delta is the pointer's travel in points, always measured from where the drag started, so a
long drag cannot accumulate rounding drift. Deliberately unrounded, like boxFromDrag:
rounding here breaks the points -> screen -> points round-trip that keeps a box where the user
dropped it.
deltais the pointer's travel in points, always measured from where the drag started, so a long drag cannot accumulate rounding drift. Deliberately unrounded, likeboxFromDrag: rounding here breaks the points -> screen -> points round-trip that keeps a box where the user dropped it.