No photo evidence exists to show that window, to be open or closed, pre shooting, there are one or two but they are blurry.
The reason this scenario would be used would be to create the impression that the shooter was at the 6th floor window, to frame Oswald, and it allowed the real shooters on the Daltex escape because all eyes were off that building.
The view from the Daltex via the tsbd 6th floor, makes the road on Elm visible from the tree right up to the underpass.