When a top-level Window such as a Frame is dragged from one screen to another of a multi-monitor system, it's GraphicsConfiguration is updated to reflect the new device it is on. Under XAWT, this updating doesn't happen. The test below demonstrates this. Run it and drag the Frame to various screens and dump the GC. You'll notice it isn't updated.
// Get the GraphicsConfiguration of a Frame

import java.awt.*;
import java.awt.event.*;

public class GetFrameGC extends Frame implements ActionListener {
    public GetFrameGC() {
        super("GetFrameGC");
        Button b = new Button("getGC");
        b.addActionListener(this);
        add(b);
        setSize(100, 100);
    }
    public void actionPerformed(ActionEvent e) {
        GraphicsConfiguration gc = getGraphicsConfiguration();
        System.out.println("gc:" + gc + gc.getBounds());
    }
    public static void main(String[] args) {
        GetFrameGC f = new GetFrameGC();
        f.setVisible(true);
    }
}