

This indicates that tooltips in general cannot be created off the FXApplicationThread.