According to the official documentation (here), measuring viewability outside the ad iframe is not possible :
Custom JavaScript executed as the creative payload that inserts the actual content somewhere else within the ad iframe, but outside of the creative . Active View will search for a second with the class attribute GoogleActiveViewInnerContainer. This expands to the size of the ad container, but allows the container size to be measured from within the ad iframe.
If you find a way to render your creative within the ad iframe by forcing a new iframe size, you could wrap your rendered creative inside a div class="GoogleActiveViewInnerContainer" to tell Active View where to watch the viewability. Sadly and as detailed, it would only works within the ad iframe.
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