Gtk: Undecorated stages shows a bar increasing the defined height when using an image as background

On Ubuntu 13.10, using Unity session, all undecorated stages shows a bar at the botton, increasing the defined height, when using an image as background. This bug not happens using Ubuntu 12.04, 12.10 or 13.04, or even using Ubuntu 13.10 with Gnome Classic session.

Steps to reproduce:

1- Create a sample project using FXML to define de view;
2- Define the stage as StageStyle.UNDECORATED.
3- Use an AnchorPane as the root element in FXML and define prefHeight and prefWidth with the same size as an sample image.
4- Define an image as background with the rule "-fx-background-image" in a CSS file.

FXMLDocument.fxml:

<AnchorPane id="AnchorPane" prefHeight="480" prefWidth="600" xmlns:fx="http://javafx.com/fxml">
    <stylesheets>
        <URL value="@FXMLDocument.css" />
    </stylesheets>
</AnchorPane>

FXMLDocument.css:

.root {
    -fx-background-image: url("image.png");
}

JavaFXApplication.java:

...
Parent root = FXMLLoader.load(getClass().getResource("FXMLDocument.fxml"));
Scene scene = new Scene(root);
stage.initStyle(StageStyle.UNDECORATED);
stage.setScene(scene);
stage.show();
...