See attached image. Comment out setPlaceholder call in the sample below and the ellipsis goes away.

    @Override
    public void start(Stage stage) throws Exception {

        ComboBox emptyPromptComboBox = new ComboBox();
        emptyPromptComboBox.setPromptText("Prompting you here");
        emptyPromptComboBox.setPlaceholder(new Label("There are no options available!"));

        StackPane root = new StackPane();
        root.getChildren().add(emptyPromptComboBox);

        Scene scene = new Scene(root, 400, 300);
        stage.setScene(scene);
        stage.show();
    }