When I launch this application:

import javafx.application.Application;
import javafx.collections.FXCollections;
import javafx.collections.ObservableList;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.MenuButton;
import javafx.scene.control.MenuItem;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class JavaApplication6 extends Application {

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

    @Override
    public void start(Stage stage) throws Exception {
        final MenuButton testMenuButton = new MenuButton();

        ObservableList<MenuItem> testMenuItems = FXCollections.observableArrayList();

        for (int indx = 1; indx <= 5; indx++) {
            MenuItem mi = new MenuItem();
            mi.setText("Test Menu Item-" + indx);
            testMenuItems.add(mi);
        }

        testMenuButton.getItems().addAll(testMenuItems);
        testMenuButton.setOnMousePressed(new EventHandler<MouseEvent>() {

            @Override
            public void handle(MouseEvent t) {
                System.out.println("A");
                testMenuButton.show();
            }
        });
        testMenuButton.setOnMouseReleased(new EventHandler<MouseEvent>() {

            @Override
            public void handle(MouseEvent me) {
                System.out.println("B");
                testMenuButton.hide();
            }
        });
        
        VBox vb = new VBox();
        vb.getChildren().addAll(testMenuButton);
        Scene scene = new Scene(vb, 300, 300);
        stage.setScene(scene);
        stage.show();
    }
}


I see an output:

Unknown view event type: 423
Unknown view event type: 423
Unknown view event type: 423
Unknown view event type: 423
Unknown view event type: 423

Not reproducible with 8.0b64.