My point is that the Designer uses a "fake" or "moked" page, and so it doesn't behave the same as what a true session in a browser would.
I would not expect you to need a pageId in the browser at all.
The pageID is only needed if you are attempting to show the pop-up only on a specific page, and that would only matter if you happen to have multiple pages (tabs) open at once, or if you are attempting to open a popup on a remote session.