'No boxes were moved'
I get the perspective change thing, but I thought the moved/not moved boxes thing was about the boxes right at the window. Yet your gif appears to show the wall of boxes behind the immediate cuckoo nest, and only the tip of a top window box.
While today we know the boxes in question to be a few feet from the window, with some receiving sunlight through the window, the HSCA Photographic Panel in 1977 thought the boxes sat "close to the plane of the window") in order to receive sunlight, and that they weren't being lit though the window because the boxes supposedly had no lattice shadows.
"Since the precise positions of Dillard and Powell at the time of the photographs were unknown,
it was not possible to calculate precisely the region within the sixth floor room that would have
been visible to each photographer. In the Dillard photograph, the two the left and right of the
window frame appear to be in the full light of the Sun, with no shadows cast on them by the frame
of the partially opened window. In the Powell photograph, it also appears that the boxes are in full
sunlight, with no shadow cast on them by the window frame."
With 3D and John's animation, we can demonstrate that the box seen in the open window area of the Dillard photo is the high box seen in Powell.
Not perfect but gives the idea.
