FULL PRODUCT VERSION :
java version "1.8.0_91"
Java(TM) SE Runtime Environment (build 1.8.0_91-b15)
Java HotSpot(TM) 64-Bit Server VM (build 25.91-b15, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.1.7601]

A DESCRIPTION OF THE PROBLEM :
A JMenu inside a frame with GridBagLayout partly vanishes and reappears when the frame also contains a label which text is updated frequently and the text is sometimes shortened with "..." and sometimes not.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Execute test program.


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The menu remains stable.
ACTUAL -
The menu partly vanishes when the long shortened text is shown in the label at the bottom. It reappears when the short text is shown in the label.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JPanel;
import javax.swing.SwingConstants;


public class LabelDotTest
{
// private final static String longText = "nur anders";
    private final static String longText = "show a very long text to have it automatically shortened";
    private final static String shortText = "show short text";

    private JLabel label = null;
    private JMenu menu = null;
    
    public LabelDotTest()
    {
        System.out.println("BEFORE CREATION");
        JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(new Dimension(50, 150));
        frame.setLocationRelativeTo(null);

        frame.setLayout(new GridBagLayout());
        GridBagConstraints c = new GridBagConstraints();
        c.fill = GridBagConstraints.BOTH;
        c.weightx = 1.0;
        c.weighty = 0.0;
        c.gridwidth = GridBagConstraints.REMAINDER;

        JMenuBar menuBar = new JMenuBar();
        menu = new JMenu("Menu");
        menuBar.add(menu);
        frame.add(menuBar, c);
        
        frame.add(new JLabel("Title", SwingConstants.CENTER), c);

        c.weighty = 1.0;
        frame.add(new JPanel(new GridBagLayout()), c);
        c.weighty = 0.0;

        label = new JLabel(shortText);
        frame.add(label, c);

        frame.setVisible(true);

        // Thread starten, der ständig den Text im Label größer und kleiner macht
        new Thread(new Runnable()
        {
            public void run()
            {
                int i = 0;
                while ( i < 50 )
                {
                    i++;

                    if ( label.getText().equals(shortText) )
                    {
                        label.setText(longText);
                    }
                    else
                    {
                        label.setText(shortText);
                    }

                    try
                    {
                        Thread.sleep(1000);
                    }
                    catch( InterruptedException e ) { }
                }
            }
        } ).start();
    }

    public static void main(String[] args)
    {
        new LabelDotTest();
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Add the menu via frame.setJMenu(menubar) to the frame.
That requires an additional line and space is rare in my use case.