Initiates immediately after a form filler performs an action that causes the data in a drop-down list to appear, such as clicking the arrow icon on the drop-down list or tabbing into the drop-down list and then using the down arrow. This event initiates after the contents of the drop-down list are displayed.
You can use this event to handle errors or unexpected outcomes as a result of processing the opening of the drop-down list. For example, if the preOpen event is dispatched via scripting instead of user interaction, or if the opening of the drop-down list data does not occur as a result of an error, the postOpen event is still dispatched to let error handling scripts execute.
See also 

postOpen event