Window magic

This is how I think they did the window scene filming, putting Davey in an impossible perspective, and Jack Flack not visible:
-Henry was filmed from both sides simultaneously, one from the view we get, from behind Davey in his stairwell, the other from out front, looking back at the window with Davey in it.

-The out-front window view was isolated and composited into the picture, with frame-by-frame synchronization, it would match the two Davey views perfectly.

-Jack was not composited in. And it looks to me like Davey's path to the other side of the window had a separate view of Davey, which did not need to be perfectly matched, and did not appear to be so.

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The story is king.