I have the following containment hierarchy:Frame-->JPanel-->JButtons. The buttons have a tooltip for each of them.
 
Platform in which occurs: Solaris Sparc; does not occur in Windows.

% java -version
java version "1.4.0-internal"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.0-internal-b57)
Java HotSpot(TM) Client VM (build 1.4.0-beta-b57, mixed mode)

Steps to reproduce:
1. Run the following program.
2. Focus mouse pointer on one of the buttons until the tooltip appears.
3. Move the mouse clock/anti-clockwise over the buttons, staying in each button very briefly instead of continuously moving.
4. The app will lose focus after moving 2 buttons this way and will gain & lose focus alternately.

Code used:
import java.awt.*;
import javax.swing.*;

public class testHeavyPop {
  Frame heavy_pop;
  JPanel panel;
  JButton btn1, btn2, btn3, btn4;
  
  public testHeavyPop() {
    heavy_pop = new Frame("Heavy-weight Popup Test");
    heavy_pop.setSize(300,300);
    
    panel = new JPanel();
    panel.setLayout(new GridLayout(2,2));
    heavy_pop.add(panel);
    
    btn1 = new JButton("Button 1");
    btn1.setToolTipText("I am tooltip over button 1!");
    panel.add(btn1);
    
    btn2 = new JButton("Button 2");
    btn2.setToolTipText("I am tooltip over button 2!");
    panel.add(btn2);
    
    btn3 = new JButton("Button 3");
    btn3.setToolTipText("I am tooltip over button 3!");
    panel.add(btn3);
    
    btn4 = new JButton("Button 4");
    btn4.setToolTipText("I am tooltip over button 4!");
    panel.add(btn4);
    
    heavy_pop.show();
  }
  
  public static void main(String args[]) {
    testHeavyPop t = new testHeavyPop();
  }
}