Menus: avoid that SubMenuUsabilityHelper can be installed multiple times, which can freeze the application caused pushing multiple event queues and popping wrong event queue first

(e.g. NetBeans Form Editor invokes `FlatLaf.initialize()` but not `uninitialize()`)

(PR #490; https://github.com/apache/netbeans/issues/4231)

CHANGELOG.md

@@ -7,6 +7,11 @@ FlatLaf Change Log
 
 - ComboBox: Fixed vertical alignment of text in popup list with text in combo
   box in IntelliJ/Darcula themes.
+- Menus: Fixed application freeze under very special conditions (invoking
+  `FlatLaf.initialize()` twice in NetBeans GUI builder) and using menu that has
+  submenus. See
+  [NetBeans issue #4231](https://github.com/apache/netbeans/issues/4231#issuecomment-1179611682)
+  for details.
 - MenuItem: Fixed sometimes wrapped HTML text on HiDPI screens on Windows.
 - TableHeader: Fixed exception when changing table structure (e.g. removing
   column) from a table header popup menu action. (issue #532)

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java

@@ -103,7 +103,7 @@ public abstract class FlatLaf
 
 	private PopupFactory oldPopupFactory;
 	private MnemonicHandler mnemonicHandler;
-	private SubMenuUsabilityHelper subMenuUsabilityHelper;
+	private boolean subMenuUsabilityHelperInstalled;
 
 	private Consumer<UIDefaults> postInitialization;
 	private List<Function<Object, Object>> uiDefaultsGetters;
@@ -246,8 +246,7 @@ public abstract class FlatLaf
 		mnemonicHandler.install();
 
 		// install submenu usability helper
-		subMenuUsabilityHelper = new SubMenuUsabilityHelper();
+		subMenuUsabilityHelperInstalled = SubMenuUsabilityHelper.install();
-		subMenuUsabilityHelper.install();
 
 		// listen to desktop property changes to update UI if system font or scaling changes
 		if( SystemInfo.isWindows ) {
@@ -329,9 +328,9 @@ public abstract class FlatLaf
 		}
 
 		// uninstall submenu usability helper
-		if( subMenuUsabilityHelper != null ) {
+		if( subMenuUsabilityHelperInstalled ) {
-			subMenuUsabilityHelper.uninstall();
+			SubMenuUsabilityHelper.uninstall();
-			subMenuUsabilityHelper = null;
+			subMenuUsabilityHelperInstalled = false;
 		}
 
 		// restore default link color

flatlaf-core/src/main/java/com/formdev/flatlaf/SubMenuUsabilityHelper.java

@@ -59,6 +59,11 @@ class SubMenuUsabilityHelper
 	private static final String KEY_USE_SAFE_TRIANGLE = "Menu.useSafeTriangle";
 	private static final String KEY_SHOW_SAFE_TRIANGLE = "FlatLaf.debug.menu.showSafeTriangle";
 
+	// Using a static field to ensure that there is only one instance in the system.
+	// Multiple instances would freeze the application.
+	// https://github.com/apache/netbeans/issues/4231#issuecomment-1179616607
+	private static SubMenuUsabilityHelper instance;
+
 	private SubMenuEventQueue subMenuEventQueue;
 	private SafeTrianglePainter safeTrianglePainter;
 	private boolean changePending;
@@ -74,13 +79,22 @@ class SubMenuUsabilityHelper
 
 	private Rectangle invokerBounds;
 
-	void install() {
+	static synchronized boolean install() {
-		MenuSelectionManager.defaultManager().addChangeListener( this );
+		if( instance != null )
+			return false;
+
+		instance = new SubMenuUsabilityHelper();
+		MenuSelectionManager.defaultManager().addChangeListener( instance );
+		return true;
 	}
 
-	void uninstall() {
+	static synchronized void uninstall() {
-		MenuSelectionManager.defaultManager().removeChangeListener( this );
+		if( instance == null )
-		uninstallEventQueue();
+			return;
+
+		MenuSelectionManager.defaultManager().removeChangeListener( instance );
+		instance.uninstallEventQueue();
+		instance = null;
 	}
 
 	@Override