FULL PRODUCT VERSION :
$ java -version
java version "1.8.0_71"
Java(TM) SE Runtime Environment (build 1.8.0_71-b15)
Java HotSpot(TM) 64-Bit Server VM (build 25.71-b15, mixed mode)


A DESCRIPTION OF THE PROBLEM :
When assigning a mnemonic to the first Letter of the second line in multi line button text, the marking underline appears at the end of the first line.

Example text: "Eingaben _Zurücksetzen"

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

public class Test extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {
        Button b = new Button("Eingaben _Zurücksetzen");
        b.setMaxWidth(110);
        b.wrapTextProperty().setValue(true);

        VBox box = new VBox();
        Scene s = new Scene(box);
        box.getChildren().add(b);
        primaryStage.setScene(s);
        primaryStage.show();
    }

    public static void main(String[] args) {
        Application.launch(Test.class, (java.lang.String[]) null);
    }
}



REPRODUCIBILITY :
This bug can be reproduced always.