If I understand you correctly you did a postback to a new browser window (by
setting the forms "target" attribute to "_blank" or something like that) and
you want to access information from the dropdown on the original page?

If that's correct, understand first that both pages are the same page,
although you made the first DIV invisible it is still there, and unless you
have re-bound it or made any changes to it, it is still the exact same way
as it was before you made it invisible so you can access it programatically
as you would if it where still visible.

The server doesn't even knows that you opened it on a new window so for all
it cares it is still the same window. Once the server sends the page to you
it forgets about it, so there's no way to access a previous page
programatically (unless it's done on the client-side with javascript).

I hope that answers your question, please post more details if it doesn't.