JavaFX Dialogs center on owner https://github.com/marcojakob/javafx-ui-sandbox/blob/master/javafx-dialogs/src/javafx/scene/control/Dialogs.java#L501

gistfile1.java

private static DialogResponse showDialog(DialogTemplate template) {
        try {

            final FXDialog dialog = template.getDialog();
            Window window = dialog.getOwner();

            // get center of window
            final double windowCenterX = window.getX() + (window.getWidth() / 2);
            final double windowCenterY = window.getY() + (window.getHeight() / 2);

            // verify
            if(!Double.isNaN(windowCenterX)){
                // set a temp position
                dialog.setX(windowCenterX);
                dialog.setY(windowCenterY);

                // Since the dialog doesn't have a width or height till it's shown, calculate its position after it's shown
                Platform.runLater(new Runnable() {
                    @Override
                    public void run() {
                        dialog.setX(windowCenterX - (dialog.getWidth() / 2));
                        dialog.setY(windowCenterY - (dialog.getHeight() / 2));
                    }
                });
            }
            else {
                template.getDialog().centerOnScreen();
            }

            template.show();
            return template.getResponse();
        } catch (Throwable e) {
            return CLOSED;
        }
    }