An IllegalComponentStateException is often thrown when switching
between lightweight and heavyweight popup menu of JComboBox:
Exception occurred during event dispatching:
java.awt.IllegalComponentStateException: component must be showing on the screen to determine its location
        at java.awt.Component.getLocationOnScreen(Compiled Code)
        at java.awt.LightweightDispatcher.eventDispatched(Compiled Code)
        at java.awt.Toolkit$SelectiveAWTEventListener.eventDispatched(Compiled Code)
        at java.awt.Toolkit$ToolkitEventMulticaster.eventDispatched(Compiled Code)
        at java.awt.Toolkit.notifyAWTEventListeners(Compiled Code)
        at java.awt.Component.dispatchEventImpl(Compiled Code)
        at java.awt.Container.dispatchEventImpl(Compiled Code)
        at java.awt.Component.dispatchEvent(Compiled Code)
        at java.awt.LightweightDispatcher.retargetMouseEvent(Compiled Code)
        at java.awt.LightweightDispatcher.processMouseEvent(Compiled Code)
        at java.awt.LightweightDispatcher.dispatchEvent(Compiled Code)
        at java.awt.Container.dispatchEventImpl(Compiled Code)
        at java.awt.Window.dispatchEventImpl(Compiled Code)
        at java.awt.Component.dispatchEvent(Compiled Code)
        at java.awt.EventQueue.dispatchEvent(Compiled Code)
        at java.awt.EventDispatchThread.run(Compiled Code)



To recreate this, bring up the following application and do the
following:
- Pop up the combo box, select an item
- De-select the "LightWeightPopup enabled" checkbox
- Pop up the combo box, select an item
- Select the "LightWeightPopup enabled" checkbox
- Pop up the combo box, and attempt to use the scrollbar of the
  popup menu

This causes the IllegalComponentStateException to be thrown.

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/*
 * JComboBoxTest.java
 * IllegalComponentStateException is often thrown when switching
 * between lightweight and heavyweight popup menu for JComboBox
 *
*/

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class JComboBoxTest extends JFrame implements ActionListener {
    JComboBox combobox;
    JCheckBox lwpopup;
    JPanel infoPanel = new JPanel();
    JPanel p1 = new JPanel();
    JPanel p2 = new JPanel();

    public JComboBoxTest() {
        Container c = getContentPane();
c.setLayout(new BorderLayout());
infoPanel.setLayout(new GridLayout(0,1));
        infoPanel.add(new JLabel("IllegalComponentStateException is often thrown when switching"));
        infoPanel.add(new JLabel("between lightweight and heavyweight popup menu of JComboBox."));
        infoPanel.add(new JLabel("Click \"LightWeightPopup enabled\" checkbox to invoke setLightWeightPopupEnabled()."));
        infoPanel.add(new JLabel("Switch between lightweight and heavyweight popup menu a few times,"));
        infoPanel.add(new JLabel("then use the popup menu's scrollbar."));
        infoPanel.add(new JLabel("This often causes an IllegalComponentStateException."));
        c.add(BorderLayout.NORTH, infoPanel);

        combobox = new JComboBox();
        for(int i = 0; i < 25; i++) {
          combobox.addItem("Row " + i);
        }

        p1.add(combobox);
        c.add(BorderLayout.CENTER, p1);
        lwpopup = new JCheckBox("LightWeightPopup enabled");
        lwpopup.setSelected(combobox.isLightWeightPopupEnabled());
        lwpopup.setToolTipText("Invokes setLightWeightPopupEnabled().");
        lwpopup.addActionListener(this);
        p2.add(lwpopup);
        p2.setBackground(Color.pink);

        c.add(BorderLayout.SOUTH, p2);
    }

    public static void main(String argv[]) {
        JFrame frame = new JComboBoxTest();
frame.setTitle("JComboBox.setLightWeightPopupEnabled() Test");
frame.pack();
frame.setSize(700,400);
frame.setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {
if (e.getSource() == lwpopup) {
          combobox.setLightWeightPopupEnabled(lwpopup.isSelected());
}
    }
}
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