What steps will reproduce the problem?
(1) Go to http://csreis.github.io/tests/cross-site-plus-same-site-iframes.html
(2) Turn on devtools emulation for a device with pinch zoom
(3) Open the date select popup in the OOPIF frame.

What is the expected result?
It opens below the date element.

What happens instead?
It opens.. somewhere else.

I think it is not respecting the scroll position or the page scale factor (ie the viewport) of the main frame - and probably not the scroll position of any ancestor local roots.