Recently (I think; maybe it was a bug fix) it was made so that MouseEntered is called when an atom moves under the mouse while the mouse is stationary. This is great, but MouseEntered isn't called in this case if the mouse is down. I'm guessing it assumes that if the mouse is down, the mouse entering something would be a drag. But an atom moving, and not the mouse, would not be a drag.
So basically what I would like is for MouseEntered to be called when the mouse is down and an atom moves under the mouse while it is stationary (not being dragged).
Edit: I suppose you could argue that the mouse moving away from the location of an object on the screen due to the screen itself moving and not the mouse would be a drag. If so, though, at least call MouseDrag when an atom moves under a stationary mouse like it does with MouseEntered.