Mandatory field and set focus does not work

All,

On validations, I created activity registration for Tasks.

On any update, an activity needs to be registered. If not, validation invoked and I want the focus to the activity field.

Validation : not null(denull($cmt.update))
Update : true
Message : Please describe your Activity.
On Failed Validation Set Focus to: $cmt.update

The activity info is on the second tab on the form.. and the focus is not jumping to the field ...

Windows client

any thoughts?

Maurice