HtmlLabelBugTest.java

package net.miginfocom.examples;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.event.ActionEvent;

import javax.swing.AbstractAction;
import javax.swing.Icon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.UIManager;

import net.miginfocom.swing.MigLayout;

public class HtmlLabelBugTest {
	public static void main(String[] args) throws Exception {
		UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
		createTestFrame();
	}

	private static void createTestFrame() {
		JFrame f1 = new HtmlLabelBugFrame();
		f1.setVisible(true);
	}

	@SuppressWarnings("serial")
	private static class HtmlLabelBugFrame extends JFrame {
		private final AnimalDTO[] shuffleAnimals;
		private final HtmlLabelBugPanel bugPanel;

		private volatile int currentShuffle;

		public HtmlLabelBugFrame() {
			super(HtmlLabelBugTest.class.getSimpleName());
			this.shuffleAnimals = new AnimalDTO[] {
					new AnimalDTO("9b4c3a229ecb436bf", "very very very long string",
							"another very very very long string"),
					new AnimalDTO("995689c0b627d9621", "short s", "short n") };
			bugPanel = new HtmlLabelBugPanel();
			JButton changeButton = new JButton(new AbstractAction("Switch") {
				@Override
				public void actionPerformed(ActionEvent arg0) {
					AnimalDTO animal = shuffleAnimals[currentShuffle];
					bugPanel.setAnimal(animal);
					System.out.println(animal.toCodeString());
					currentShuffle++;
					if (currentShuffle >= shuffleAnimals.length) {
						currentShuffle = 0;
					}
				}
			});
			JPanel buttonPanel = new JPanel(new FlowLayout());
			buttonPanel.add(changeButton);

			JPanel testPanel = new JPanel(new BorderLayout());
			testPanel.add(bugPanel, BorderLayout.CENTER);
			testPanel.add(buttonPanel, BorderLayout.SOUTH);

			setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
			getContentPane().add(testPanel);
			setSize(new Dimension(720, 250));
			setLocationRelativeTo(null);

			shuffleAnimal();
		}

		private void shuffleAnimal() {
			AnimalDTO animal = shuffleAnimals[currentShuffle];
			bugPanel.setAnimal(animal);
			System.out.println(animal.toCodeString());
			System.out.println("Height: " + bugPanel.getHeight());
			currentShuffle++;
			if (currentShuffle >= shuffleAnimals.length) {
				currentShuffle = 0;
			}
		}
	}

	@SuppressWarnings("serial")
	private static class HtmlLabelBugPanel extends JPanel {
		private final JLabel bornTf;
		private final JLabel deliveredTf;
		private final JLabel animalname;
		private final JLabel animalLabel;

		public HtmlLabelBugPanel() {
			super(new MigLayout("debug, insets 20 10 20 10", "[]25[]10[]20[]10[]20[]10[]", "[]15[]5[]"));

			Icon testIcon = new Icon() {
				@Override
				public void paintIcon(Component c, Graphics g, int x, int y) {
					g.setColor(Color.RED);
					g.fillRect(x, y, this.getIconWidth(), this.getIconHeight());
				}

				@Override
				public int getIconWidth() {
					return 64;
				}

				@Override
				public int getIconHeight() {
					return 64;
				}
			};
			JLabel icon = new JLabel(testIcon);
			add(icon, "spany");
			animalLabel = new HtmlLabel("Animal:");
			animalLabel.setFont(animalLabel.getFont().deriveFont(24f));
			add(animalLabel, "spanx, split 2");
			animalname = new HtmlLabel();
			animalname.setFont(animalLabel.getFont().deriveFont(Font.BOLD));
			add(animalname, "growx, al left, wrap");
			bornTf = new HtmlLabel();
			bornTf.setFont(bornTf.getFont().deriveFont(Font.BOLD));
			deliveredTf = new HtmlLabel();
			deliveredTf.setFont(bornTf.getFont());
			JLabel bornLabel = new HtmlLabel("Born on:");
			JLabel deliveredLabel = new HtmlLabel("Delivered on:");
			add(bornLabel);
			add(bornTf);
			add(deliveredLabel);
			add(deliveredTf, "wrap");
		}

		public void setAnimal(AnimalDTO animal) {
			animalLabel.setVisible(true);
			animalname.setText(animal == null ? "" : animal.getDisplayName());
			String bornOn = animal == null ? null : animal.getDateOfBirth();
			String deliveredOn = animal == null ? null : animal.getDeliveryDate();
			if (bornOn == null) {
				bornTf.setText("?");
			} else {
				bornTf.setText(bornOn);
			}
			if (deliveredOn == null) {
				deliveredTf.setText("?");
			} else {
				deliveredTf.setText(deliveredOn);
			}
			this.repaint();
		}
	}

	@SuppressWarnings("serial")
	private static class HtmlLabel extends JLabel {
		private static final boolean WITH_HTML = true;

		public HtmlLabel(String text) {
			super(text);
		}

		@SuppressWarnings("unused")
		public HtmlLabel(Icon image) {
			super(image);
		}

		public HtmlLabel() {
			super();
		}

		@Override
		public void setText(String text) {
			if (!WITH_HTML) {
				super.setText(text);
			} else {
				super.setText("<html><p>" + text + "</p></html>");
			}
		}

		@Override
		public int getBaseline(int width, int height) {
			int origBaseLine = super.getBaseline(width, height);
			System.out.println("Input: w:" + width + ", h:" + height);
			System.out.println("Baseline: " + origBaseLine);
			return origBaseLine;
		}
	}

	private static class AnimalDTO {
		private final String name;
		private final String bornOn;
		private final String deliveredOn;

		public AnimalDTO(String name, String bornOn, String deliveredOn) {
			super();
			this.name = name;
			this.bornOn = bornOn;
			this.deliveredOn = deliveredOn;
		}

		public String getDisplayName() {
			return name;
		}

		public String getDateOfBirth() {
			return bornOn;
		}

		public String getDeliveryDate() {
			return deliveredOn;
		}

		public String toCodeString() {
			final StringBuilder sb = new StringBuilder();
			sb.append("new AnimalDTO(\"");
			sb.append(this.getDisplayName()).append("\",");
			sb.append("\"").append(this.getDateOfBirth()).append("\",");
			sb.append("\"").append(this.getDeliveryDate());
			sb.append(");");
			return sb.toString();
		}
	}
}

Since Swing has such a bad spec on these things and what to do when width and height are connected these kinds of bugs are just fix one and get two more. If you find a simple fix where it is clear that MigLayout breaks the contract, please submit a pull request and I'll look. But otherwise the effort/time is just too large for trying to circumvent Swing's shortcomings. Or just do a local fix for your version, which might be easiest.