call __mg_join_all_message_threads()

src/kernel/init-lite.c

@@ -557,6 +557,11 @@ int InitGUI (int argc, const char* agr[])
 
 void TerminateGUI (int rcByGUI)
 {
+#ifdef _MGHAVE_VIRTUAL_WINDOW
+    /* Since 5.0.0 */
+    __mg_join_all_message_threads ();
+#endif
+
     mg_DestroyFreeQMSGList ();
     mg_TerminateAccel ();
     mg_TerminateControlClass ();

src/kernel/init.c

@@ -590,6 +590,9 @@ void GUIAPI TerminateGUI (int not_used)
 {
     __mg_enter_terminategui = 1;
 
+    /* Since 5.0.0 */
+    __mg_join_all_message_threads ();
+
     pthread_join (__mg_desktop, NULL);
 
     /* DesktopProc() will set __mg_quiting_stage to _MG_QUITING_STAGE_EVENT */