ADDITIONAL SYSTEM INFORMATION :
* OpenJDK 11.0.2
* Redhat 6.8 and Windows 7 64bit. Confident this is agnostic of OS

A DESCRIPTION OF THE PROBLEM :
When the skin of a control is changed the old skin is not garbage collected.

I've traced this to strong reference in an event listener

Specifically com.sun.javafx.scene.control.skin.BehaviorSkinBase adds a listener for context menu request in its constructor. i.e. this line

control.addEventHandler(ContextMenuEvent.CONTEXT_MENU_REQUESTED, contextMenuHandler);

However this listener is never removed in dispose() 

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the test application
Monitor number of skin instances with JVisualVM or other profiler

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Only single instance of skin object, unused instances should be garbage collected
ACTUAL -
Constant increase in number of skin objects (i.e. they're not being garbage collected)

---------- BEGIN SOURCE ----------
import com.sun.javafx.scene.control.skin.ButtonSkin;

import javafx.animation.Animation;
import javafx.animation.KeyFrame;
import javafx.animation.Timeline;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.stage.Stage;
import javafx.util.Duration;

public class SkinMemoryLeak extends Application {

    public static class ButtonSkinCustom1 extends ButtonSkin {
        public ButtonSkinCustom1(Button button) {
            super(button);
        }
    }

    public static class ButtonSkinCustom2 extends ButtonSkin {
        public ButtonSkinCustom2(Button button) {
            super(button);
        }
    }

    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage primaryStage) throws Exception {
        Button button = new Button("Foo");
        Scene scene = new Scene(button);
        primaryStage.setScene(scene);
        primaryStage.show();
        Timeline animation = new Timeline();
        animation.getKeyFrames().add(new KeyFrame(Duration.seconds(1), event -> {
            if (button.getSkin() instanceof ButtonSkinCustom2) {
                button.setSkin(new ButtonSkinCustom1(button));
            } else {
                button.setSkin(new ButtonSkinCustom2(button));
            }
        }));
        animation.setCycleCount(Animation.INDEFINITE);
        animation.play();
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Can be monkey patched by following in affected skin

@Override
public void dispose() {
    try {
        Field field = BehaviorSkinBase.class.getDeclaredField("contextMenuHandler");
        field.setAccessible(true);
        EventHandler handler = (EventHandler) field.get(this);
        getSkinnable().removeEventHandler(ContextMenuEvent.CONTEXT_MENU_REQUESTED, handler);
    } catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {
        e.printStackTrace();
    }
    super.dispose();
}

FREQUENCY : always