If it is correct for a scrollbar to form when a shadow falls outside the right or bottom edge of the window (so the entire shadow can be seen), then shouldn't this object be pushed to the right and down when its shadow falls outside the left and top edge of the window (so its entire shadow can be seen)?
Shadows and glows and fuzzing are effects that simulate phenomena. They're not material entities. It's not logical for them to trigger either horizontal or vertical scrolling.
According to the CSS spec, “...an outline doesn't influence the position or size of a box or of any other boxes” and “...does not affect formatting (i.e., no space is left for it in the box model).” Consequently, hovering over the button at right should not affect the horizontal scrollbar.