It took me 3 hours to understand this (I think I did). So what you're saying is that as you focus on the combo box and press the down arrow to scroll down the list of combo box items, after you reach the last entry (December in this case), and you press the down button again, the focus jumps to the next control in the tab index (the Conferma button in this case), correct?

I guess that's how all combo boxes work, and I do not understand what's the disadvantage in that. On the contrary, I find that as an advantage, instead of having to work on the tab button, or use the cursor.