Floating / non-active pop-up page

Hi all, I have the following use case: We would like to be able to have a pop-up page on the screen that is 'not active’ (I don't know the right word for it, sorry). When we see the pop-up on your screen, we still want to be able to work on the 'overview’ page behind it. How would we solve this? Thanks for your help!      
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