Add test and fix

Add a test for the behavior described in issue 4092, and fix the
reference counting logic so that the reference count is decremented from
even from one to zero.  This ensures that the reference count is correct
even if the desktop instance is not design-initialized and so is not
released by Design.__exit__.

_unittest/test_01_Design.py

@@ -410,3 +410,12 @@ def test_40_get_design_settings(self, add_app):
         assert "AmbRadTemp" in design_settings_dict
         assert "GravityVec" in design_settings_dict
         assert "GravityDir" in design_settings_dict
+
+    def test_41_desktop_reference_counting(self, desktop):
+        num_references = desktop._connected_app_instances
+        with Hfss() as hfss:
+            assert hfss
+            assert desktop._connected_app_instances == num_references + 1
+            hfss.set_active_design(hfss.design_name)
+            assert desktop._connected_app_instances == num_references + 1
+        assert desktop._connected_app_instances == num_references

pyaedt/application/Design.py

@@ -137,9 +137,9 @@ def __str__(self):
     def __exit__(self, ex_type, ex_value, ex_traceback):
         if ex_type:
             exception_to_desktop(ex_value, ex_traceback)
-        if self._desktop_class._connected_app_instances > 1:
+        if self._desktop_class._connected_app_instances > 0:
             self._desktop_class._connected_app_instances -= 1
-        elif self._desktop_class._initialized_from_design:
+        if self._desktop_class._connected_app_instances < 1 and self._desktop_class._initialized_from_design:
             self.release_desktop(self.close_on_exit, self.close_on_exit)
 
     def __enter__(self):