https://github.com/xceedsoftware/wpftoolkit/issues/1400

Checking if the mouse is outside the bounds of the DropDownButton when the PreviewMouseDownOutsideCapturedElement event tunnels through it to prevent the DropDownButton's Popup from closing if the event is caused by a child Popup

source/DropDownButtonLib/Controls/DropDownButton.xaml.cs

@@ -432,7 +432,8 @@ private void OnKeyDown(object sender, KeyEventArgs e)
     /// <param name="e"></param>
     private void OnMouseDownOutsideCapturedElement(object sender, MouseButtonEventArgs e)
     {
-      this.CloseDropDown(false);
+      if( !IsMouseOver )
+        CloseDropDown( true );
     }
 
     /// <summary>