A DESCRIPTION OF THE PROBLEM :
When I set the background color for a Tooltip, the pixels on the right edge do not match the color I set. Here is the code to reproduce the issue:
```
public class App extends Application {
    @Override
    public void start(Stage stage) throws Exception {
        Tooltip tooltip = new Tooltip("This is a tooltip.");
        tooltip.getStyleClass().clear();
        tooltip.setStyle("-fx-background-color: black; -fx-text-fill: white;");

        Button button = new Button("Hover me");
        button.setTooltip(tooltip);

        Scene scene = new Scene(button);
        scene.setFill(Color.WHITE);

        stage.setScene(scene);
        stage.setWidth(600);
        stage.setHeight(600);
        stage.show();
    }
}
```


FREQUENCY : always