diff -r db324b2feaf3 glass/glass-mat-lib-gtk/src/com/sun/glass/ui/GlassApplication.cpp
--- a/glass/glass-mat-lib-gtk/src/com/sun/glass/ui/GlassApplication.cpp	Wed May 23 13:37:48 2012 +0200
+++ b/glass/glass-mat-lib-gtk/src/com/sun/glass/ui/GlassApplication.cpp	Wed May 23 14:49:27 2012 +0200
@@ -201,31 +201,44 @@
 /*
  * Class:     com_sun_glass_ui_gtk_GtkApplication
  * Method:    _runLoop
- * Signature: ([Ljava/lang/String;Lcom/sun/glass/ui/Launchable;)V
+ * Signature: ([Ljava/lang/String;Lcom/sun/glass/ui/Launchable;Z)V
  */
 JNIEXPORT void JNICALL Java_com_sun_glass_ui_gtk_GtkApplication__1runLoop
-  (JNIEnv * env, jobject obj, jobjectArray arguments, jobject launchable)
+  (JNIEnv * env, jobject obj, jobjectArray arguments, jobject launchable, jboolean embedded)
 {
     jclass jLaunchable = env->FindClass("com/sun/glass/ui/Launchable");
     jmethodID jLaunchableFinishLaunching = env->GetMethodID(jLaunchable, "finishLaunching", "([Ljava/lang/String;)V");
     env->CallVoidMethod(launchable, jLaunchableFinishLaunching, arguments);
     CHECK_JNI_EXCEPTION(env);
 
-	// GTK installs its own X error handler that conflicts with AWT.
-    // During drag and drop, AWT hides errors so we need to hide them
-    // to avoid exit()'ing.  It's not clear that we don't want to hide
-    // X error all the time, otherwise FX will exit().
-    //
-    // A better solution would be to coordinate with AWT and save and
-    // restore the X handler.
+    if (embedded) {
+        // GTK installs its own X error handler that conflicts with AWT.
+        // During drag and drop, AWT hides errors so we need to hide them
+        // to avoid exit()'ing.  It's not clear that we don't want to hide
+        // X error all the time, otherwise FX will exit().
+        //
+        // A better solution would be to coordinate with AWT and save and
+        // restore the X handler.
+        //
 
-    // Disable X error handling
-    gdk_error_trap_push();
-    
+        // Disable X error handling
+#ifndef VERBOSE
+        gdk_error_trap_push();
+#endif
+    }
     gtk_main();
     
+    //// When the last JFrame closes and DISPOSE_ON_CLOSE is specified, 
+    // Java exits with an X error. X error are hidden during the FX 
+    // event loop and should be restored when the event loop exits. Unfortunately, 
+    // this is too early. The fix is to never restore X errors. 
+    // 
+    // See RT-21408 & RT-20756
+    
     // Restore X error handling
-    gdk_error_trap_pop();
+//    if (embedded) {
+//        gdk_error_trap_pop();
+//    }
 
     gdk_threads_leave();
 
diff -r db324b2feaf3 glass/glass-mat/src/com/sun/glass/ui/gtk/GtkApplication.java
--- a/glass/glass-mat/src/com/sun/glass/ui/gtk/GtkApplication.java	Wed May 23 13:37:48 2012 +0200
+++ b/glass/glass-mat/src/com/sun/glass/ui/gtk/GtkApplication.java	Wed May 23 14:49:27 2012 +0200
@@ -49,14 +49,22 @@
     }
 
     protected void runLoop(final String args[], final Launchable launchable) {
-        boolean isEventThread = AccessController
+        final boolean isEventThread = AccessController
             .doPrivileged(new PrivilegedAction<Boolean>() {
                 public Boolean run() {
-                    Boolean result = Boolean.getBoolean("javafx.embed.isEventThread");
-                    return result == null ? false : result.booleanValue();
+                    // Embedded in SWT, with shared event thread
+                    return Boolean.getBoolean("javafx.embed.isEventThread");
                 }
             });
-
+        
+        final boolean isEmbedded = AccessController
+            .doPrivileged(new PrivilegedAction<Boolean>() {
+                public Boolean run() {
+                    // Embedded in Swing
+                    return Boolean.getBoolean("javafx.macosx.embedded");
+                }
+            });
+        
         if (isEventThread) {
             init();
             setEventThread(Thread.currentThread());
@@ -71,7 +79,7 @@
                 Thread th = new Thread(new Runnable() {
                     public void run() {
                         init();
-                        _runLoop(args, launchable);
+                        _runLoop(args, launchable, isEmbedded);
                     }
                 }, "GtkNativeMainLoopThread");
 
@@ -100,7 +108,7 @@
     
     private native void _init();
     
-    protected native void _runLoop(String[] args, Launchable launchable);
+    protected native void _runLoop(String[] args, Launchable launchable, boolean embedded);
 
     @Override
     protected native void _invokeAndWait(Runnable runnable);