ADDITIONAL SYSTEM INFORMATION :
This is a very intermittent problem but could be successfully reproduced on Windows 10 using latest available versions of Java 10 and Java 11 with multiple tries and the right conditions (may help to have high background CPU load).


A DESCRIPTION OF THE PROBLEM :
Call to JPanel.pack() from thread main gets stuck waiting on HTMLDocument while AWT-EventQueue-0] is stuck waiting for: Component$AWTTreeLock , which is owned by thread main.

com.test.Main at localhost:50095 (Suspended)
Thread [main] (Suspended)
owns: BoxLayout (id=24)
owns: Component$AWTTreeLock (id=25)
waited by: Thread [AWT-EventQueue-0] (Suspended)
waiting for: HTMLDocument (id=23)
Object.wait(long) line: not available [native method]
HTMLDocument(Object).wait() line: 328
HTMLDocument(AbstractDocument).readLock() line: 1398
BasicEditorPaneUI(BasicTextUI).getMinimumSize(JComponent) line: 980
JEditorPane(JComponent).getMinimumSize() line: 1760
BoxLayout.checkRequests() line: 479
BoxLayout.layoutContainer(Container) line: 421
JPanel(Container).layout() line: 1537
JPanel(Container).doLayout() line: 1526
JPanel(Container).validateTree() line: 1722
JPanel(Container).validateTree() line: 1731
JPanel(Container).validateTree() line: 1731
JLayeredPane(Container).validateTree() line: 1731
JRootPane(Container).validateTree() line: 1731
Main$testThreadLockPanel(Container).validateTree() line: 1731
Main$testThreadLockPanel(Container).validate() line: 1657
Main$testThreadLockPanel(Container).validateUnconditionally() line: 1694
Main$testThreadLockPanel(Window).pack() line: 835
Main$testThreadLockPanel.<init>(Main) line: 123
Main.<init>() line: 48
Main.main(String[]) line: 37
Daemon System Thread [Reference Handler] (Suspended)
Reference<T>.waitForReferencePendingList() line: not available [native method]
Reference<T>.processPendingReferences() line: 241
Reference$ReferenceHandler.run() line: 213
Daemon System Thread [Finalizer] (Suspended)
waiting for: ReferenceQueue$Lock (id=26)
Object.wait(long) line: not available [native method]
ReferenceQueue<T>.remove(long) line: 155
ReferenceQueue<T>.remove() line: 176
Finalizer$FinalizerThread.run() line: 170
Daemon System Thread [Signal Dispatcher] (Suspended)
Daemon System Thread [Attach Listener] (Suspended)
Daemon System Thread [Common-Cleaner] (Suspended)
waiting for: ReferenceQueue$Lock (id=51)
Object.wait(long) line: not available [native method]
ReferenceQueue<T>.remove(long) line: 155
CleanerImpl.run() line: 148
InnocuousThread(Thread).run() line: 834
InnocuousThread.run() line: 134
Daemon System Thread [Java2D Disposer] (Suspended)
waiting for: ReferenceQueue$Lock (id=52)
Object.wait(long) line: not available [native method]
ReferenceQueue<T>.remove(long) line: 155
ReferenceQueue<T>.remove() line: 176
Disposer.run() line: 144
Thread.run() line: 834
System Thread [AWT-Shutdown] (Suspended)
waiting for: Object (id=53)
Object.wait(long) line: not available [native method]
Object.wait() line: 328
AWTAutoShutdown.run() line: 291
Thread.run() line: 834
Daemon System Thread [AWT-Windows] (Suspended)
WToolkit.eventLoop() line: not available [native method]
WToolkit.run() line: 303
Thread.run() line: 834
Thread [AWT-EventQueue-0] (Suspended)
waiting for: Component$AWTTreeLock (id=25)
JEditorPane(Component).invalidate() line: 3016
JEditorPane(Container).invalidate() line: 1614
JEditorPane(JComponent).revalidate() line: 4891
BasicTextUI$RootView.preferenceChanged(View, boolean, boolean) line: 1505
BlockView(View).preferenceChanged(View, boolean, boolean) line: 287
BlockView(BoxView).preferenceChanged(View, boolean, boolean) line: 287
HTMLEditorKit$HTMLFactory$BodyBlockView(View).preferenceChanged(View, boolean, boolean) line: 287
HTMLEditorKit$HTMLFactory$BodyBlockView(BoxView).preferenceChanged(View, boolean, boolean) line: 287
ParagraphView(View).preferenceChanged(View, boolean, boolean) line: 287
ParagraphView(BoxView).preferenceChanged(View, boolean, boolean) line: 287
FlowView$LogicalView(View).preferenceChanged(View, boolean, boolean) line: 287
BRView(View).preferenceChanged(View, boolean, boolean) line: 287
BRView(GlyphView).changedUpdate(DocumentEvent, Shape, ViewFactory) line: 955
BRView(LabelView).changedUpdate(DocumentEvent, Shape, ViewFactory) line: 304
BRView(InlineView).changedUpdate(DocumentEvent, Shape, ViewFactory) line: 93
FlowView$LogicalView(View).forwardUpdateToView(View, DocumentEvent, Shape, ViewFactory) line: 1241
FlowView$LogicalView.forwardUpdateToView(View, DocumentEvent, Shape, ViewFactory) line: 811
FlowView$LogicalView(View).forwardUpdate(DocumentEvent$ElementChange, DocumentEvent, Shape, ViewFactory) line: 1172
FlowView$LogicalView.forwardUpdate(DocumentEvent$ElementChange, DocumentEvent, Shape, ViewFactory) line: 821
FlowView$LogicalView(View).changedUpdate(DocumentEvent, Shape, ViewFactory) line: 797
ParagraphView(FlowView).changedUpdate(DocumentEvent, Shape, ViewFactory) line: 291
ParagraphView(ParagraphView).changedUpdate(DocumentEvent, Shape, ViewFactory) line: 781
HTMLEditorKit$HTMLFactory$BodyBlockView(View).forwardUpdateToView(View, DocumentEvent, Shape, ViewFactory) line: 1241
HTMLEditorKit$HTMLFactory$BodyBlockView(View).forwardUpdate(DocumentEvent$ElementChange, DocumentEvent, Shape, ViewFactory) line: 1172
HTMLEditorKit$HTMLFactory$BodyBlockView(BoxView).forwardUpdate(DocumentEvent$ElementChange, DocumentEvent, Shape, ViewFactory) line: 241
HTMLEditorKit$HTMLFactory$BodyBlockView(View).changedUpdate(DocumentEvent, Shape, ViewFactory) line: 797
HTMLEditorKit$HTMLFactory$BodyBlockView(BlockView).changedUpdate(DocumentEvent, Shape, ViewFactory) line: 341
BlockView(View).forwardUpdateToView(View, DocumentEvent, Shape, ViewFactory) line: 1241
BlockView(View).forwardUpdate(DocumentEvent$ElementChange, DocumentEvent, Shape, ViewFactory) line: 1172
BlockView(BoxView).forwardUpdate(DocumentEvent$ElementChange, DocumentEvent, Shape, ViewFactory) line: 241
BlockView(View).changedUpdate(DocumentEvent, Shape, ViewFactory) line: 797
BlockView.changedUpdate(DocumentEvent, Shape, ViewFactory) line: 341
BasicTextUI$RootView.changedUpdate(DocumentEvent, Shape, ViewFactory) line: 1737
BasicTextUI$UpdateHandler.changedUpdate(DocumentEvent) line: 1998
HTMLDocument(AbstractDocument).fireChangedUpdate(DocumentEvent) line: 232
HTMLDocument.fireChangedUpdate(DocumentEvent) line: 1763
DefaultStyledDocument$ChangeUpdateRunnable.run() line: 2761
InvocationEvent.dispatch() line: 313
EventQueue.dispatchEventImpl(AWTEvent, Object) line: 770
EventQueue$4.run() line: 721
EventQueue$4.run() line: 715
AccessController.doPrivileged(PrivilegedAction<T>, AccessControlContext) line: not available [native method]
ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(PrivilegedAction<T>, AccessControlContext, AccessControlContext) line: 85
EventQueue.dispatchEvent(AWTEvent) line: 740
EventDispatchThread.pumpOneEventForFilters(int) line: 203
EventDispatchThread.pumpEventsForFilter(int, Conditional, EventFilter) line: 124
EventDispatchThread.pumpEventsForHierarchy(int, Conditional, Component) line: 113
EventDispatchThread.pumpEvents(int, Conditional) line: 109
EventDispatchThread.pumpEvents(Conditional) line: 101
EventDispatchThread.run() line: 90


REGRESSION : Last worked in version 11

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
This issue is very sensitive to system configuration and system performance level. It may to be easier to reproduce on slower CPUs, or on CPUs with lots of background CPU load.

1 - Load example source code into Eclipse IDE (Java Photon)
2 - Click Debug button to start program in debug session. If program window appears after launch, problem did not reproduce. If this happens, try clicking on Eclipse debug button several times (up to 5-10) in quick succession to launch multiple simultaneous instantiations of program.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
One program window should appear for each attempted launch of the program.

ACTUAL -
Some program launches may result in main thread and AWT Event Queue thread being locked and no window appearing.

---------- BEGIN SOURCE ----------
package com.test;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.WindowEvent;
import java.text.NumberFormat;
import javax.swing.BorderFactory;
import javax.swing.Box;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JEditorPane;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.UIManager;
import javax.swing.border.EtchedBorder;
import javax.swing.event.ChangeEvent;
import javax.swing.event.ChangeListener;

public class Main extends JFrame {

private testThreadLockPanel tstPanel;
public static void main(String[] args) {
new Main();
}
  
public Main() {

//for (int i=0; i<0; i++) {
// KeepCPUBusyThread t = new KeepCPUBusyThread();
// t.start();
//}
//try { Thread.sleep(1000); } catch (InterruptedException e) {}

tstPanel = new testThreadLockPanel();

}

//class KeepCPUBusyThread extends Thread {
// public void run() {
// float a;
// for (int k = 0; k < 2; k++)
// for (int j = 0; j < 500000000; j++)
// for (int i = 0; i < 500000000; i++)
// a = (i*j)/1000;
// }
//}

private class testThreadLockPanel extends JFrame implements ActionListener {

private JTextField displayScaleField = new JTextField(5);

public testThreadLockPanel() {

this.setTitle("Test Panel");
setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
setBackground(new Color (217, 227, 232));

JPanel topPanel = new JPanel();
topPanel.setBackground(new Color (217, 227, 232));
topPanel.setOpaque(true);

JPanel boxPanel = new JPanel();
boxPanel.setOpaque(false);
boxPanel.setLayout(new BoxLayout(boxPanel, BoxLayout.Y_AXIS));
boxPanel.setBorder(BorderFactory.createCompoundBorder(BorderFactory
.createEmptyBorder(0, 0, 0, 0), BorderFactory
.createEtchedBorder(EtchedBorder.LOWERED)));

JPanel headerPanel = new JPanel();
headerPanel.setLayout(new FlowLayout(FlowLayout.LEFT));
headerPanel.setBackground(new Color(171, 194, 211));
headerPanel.setOpaque(true);
JLabel text = new JLabel("Configure Display Scaling:", JLabel.LEFT);
text.setFont(text.getFont().deriveFont(12.0f).deriveFont(Font.BOLD));
headerPanel.add(text);
boxPanel.add(headerPanel);

NumberFormat nf = NumberFormat.getInstance(); // otherwise it's a decimal number eg 1.2
nf.setMinimumFractionDigits(1);
String text2 = "Display scaling can be used to increase the size of program windows and fonts for improved readability on high resolution displays. <br><br>By default, the scale is set automatically based on the display resolution. To override this behavior, enter a value below between " + nf.format(1.0) + " and " + nf.format(3.0) + ". To restore the default behavior, leave this field blank.";
JEditorPane scalingMsgArea = new JEditorPane("text/html", text2);
scalingMsgArea.setEditable(false);
scalingMsgArea.putClientProperty(JEditorPane.HONOR_DISPLAY_PROPERTIES, Boolean.TRUE);
scalingMsgArea.setFont(UIManager.getFont("Label.font").deriveFont(12.0f).deriveFont(Font.PLAIN));
scalingMsgArea.setOpaque(false);
scalingMsgArea.setBorder(
BorderFactory.createEmptyBorder(25, 20, 10, 20));

// uncomment sleep to let object setup complete, which prevents thread lock
//try { Thread.sleep(50); } catch (InterruptedException e) { e.printStackTrace(); }

boxPanel.add(scalingMsgArea);


JPanel footerPanel = new JPanel();
footerPanel.setLayout(new FlowLayout(FlowLayout.LEFT));
footerPanel.setBorder(
BorderFactory.createEmptyBorder(10,20,10,20));
footerPanel.setOpaque(false);
JLabel text3 = new JLabel("Note: Changes do not take effect until the program is restarted.", JLabel.LEFT);
text3.setFont(text3.getFont().deriveFont(12.0f).deriveFont(Font.PLAIN));
footerPanel.add(text3);
boxPanel.add(footerPanel);

topPanel.add(boxPanel);
boxPanel.setPreferredSize(new Dimension(520, 330));
this.add(topPanel);
this.pack();
this.setLocationRelativeTo(null);
this.setVisible(true);
}

protected void processWindowEvent(WindowEvent e) {
if (e.getID() == WindowEvent.WINDOW_CLOSING) {
System.exit(0);
}
}
public void actionPerformed(ActionEvent evt) { }
}
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Adding a Thread.sleep(50) after instantiation of JEditorPane object and before calling pack() seems to make it impossible to reproduce.

FREQUENCY : often