I want to display Progressbar into Combobox. I tested this code with Java 8b119

import javafx.application.Application;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;
import javafx.scene.control.ListCell;
import javafx.scene.control.ListView;
import javafx.scene.control.ProgressBar;
import javafx.scene.layout.HBox;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
import javafx.util.Callback;

public class JavaFXApplication4 extends Application
{

    @Override
    public void start(Stage primaryStage)
    {

        double y1 = 15;
        ProgressBar p1 = new ProgressBar();
        p1.setLayoutY(y1);
        HBox vb1 = new HBox(10);
        vb1.getChildren().addAll(new Label("Progressbar 1"), p1);

        double y2 = 15;
        ProgressBar p2 = new ProgressBar();
        p2.setLayoutY(y2);
        HBox vb2 = new HBox(10);
        vb2.getChildren().addAll(new Label("Progressbar 2"), p2);

        double y3 = 15;
        ProgressBar p3 = new ProgressBar();
        p3.setLayoutY(y3);
        HBox vb3 = new HBox(10);
        vb3.getChildren().addAll(new Label("Progressbar 3"), p3);

        TextChooser textChooser = new TextChooser(
            vb1, vb2, vb3
        );

        textChooser.setStyle("-fx-font: 10px \"Verdana\";");

        StackPane root = new StackPane();
        root.getChildren().add(textChooser);

        Scene scene = new Scene(root, 300, 250);

        primaryStage.setTitle("Hello World!");
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    public static class TextChooser extends StackPane
    {
        private Label label = new Label();

        private ComboBox<HBox> combo = new ComboBox<>();

        public TextChooser(HBox... options)
        {
            StackPane.setAlignment(label, Pos.CENTER_LEFT);
            StackPane.setAlignment(combo, Pos.CENTER_LEFT);

            label.graphicProperty().bind(
                //combo.getSelectionModel().selectedItemProperty()
                combo.getSelectionModel().selectedItemProperty()
            );
            label.visibleProperty().bind(
                combo.visibleProperty().not()
            );
            //label.setPadding(new Insets(0, 0, 0, 10));

            combo.getItems().setAll(options);

            //!!!!!!!!!!!!!!!!! combo.setButtonCell(null);

            combo.setCellFactory(new Callback<ListView<HBox>, ListCell<HBox>>()
            {
                @Override
                public ListCell<HBox> call(ListView<HBox> p)
                {
                    return new ListCell<HBox>()
                    {
                        @Override
                        protected void updateItem(HBox item, boolean empty)
                        {
                            super.updateItem(item, empty);

                            if (item == null || empty)
                            {
                                setGraphic(null);
                            }
                            else
                            {
                                setGraphic(item);
                            }
                        }
                    };
                }
            });

            combo.getSelectionModel().select(0);
            combo.setVisible(true);

            label.setOnMouseEntered(event -> combo.setVisible(true));
            combo.showingProperty().addListener(observable ->
            {
                if (!combo.isShowing())
                {
                    combo.setVisible(false);
                }
            });
            combo.setOnMouseExited(event ->
            {
                if (!combo.isShowing())
                {
                    combo.setVisible(false);
                }
            });

            getChildren().setAll(label, combo);
        }

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

I also tested the same code with JavaFX 2.2:

import javafx.application.Application;
import javafx.beans.InvalidationListener;
import javafx.beans.Observable;
import javafx.event.EventHandler;
import javafx.geometry.Pos;
import javafx.scene.Scene;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;
import javafx.scene.control.ListCell;
import javafx.scene.control.ListView;
import javafx.scene.control.ProgressBar;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.HBox;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
import javafx.util.Callback;

public class JavaFXApplication5 extends Application
{
public static void main( String[] args )
{
launch( args );
}

@Override
public void start( Stage primaryStage )
{
double y1 = 15;
ProgressBar p1 = new ProgressBar();
p1.setLayoutY( y1 );
HBox vb1 = new HBox( 10 );
vb1.getChildren().addAll( new Label( "Progressbar 1" ), p1 );

double y2 = 15;
ProgressBar p2 = new ProgressBar();
p2.setLayoutY( y2 );
HBox vb2 = new HBox( 10 );
vb2.getChildren().addAll( new Label( "Progressbar 2" ), p2 );

double y3 = 15;
ProgressBar p3 = new ProgressBar();
p3.setLayoutY( y3 );
HBox vb3 = new HBox( 10 );
vb3.getChildren().addAll( new Label( "Progressbar 3" ), p3 );

TextChooser textChooser = new TextChooser( vb1, vb2, vb3 );

textChooser.setStyle( "-fx-font: 10px \"Verdana\";" );

StackPane root = new StackPane();
root.getChildren().add( textChooser );

Scene scene = new Scene( root, 300, 250 );

primaryStage.setTitle( "Hello World!" );
primaryStage.setScene( scene );
primaryStage.show();
}

public static class TextChooser extends StackPane
{
private final Label label = new Label();

private final ComboBox<HBox> combo = new ComboBox<>();

public TextChooser(HBox... options)
{
StackPane.setAlignment( label, Pos.CENTER_LEFT );
StackPane.setAlignment( combo, Pos.CENTER_LEFT );

label.graphicProperty().bind(
// combo.getSelectionModel().selectedItemProperty()
combo.getSelectionModel().selectedItemProperty()
);
label.visibleProperty().bind(
combo.visibleProperty().not()
);
// label.setPadding(new Insets(0, 0, 0, 10));

combo.getItems().setAll( options );

combo.setCellFactory( new Callback<ListView<HBox>, ListCell<HBox>>()
{
@Override
public ListCell<HBox> call( ListView<HBox> p )
{
return new ListCell<HBox>()
{
@Override
protected void updateItem( HBox item, boolean empty )
{
super.updateItem( item, empty );

if ( item == null || empty )
{
setGraphic( null );
}
else
{
setGraphic( item );
}
}
};
}
} );

combo.getSelectionModel().select( 0 );
combo.setVisible( true );

label.setOnMouseEntered( new EventHandler<MouseEvent>()
{
@Override
public void handle( MouseEvent event )
{
combo.setVisible( true );
}
} );

combo.showingProperty().addListener( new InvalidationListener()
{
@Override
public void invalidated( Observable observable )
{
if ( !combo.isShowing() )
{
combo.setVisible( false );
}
}
} );

combo.setOnMouseExited( new EventHandler<MouseEvent>()
{
@Override
public void handle( MouseEvent event )
{
if ( !combo.isShowing() )
{
combo.setVisible( false );
}
}
} );

getChildren().setAll( label, combo );
}

}

}

I cannot display any Progressbar into the Combobox label but this works well in JavaFX 2.2. Maybe this is a bug into Java 8.