JScrollPane with scrollbar as needed packs to the wrong size.

Question

Hi!
I just noticed a strange problem when packing a JScrollPane with scrollbar policy "as needed" and wondered if it's me or if it is a real bug in Swing.
I create a JFrame with a JScrollPane in it and a JPanel as viewportview. The JPanel (called "stuffPanel") contains a few components which give it a preferred size, and the JScrollPane is set to grow as needed if the window grows. A button in the center of the stuffPanel will invoke pack() on the window when clicked.
Now, if I resize the window such that the JScrollPane can't display the entire stuffPanel, I get scrollbars as expected. If I click the pack button the window correctly resizes to the preferred size (without scrollbars).
On the other hand, if I resize the window to become larger than the preferred size and then click the pack button, the window (and the stuffPanel) is resized to include 17 pixels of extra size around the edges.
Put simply, pack() will resize the JPanel to different sizes depending on whether the scrollbars of the JScrollPane were visible at the time. Note that this only occurs when you're using the "scrollbars as needed" scrollbar policy.
An even more interesting situation occurs if you resize the window to include (for example) only horizontal scrollbars and then packs the window repeatedly...
Code for a Java app which displays the strange behaviour is included below:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class TestJScrollPane {
  static {
    try {
      UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
    catch (Exception e) { }
  private JFrame mainWindow;
  public TestJScrollPane() {
    mainWindow = createMainWindow();
    mainWindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    mainWindow.pack();
    mainWindow.show();
  public JFrame createMainWindow() {
    JFrame window = new JFrame("Test of scrollpane");
    window.getContentPane().setLayout(new GridBagLayout());
    final JPanel stuffPanel = new JPanel();
    stuffPanel.setBorder(BorderFactory.createTitledBorder("Collection of stuff"));
    stuffPanel.setLayout(new GridBagLayout());
    JLabel stuffLabel = new JLabel("A label");
    JTextField stuffField = new JTextField(10);
    JButton stuffButton = new JButton("Pack the window");
    JList listOfStuff = new JList();
    listOfStuff.setListData(new String[] {"Fish", "Bird", "Mammal", "Insect", "Spider"});
    GridBagConstraints gbc = new GridBagConstraints();
    gbc.gridx = 0;
    gbc.gridy = 0;
    stuffPanel.add(stuffLabel, gbc);
    gbc.gridx = 1;
    stuffPanel.add(stuffField, gbc);
    gbc.gridy = 1;
    gbc.gridx = 0;
    gbc.gridwidth = 2;
    stuffPanel.add(stuffButton, gbc);
    gbc.gridy = 2;
    gbc.fill = GridBagConstraints.BOTH;
    stuffPanel.add(listOfStuff, gbc);
    final JScrollPane scroller = new JScrollPane(JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED, JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
    scroller.setViewportView(stuffPanel);
    gbc = new GridBagConstraints();
    gbc.fill = GridBagConstraints.BOTH;
    gbc.weightx = 1;
    gbc.weighty = 1;
    window.getContentPane().add(scroller, gbc);
    stuffButton.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        mainWindow.pack();
        System.out.println("Stuff panel size: " + stuffPanel.getSize());
        System.out.println("Stuff panel preferred size:" + stuffPanel.getPreferredSize());
        System.out.println("Scroller size: " + scroller.getSize());
        System.out.println("Scroller preferred size: " + scroller.getPreferredSize());
    return window;
  public static void main(String[] args) {
    new TestJScrollPane();
}

Answer 1

Why would the JScrollPane be as big as the entire
frame when you added 800x600 panels on all sides of
it? It's going to be in the center surrounded by
huge panels. Thanks, kablair. However, if you look at the code, these panels are not 800x600 on all sides. The north and south panels only add a combined height of 100, and the east and west panels only add a combined width of 100. so the whole window, minus insets, should be 900x700.
Anyway, i took the advice of the guy above, and that has gotten me much closer. In the bigger app that I'm trying to calculate the size, now I'm only 2 pixels away from the true size, although I can't figure out what I'm forgetting.