caveats: - it drops wherever the y of the cursor is no matter the x for
the cursor. I've constrained the draggables to never change x; so, the
drop doesn't care where the x of the cursor is only the y.
- if you force auto scroll, the visual thing you'll dragging gets out of
sync w/ the mouse cursor; however, the mouse cursor is the "true"
location; so, the drop's posit is the mouse cursor y not the visual
clone's y.