Testcase:
Run it and you can see in output, that selection is 0,24, but it become blue after minimazing/opening window (I mean, taskbar). You can see on attached movie.

import javafx.application.Application;
import javafx.beans.InvalidationListener;
import javafx.beans.Observable;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.TextField;
import javafx.stage.Stage;

public class HelloTextField extends Application {
    private Group content;

    @Override
    public void start(Stage stage) throws Exception {
        stage.setScene(new Scene(content = new Group(), 500, 500));
        stage.centerOnScreen();
        stage.show();

        final TextField label = new TextField();
        label.setPromptText("Enter Pithy Pun");
        label.setText("bbbbbbbbbbbbbbbbbbbbb");
        label.selectionProperty().addListener(new InvalidationListener() {
            public void invalidated(Observable o) {
                System.out.println(label.getSelection());
            }
        });
        label.selectAll();
        content.getChildren().add(label);
        //label.setText("aaa");
    }

    public static void main(String[] args) {
        Application.launch(HelloTextField.class, args);
    }
}