Thank again very much Nasko. It's much easier when one knows the purpose of this standard DragDropCompleted method. With your help it crystal clear indeed. (a quick search on the web does not provide something as good)
Now, with my colleague, thinking on how long we have spent on this issue (we won't dare to think too long about it), and how simple the solution is... we just wish the doc would have said in first place something as good as you explain on this forum. Indeed, as said in my previous entry, what the doc says currently looks a bit like a typical documentation point "click on the Button Cancel to cancel" or "Click on Validate to Validate" ...
May I kindly suggest to improve the documentation about this method with what you just said?