I have a trick for this that I love that is very general:

1. When a user begins dragging, calculate the layouts for all possible drop targets (or perhaps just those that are currently visible). 2. For each of those layouts record the position the dragged objects ends up in. 3. On each mouse movement, select from those positions the one that's closest to the dragged object's current position 4. Render the selected layout

This ends up feeling really good and works for any kind of complex layout / reflow.