The shadowbox content might visually pop up "on the same page", but that pop-up box will be indexed as a separate URL.
I've recently met that problem on a shopping site where the product reviews popped up on a shadowbox effect on the product description page. The reviews themselves then also appeared as a separate page in the SERPs. Visiting any of the review pages from the SERPs was problematical as there was no navigation back to the product page or indeed to any place on the site.
That was solved by adding a tiny navigation bar to the reviews page.
There were other challenges to ensure that products with no reviews didn't expose a multitude of blank pages. Likewise for the 'write a review' pages, those contain only a form and had to be excluded from indexing from the start.