I can do this on my mac 10.9.4.
The landing area for scrolling while selected at the top and bottom of the grid is very small (about half the height of the arrow cursor)
Hi Rikk, thanks for the help. Yes, after playing with it quite a bit I noticed that it is possible. However, as you mentioned the landing area is quite small and in my experience that makes this operation mostly unusable. It's odd that on the Windows version this is pretty easy to execute. Are you able to do this reliably and with decent scroll speed? Whenever I try I can only get a fairly slow scroll. Moving up more often terminates the scroll.
I've found a few workarounds that work better for me -- the main one being starting the drag and then using the scroll wheel on the mouse to find the new location for the photos. However, It'd be great if I could use the more natural mouse gesture scroll.
After playing just a bit more, I noticed two things that have helped quite a bit. The landing area for the scroll is actually farther from the grid edge than I had expected (more than half the dragged thumbnail height) and as you said, is rather small. Also, this works much better in the upwards direction when the module picker is not visible. With it visible, it seems to greatly reduce the landing area in the up direction.