Finally I was able to create a testcase.
Is my code cleaned from every non smart element component and reproduces the error.
I run it from my application server tomcat (I don’t know if it could be relevant).
Not every combobox produces the wrong situation (6 over 9).
As You can observe the alert message, connected to the onchange is executed before the combobox is shown.
On chrome it works, on firefox no.
Tks foy your help
I sent it You by mail