Treat the popunder as a browser-mediated path
Map the attempted event from a publisher page to the secondary browsing context, the requested URL, redirects, final destination and any accepted event. Record what the buying platform counts at each stage. The word popunder does not establish identical behavior across browsers, operating systems, settings, extensions, security products or publisher implementations.
Test representative environments using harmless accounts and a controlled destination. Observe whether the new context opens, where it appears, whether focus changes, which URL finally loads, what happens when the user closes it and whether navigation controls remain available. Store the browser version, device class and time with the observation.
Do not disguise the destination or simulate a browser control. A campaign must not depend on a visitor confusing an advertisement with a security message, system alert, close button or site navigation. If the rendered experience differs materially from the approved review, quarantine the source and preserve evidence before changing settings.