This is reproducible with marina b05.

I'm running the below program to show a transparent Stage.

import javafx.stage.*;
import javafx.scene.*;
import javafx.scene.shape.*;
import javafx.scene.paint.*;

Stage {
    style: StageStyle.TRANSPARENT
    scene: Scene {
        fill: Color.TRANSPARENT
        content: Rectangle {
            x: 30
            y: 30
            width: 50
            height: 60
            fill: Color.BLUE
        }
    }
}

It throws the below exception and shows up as an undecorated Stage.

Error setting window transparency using AWTUtilities
java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.sun.javafx.tk.swing.WindowImpl.setWindowTransparency(WindowImpl.java:199)
at com.sun.javafx.tk.swing.WindowStage.initializeStyle(WindowStage.java:142)
at com.sun.javafx.tk.swing.FrameStage.initializeStyle(FrameStage.java:74)
at com.sun.javafx.tk.swing.WindowStage.<init>(WindowStage.java:80)
at com.sun.javafx.tk.swing.FrameStage.<init>(FrameStage.java:51)
at com.sun.javafx.tk.swing.SwingToolkit.createTKStage$impl(SwingToolkit.fx:118)
at com.sun.javafx.tk.swing.SwingToolkit.createTKStage(SwingToolkit.fx:71)
at javafx.stage.Stage.applyDefaults$javafx$stage$Stage$impl_peer(Stage.fx:109)
at javafx.stage.Stage.initialize$(Stage.fx:108)
at Test.javafx$run$(Test.fx:6)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.sun.javafx.runtime.provider.GUIRuntimeProvider$1.run(GUIRuntimeProvider.java:66)
at com.sun.javafx.tk.swing.SwingToolkit$StartupRoutine.run$impl(SwingToolkit.fx:754)
at com.sun.javafx.tk.swing.SwingToolkit$StartupRoutine.run(SwingToolkit.fx:707)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
Caused by: java.lang.IllegalArgumentException: The effects for decorated windows are not supported.
at com.sun.awt.AWTUtilities.setWindowOpaque(AWTUtilities.java:388)
... 29 more