MigLayout span components overflowing from cell

Question 1

Thomas' answer works as long as the panel has a static number of rows, which unfortunately our program doesn't. To get around this I keep track of how many rows I'm going to create and then dynamically generate the row constraints at the end. (This is a bit messy but not as bad as I expected. If there's a way to avoid the row constraints I'll update the code accordingly.) I also used a dummy row between the components beside the button panel and the list and textfield at the bottom to take up the extra space keeping everything aligned properly without resizing.

import net.miginfocom.swing.MigLayout;

import javax.swing.*;
import javax.swing.border.EmptyBorder;
import java.awt.*;

public class MigLayoutTest extends JPanel
{
    private MigLayoutTest(boolean addExtraRow1, boolean addExtraRow2)
    {
        JFrame frame = new JFrame("MigLayout Test");
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setContentPane(new JScrollPane(getPage(addExtraRow1, addExtraRow2)));
        frame.getContentPane().setMinimumSize(new Dimension(650, 336));
        frame.getContentPane().setPreferredSize(new Dimension(890, 562));
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    private JPanel getPage(boolean addExtraRow1, boolean addExtraRow2)
    {
        // Create buttons panel
        JPanel listButtonPanel = new JPanel(new MigLayout("ins 0, wrap 1, aligny top"));
        JButton addBtn = new JButton("Add");
        listButtonPanel.add(addBtn, "w 105px, h 25px, sg btns");

        JButton removeBtn = new JButton("Remove");
        listButtonPanel.add(removeBtn, "sg btns");

        JButton copyBtn = new JButton("Copy");
        listButtonPanel.add(copyBtn, "sg btns");

        // Create other components
        int rowCount = 1; // make a dummy row at the bottom to push all the components beside the button panel up

        JTextField txtField = new JTextField();
        rowCount++;

        JComboBox comboBox = new JComboBox(new String[] {"cbx itm 1", "cbx itm 2", "cbx itm 3"});
        comboBox.setEditable(true);
        comboBox.setSelectedItem("");
        rowCount++;

        JCheckBox checkBox = null;
        if (addExtraRow1)
        {
            checkBox = new JCheckBox();
            rowCount++;

        }

        JTextField optTxtField = null;
        if (addExtraRow2)
        {
            optTxtField = new JTextField();
            rowCount++;
        }

        rowCount++;

        JList list = new JList(new DefaultListModel());
        list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        list.setVisibleRowCount(-1);
        String[] names = new String[] {"Stuff to", "fill this", "JList..."};
        DefaultListModel model = (DefaultListModel)list.getModel();
        for (String name : names)
            model.addElement(name);

        JTextArea textArea = new JTextArea();

        // Generate row constraints
        StringBuilder rowConstraints = new StringBuilder();
        for (int i=0; i<rowCount; i++)
            rowConstraints.append("[]");
        rowConstraints.append("[grow, fill]");

        // Create main panel and add components
        JPanel panel = new JPanel(new MigLayout("fill, wrap 3, debug", "[][][grow, fill, align left]", rowConstraints.toString()));
        panel.setBorder(new EmptyBorder(10, 10, 10, 10));

        panel.add(listButtonPanel, "spany " + --rowCount + ", aligny top, hmax 100%"); // decrement rowCount because the buttons should be above the bottom components' labels

        addComponent(panel, new JLabel("Property 1"), txtField);
        addComponent(panel, new JLabel("ComboBox Property"), comboBox);
        if (checkBox != null)
            addComponent(panel, new JLabel("Extra comp 1"), checkBox);
        if (optTxtField != null)
            addComponent(panel, new JLabel("Extra comp 2"), optTxtField);

        panel.add(new JLabel("A big JList"), "skip 3, spanx 2, grow"); // skip the dummy row before adding this
        panel.add(new JLabel("A big JTextArea"));

        panel.add(new JScrollPane(list), "hmin 100px, spanx 2, grow");
        panel.add(new JScrollPane(textArea), "hmin 100px, grow");

        return panel;
    }

    private void addComponent(JPanel panel, JLabel label, Component component)
    {
        label.setLabelFor(component);
        panel.add(label, "align right");
        panel.add(component);
    }

    public static void main(final String[] args)
    {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() { new MigLayoutTest(args.length > 1, args.length == 2); }
        });
    }
}

Question 2

I think the "split and span cell button panel" in combination with push messes the thing up. I'm sorry but I don't know why. Some optional tips:

I like to seperate declaration, initialisation and adding of all of the components. I think it is more readable and easier to maintain.
Avoid hardcoded setSize functions. I don't remember that I ever got what I wanted. I like the sizegroup contraint with MigLayout.
Do not use a variable for two object instances (e.g. scroller). It often leads to mistakes using the object references.

Here is another possible solution (I hope this still satisfies your requirements):

    private JPanel getPage() {
    JPanel panel = new JPanel(new MigLayout("wrap 3, debug", "[][][grow, fill]", "[][][][grow, fill]"));
    panel.setBorder(new EmptyBorder(10, 10, 10, 10));

    JButton addBtn, removeBtn, copyBtn;
    JTextField txtField;
    JTextArea textArea;
    JComboBox<String> comboBox;
    JList<String> list;
    JLabel property, comboboxLabel, listLabel, textAreaLabel;
    JScrollPane listScroller, textFieldScroller;

    addBtn = new JButton("Add");
    removeBtn = new JButton("Remove");
    copyBtn = new JButton("Copy");
    txtField = new JTextField();
    textArea = new JTextArea();
    property = new JLabel("Property 1");
    listLabel = new JLabel("A big JList");
    textAreaLabel = new JLabel("A big JTextArea");
    comboboxLabel = new JLabel("ComboBox Property");


    comboBox = new JComboBox<String>(new String[] { "cbx itm 1", "cbx itm 2", "cbx itm 3" });
    comboBox.setEditable(true);
    comboBox.setSelectedItem("");

    list = new JList<String>();
    list.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
    list.setVisibleRowCount(-1);

    String[] names = new String[] { "Stuff to", "fill this", "JList..." };
    DefaultListModel<String> model = new DefaultListModel<String>();
    for (String name : names) {
        model.addElement(name);
    }
    list.setModel(model);


    listScroller = new JScrollPane(list);
    textFieldScroller = new JScrollPane(textArea);

    property.setLabelFor(txtField);
    comboboxLabel.setLabelFor(comboBox);
    textAreaLabel.setLabelFor(textArea);
    listLabel.setLabelFor(list);


    panel.add(addBtn, "split 3, flowy, spany 2, sizegroup buttons");
    panel.add(removeBtn, "sizegroup buttons");
    panel.add(copyBtn, "sizegroup buttons");

    panel.add(property, "al right top, sgy components");
    panel.add(txtField, "al right top, sgy components");
    panel.add(comboboxLabel, "al right top, sgy components");
    panel.add(comboBox, "al right top, sgy components");

    panel.add(listLabel, "spanx 2, sgy components");
    panel.add(textAreaLabel, "sgy components");
    panel.add(listScroller, "spanx 2, grow");
    panel.add(textFieldScroller, "");

    return panel;
}