The OnObjectEnter trigger is one which executes POST-event, so this sounds like expected behavior -- entering the field causes the list popup, THEN the script trigger executes.
FM's KnowledgeBase article #7465 provides a list of when the various triggers are executed compared to the completion of the process on which they trigger.
Agree on the post-event, should have paid attention, my question is then, why, when I used the tab from going to the second field I don't have the message? thanks
Guessing the bevavior
When click the field, OS get the event and do the things on the UI object (show dropdown list etc.)
then FM know the object is activated.
When type tab (on previous field), FM select next field acording to tab order and goto it, and FM itself can do the things (fire event etc).