For MQTT.fx I wanted to use ToggleButtons to e.g. choose the de
coding of a MQTT Message or the QoS Level:

 

I found out that in context of a ToggleGroup ToggleButtons behave different than RadioButtons in terms of selection/deselection: unlike RadioButtons ToggleButtons can still be set to unselected state.

 

A RadioButton extends ToggleButton and overrides fire() (which is invoked when a user gesture indicates that an event for this ButtonBase should occur aka “Button was clicked”):
RadioButton:

@Override public void fire() {
  // we don't toggle from selected to not selected if part of a group
  if (getToggleGroup() == null || !isSelected()) {
    super.fire();
  }
}

ToggleButton:

@Override public void fire() {
  setSelected(!isSelected());
  fireEvent(new ActionEvent());
}

In a ToogleGroup ToggleButtons should behave like RadioButtons, so IMHO this is a bug worth a pull request for ToggleButton 😉

One way to handle this is for sure to create a custom extension of ToggleButton implementing fire() in respect to the RadioButton.
But I like more to add behavior to existing controls.
This is my tweak to modify default behavior by adding filters to all ToogleButtons of a ToggleGroup consuming unwanted MouseEvents:

public class JavaFXUtil {

    private static JavaFXUtil me;

    private JavaFXUtil() {
    }

    public static JavaFXUtil get() {
        if (me == null) {
            me = new JavaFXUtil();
        }
        return me;
    }

    public EventHandler<MouseEvent> consumeMouseEventfilter = (MouseEvent mouseEvent) -> {
        if (((Toggle) mouseEvent.getSource()).isSelected()) {
            mouseEvent.consume();
        }
    };

    public void addAlwaysOneSelectedSupport(final ToggleGroup toggleGroup) {
        toggleGroup.getToggles().addListener((Change<? extends Toggle> c) -> {
            while (c.next()) {
                for (final Toggle addedToggle : c.getAddedSubList()) {
                    addConsumeMouseEventfilter(addedToggle);
                }
            }
        });
        toggleGroup.getToggles().forEach(t -> {
            addConsumeMouseEventfilter(t);
        });
    }

    private void addConsumeMouseEventfilter(Toggle toggle) {
        ((ToggleButton) toggle).addEventFilter(MouseEvent.MOUSE_PRESSED, consumeMouseEventfilter);
        ((ToggleButton) toggle).addEventFilter(MouseEvent.MOUSE_RELEASED, consumeMouseEventfilter);
        ((ToggleButton) toggle).addEventFilter(MouseEvent.MOUSE_CLICKED, consumeMouseEventfilter);
    }

}

public class ButtonDemoController {
    @FXML
    private ToggleGroup g2;

    @FXML
    private ToggleGroup g3;

    public void initialize() {
        JavaFXUtil.get().addAlwaysOneSelectedSupport(g2);
        JavaFXUtil.get().addAlwaysOneSelectedSupport(g3);
    }

}

Example code at GitHub.