I'm trying to explane with more details.
My goal is have a control as Simple Searcher.
The user wants to search particular item, but he don't remeber completly name, thus he try to write in "searcher box" a portion of name.
When app started, it has already populated the collection using MVVM pattern.
When he's typing into box, list dropdown appers and filters items.
He scroll through the items and click (he can only click) item searched.
At this point control clear textbox (of searcherbox) and close dropdown.
I think that I found solution, that's it.
In this way, it seems that SelectedItem not change until user click, in second time into setter of SelectedItem, it clear Text property of ComboBox
"Cerca una voce nel menu"