The problem was that this was all done in the single thread and the information about layout was not yet known... But using the Platform.runLater(...) I was able to solve the problem.
At the end it is pretty elegant when you know how to solve it...
public void showDialog(final Dialog dialog) {
dialogStack.add(dialog);
dialogCanvas.toFront();
dialog.toFront();
dialogCanvas.setVisible(true);
dialog.setVisible(true);
getChildren().add(dialog);
dialog.widthProperty().addListener(new ChangeListener<Number>() {
@Override
public void changed(ObservableValue<? extends Number> observable, Number oldValue, Number newValue) {
if(newValue.doubleValue() > 0) {
centerDialog(dialog);
}
}
});
}
public void centerDialog(final Dialog dialog) {
Platform.runLater(new Runnable() {
@Override
public void run() {
double width = getWidth();
double height = getHeight();
dialog.setLayoutX(width / 2 - dialog.getWidth() / 2);
dialog.setLayoutY(height / 2 - dialog.getHeight() / 2);
}
});
}