I've got a better idea.
Don't use popup windows.
I hate those things. I also have my browser set to open in tabs, not in new windows. If your webapp is trying to sell me something, opening a popup drastically reduces the chance you'll make a sale. Although as far as that goes, gratuitious use of Flash will almost certainly kill the sale. Doubly-so if it starts a lot of noise and motion without being asked to.
I've simply got too many windows floating around open on my desktop to put up with that sort of thing.
OK, end of rant. Now ask me about Do It Yourself Login Systems
I'm not clear what you mean by "panel", here. Is this a case where you have some sort of top or sidebar menu on the web page that you want to carry over from one page to another?