To reproduce, run the example below and click the button to open an alert - it's implemented to have a onShown handler which checks the value of the alert's showingProperty

- expected: true
- actual: false

Happens only if opening the dialog with showAndWait.

public class DialogOnShown extends Application {

    @Override
    public void start(Stage primaryStage) {
        Button btn = new Button();
        btn.setText("Say 'Hello World'");
        btn.setOnAction(e -> {
            Alert alert = new Alert(Alert.AlertType.INFORMATION);
            alert.setHeaderText("This is an alert!");
            alert.setContentText("hello again!");
            alert.initOwner(primaryStage);
            alert.setOnShown(shown -> {
                System.out.println("alert must be showing:" + alert.isShowing());
            });
            alert.showAndWait();
        });

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

        Scene scene = new Scene(root, 300, 250);

        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

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

}

The issue bubbled up at SO https://stackoverflow.com/q/52062291/203657 in a use-case to size the dialog after it has been shown. A hack around is to listen to the showing property directly and do stuff as needed. On the other hand: that's exactly what the event handler hook is intended for. There must be a way to fire the event at the correct time, even though the dialog blocks.