A DESCRIPTION OF THE PROBLEM :
The promptText for the comboBox is displayed only on load. When a value is selected and cleared, the prompt text is not displayed. This used to work in JavaFX 8.

REGRESSION : Last worked in version 8

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Set promptText to a comboBox which has no value by default.
2. Select an option from the comboBox. The promptText is removed and a value is displayed.
3. Clear the value of the comboBox. (using some external call)


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The promptText should be displayed in the ComboBox.
ACTUAL -
The promptText is NOT displayed in the ComboBox.

---------- BEGIN SOURCE ----------
import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.ComboBox;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;
public class ComboBoxPromptTextIssue extends Application {
    @Override
    public void start(Stage primaryStage) {
        ComboBox<String> comboBox = new ComboBox<>();
        comboBox.setPromptText("Select Value");
        comboBox.getItems().addAll("One", "Two", "Three", "Four");

        Button clear = new Button("Clear value");
        clear.setOnAction(e->comboBox.setValue(null));
        VBox root = new VBox(comboBox,clear);
        root.setAlignment(Pos.CENTER);
        root.setSpacing(20);

        Scene scene = new Scene(root, 300, 200);
        primaryStage.setTitle("ComboBox FX " + System.getProperty("javafx.runtime.version"));
        primaryStage.setScene(scene);
        primaryStage.show();
    }
}
---------- END SOURCE ----------

FREQUENCY : always