When setting content text into an Alert dialog, the Label will be cut off (using ellipsis) if the text is too long (where too long is system/font/style-dependent).
 
Here is an example that demonstrates the problem:
import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Alert;
import javafx.scene.control.Alert.AlertType;
import javafx.scene.control.Menu;
import javafx.scene.control.MenuBar;
import javafx.scene.control.MenuItem;
import javafx.scene.input.KeyCombination;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class DialogCutOffBug extends Application {

public DialogCutOffBug() {
}

@Override
public void start(Stage stage) throws Exception {
stage.setTitle("Menu Sample");
Scene scene = new Scene(new VBox(), 400, 350);

MenuBar menuBar = new MenuBar();

// --- Menu File
Menu menuFile = new Menu("File");
menuBar.getMenus().addAll(menuFile);

MenuItem test = new MenuItem("Open Dialog");
test.setAccelerator(KeyCombination.keyCombination("Ctrl+O"));
test.setOnAction(new EventHandler<ActionEvent>() {
public void handle(ActionEvent t) {
Alert alert = new Alert(AlertType.INFORMATION);
alert.setTitle("Information Dialog");
alert.setHeaderText("Short");
alert.setContentText("Whenever I start to use dialogs, there is certain verboseness involved, which will make the dialog basically useless, however JavaFX should not produce ellipses here, because the Dialog text should always be displayed in Full");
alert.showAndWait();
}
});

menuFile.getItems().add(test);

((VBox) scene.getRoot()).getChildren().addAll(menuBar);

stage.setScene(scene);
stage.show();
}

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