The following class implements a chatGUI. When it runs okay the screen looks like this:

Fine ChatGUI

The problem is very often when i enter text of large length ie. 50 - 100 chars the gui goes crazy. the chat history box shrinks as shown in this image.

Any ideas regarding what is causing this?

Thank you.

package Sartre.Connect4;

import javax.swing.*;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.text.StyledDocument;
import javax.swing.text.Style;
import javax.swing.text.StyleConstants;
import javax.swing.text.BadLocationException;
import java.io.BufferedOutputStream;
import javax.swing.text.html.HTMLEditorKit;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.FileNotFoundException;
import javax.swing.filechooser.FileNameExtensionFilter;
import javax.swing.JFileChooser;


/**
 * Chat form class
 * @author iAmjad
 */
public class ChatGUI extends JDialog implements ActionListener {

/**
 * Used to hold chat history data
 */
private JTextPane textPaneHistory = new JTextPane();

/**
 * provides scrolling to chat history pane
 */
private JScrollPane scrollPaneHistory = new JScrollPane(textPaneHistory);

/**
 * used to input local message to chat history
 */
private JTextPane textPaneHome = new JTextPane();

/**
 * Provides scrolling to local chat pane
 */
private JScrollPane scrollPaneHomeText = new JScrollPane(textPaneHome);

/**
 * JLabel acting as a statusbar
 */
private JLabel statusBar = new JLabel("Ready");

/**
 * Button to clear chat history pane
 */
private JButton JBClear = new JButton("Clear");

/**
 * Button to save chat history pane
 */
private JButton JBSave = new JButton("Save");

/**
 * Holds contentPane
 */
private Container containerPane;

/**
 * Layout GridBagLayout manager
 */
private GridBagLayout gridBagLayout = new GridBagLayout();

/**
 * GridBagConstraints
 */
private GridBagConstraints constraints = new GridBagConstraints();

/**
 * Constructor for ChatGUI
 */
public ChatGUI(){

    setTitle("Chat");

    // set up dialog icon
    URL url = getClass().getResource("Resources/SartreIcon.jpg");
    ImageIcon imageIcon = new ImageIcon(url);
    Image image = imageIcon.getImage();
    this.setIconImage(image);

    this.setAlwaysOnTop(true);

    setLocationRelativeTo(this.getParent());
    //////////////// End icon and placement /////////////////////////

    // Get pane and set layout manager
    containerPane = getContentPane();
    containerPane.setLayout(gridBagLayout);
    /////////////////////////////////////////////////////////////

    //////////////// Begin Chat History //////////////////////////////

    textPaneHistory.setToolTipText("Chat History Window");
    textPaneHistory.setEditable(false);
    textPaneHistory.setPreferredSize(new Dimension(350,250));

    scrollPaneHistory.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
    scrollPaneHistory.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);

    // fill Chat History GridBagConstraints
    constraints.gridx = 0;
    constraints.gridy = 0;
    constraints.gridwidth = 10;
    constraints.gridheight = 10;
    constraints.weightx = 100;
    constraints.weighty = 100;
    constraints.fill = GridBagConstraints.BOTH;
    constraints.anchor = GridBagConstraints.CENTER;
    constraints.insets = new Insets(10,10,10,10);
    constraints.ipadx = 0;
    constraints.ipady = 0;

    gridBagLayout.setConstraints(scrollPaneHistory, constraints);

    // add to the pane
    containerPane.add(scrollPaneHistory);

    /////////////////////////////// End Chat History ///////////////////////

    ///////////////////////// Begin Home Chat //////////////////////////////

    textPaneHome.setToolTipText("Home Chat Message Window");
    textPaneHome.setPreferredSize(new Dimension(200,50));

    textPaneHome.addKeyListener(new MyKeyAdapter());

    scrollPaneHomeText.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
    scrollPaneHomeText.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);

    // fill Chat History GridBagConstraints
    constraints.gridx = 0;
    constraints.gridy = 10;
    constraints.gridwidth = 6;
    constraints.gridheight = 1;
    constraints.weightx = 100;
    constraints.weighty = 100;
    constraints.fill = GridBagConstraints.BOTH;
    constraints.anchor = GridBagConstraints.CENTER;
    constraints.insets = new Insets(10,10,10,10);
    constraints.ipadx = 0;
    constraints.ipady = 0;

    gridBagLayout.setConstraints(scrollPaneHomeText, constraints);

    // add to the pane
    containerPane.add(scrollPaneHomeText);

    ////////////////////////// End Home Chat /////////////////////////

    ///////////////////////Begin Clear Chat History ////////////////////////

    JBClear.setToolTipText("Clear Chat History");

    // fill Chat History GridBagConstraints
    constraints.gridx = 6;
    constraints.gridy = 10;
    constraints.gridwidth = 2;
    constraints.gridheight = 1;
    constraints.weightx = 100;
    constraints.weighty = 100;
    constraints.fill = GridBagConstraints.BOTH;
    constraints.anchor = GridBagConstraints.CENTER;
    constraints.insets = new Insets(10,10,10,10);
    constraints.ipadx = 0;
    constraints.ipady = 0;

    gridBagLayout.setConstraints(JBClear, constraints);

    JBClear.addActionListener(this);

    // add to the pane
    containerPane.add(JBClear);

    ///////////////// End Clear Chat History ////////////////////////

    /////////////// Begin Save Chat History //////////////////////////

    JBSave.setToolTipText("Save Chat History");

    constraints.gridx = 8;
    constraints.gridy = 10;
    constraints.gridwidth = 2;
    constraints.gridheight = 1;
    constraints.weightx = 100;
    constraints.weighty = 100;
    constraints.fill = GridBagConstraints.BOTH;
    constraints.anchor = GridBagConstraints.CENTER;
    constraints.insets = new Insets(10,10,10,10);
    constraints.ipadx = 0;
    constraints.ipady = 0;

    gridBagLayout.setConstraints(JBSave, constraints);

    JBSave.addActionListener(this);

    // add to the pane
    containerPane.add(JBSave);

    ///////////////////// End Save Chat History /////////////////////

    /////////////////// Begin Status Bar /////////////////////////////
    constraints.gridx = 0;
    constraints.gridy = 11;
    constraints.gridwidth = 10;
    constraints.gridheight = 1;
    constraints.weightx = 100;
    constraints.weighty = 50;
    constraints.fill = GridBagConstraints.BOTH;
    constraints.anchor = GridBagConstraints.CENTER;
    constraints.insets = new Insets(0,10,5,0);
    constraints.ipadx = 0;
    constraints.ipady = 0;

    gridBagLayout.setConstraints(statusBar, constraints);

    // add to the pane
    containerPane.add(statusBar);

    ////////////// End Status Bar ////////////////////////////

    // set resizable to false
    this.setResizable(false);

    // pack the GUI
    pack();
}

/**
 * Deals with necessary menu click events
 * @param event
 */
public void actionPerformed(ActionEvent event) {

    Object source = event.getSource();

    // Process Clear button event
    if (source == JBClear){

        textPaneHistory.setText(null);
        statusBar.setText("Chat History Cleared");
    }

    // Process Save button event
    if (source == JBSave){

        // process only if there is data in history pane
        if (textPaneHistory.getText().length() > 0){

            // process location where to save the chat history file
            JFileChooser chooser = new JFileChooser();

            chooser.setMultiSelectionEnabled(false);

            chooser.setAcceptAllFileFilterUsed(false);

            FileNameExtensionFilter filter = new FileNameExtensionFilter("HTML Documents", "htm", "html");

            chooser.setFileFilter(filter);

            int option = chooser.showSaveDialog(ChatGUI.this);

            if (option == JFileChooser.APPROVE_OPTION) {

                // Set up document to be parsed as HTML
                StyledDocument doc = (StyledDocument)textPaneHistory.getDocument();

                HTMLEditorKit kit = new HTMLEditorKit();

                BufferedOutputStream out;

                try {

                    // add final file name and extension
                    String filePath = chooser.getSelectedFile().getAbsoluteFile() + ".html";

                    out = new BufferedOutputStream(new FileOutputStream(filePath));

                    // write out the HTML document
                    kit.write(out, doc, doc.getStartPosition().getOffset(), doc.getLength());

                } catch (FileNotFoundException e) {

                    JOptionPane.showMessageDialog(ChatGUI.this,
                    "Application will now close. \n A restart may cure the error!\n\n"
                    + e.getMessage(),
                    "Fatal Error",
                    JOptionPane.WARNING_MESSAGE, null);

                    System.exit(2);

                } catch (IOException e){

                    JOptionPane.showMessageDialog(ChatGUI.this,
                    "Application will now close. \n A restart may cure the error!\n\n"
                    + e.getMessage(),
                    "Fatal Error",
                    JOptionPane.WARNING_MESSAGE, null);

                    System.exit(3);

                } catch (BadLocationException e){

                    JOptionPane.showMessageDialog(ChatGUI.this,
                    "Application will now close. \n A restart may cure the error!\n\n"
                    + e.getMessage(),
                    "Fatal Error",
                    JOptionPane.WARNING_MESSAGE, null);

                    System.exit(4);
                }

                statusBar.setText("Chat History Saved");
            }
        }
    }
}

/**
 * Process return key for sending the message
 */
private class MyKeyAdapter extends KeyAdapter {

    @Override
    @SuppressWarnings("static-access")
    public void keyPressed(KeyEvent ke) {

        DateTime dateTime = new DateTime();
        String nowdateTime = dateTime.getDateTime();

        int kc = ke.getKeyCode();

        if (kc == ke.VK_ENTER) {

            try {
                // Process only if there is data
                if (textPaneHome.getText().length() > 0){

                    // Add message origin formatting
                    StyledDocument doc = (StyledDocument)textPaneHistory.getDocument();

                    Style style = doc.addStyle("HomeStyle", null);

                    StyleConstants.setBold(style, true);

                    String home = "Home [" + nowdateTime + "]: ";

                    doc.insertString(doc.getLength(), home, style);

                    StyleConstants.setBold(style, false);

                    doc.insertString(doc.getLength(), textPaneHome.getText() + "\n", style);

                    // update caret location
                    textPaneHistory.setCaretPosition(doc.getLength());

                    textPaneHome.setText(null);

                    statusBar.setText("Message Sent");
                }

            } catch (BadLocationException e) {

                JOptionPane.showMessageDialog(ChatGUI.this,
                        "Application will now close. \n A restart may cure the error!\n\n"
                        + e.getMessage(),
                        "Fatal Error",
                        JOptionPane.WARNING_MESSAGE, null);

                System.exit(1);
            }
            ke.consume();
        }
    }
}

}