For the visual tool, I need 2 buttons (run and stop) that are completely exclusive.
I use 2 ToggleButtons within a ToggleGroup.
The ToggleGroup handles the fact that we can have only 1 button selected at a time but I need the reverse as well : only 1 button unselected at a time.
So when the run button is selected and the user clicks again on the button, it needs to stay selected.

To do that, I've added the following code :
        toggleGroup.selectedToggleProperty().addListener(new ChangeListener<Toggle>() {
            public void changed(ObservableValue<? extends Toggle> observable, Toggle oldValue, Toggle newValue) {
                if (newValue == null) {
                    toggleGroup.selectToggle(oldValue);
                }
            }
        });
but it works half the time :
I click on the run button, then click on it again (OK, the button keeps selected), then click on it again (KO, the button becomes unselected).

Attached simple test to reproduce.