I'm trying to create something like this: https://scratch.mit.edu/projects/editor/?tip_bar=getStarted
so when dragging a block next to another one on the canvas, the snap starts *before the elements touch.
Basically, a negative threshold.
This got me a good way there: http://greensock.com/forums/topic/9265-draggable-snapping-to-specific-points-with-sensitivity/
looping through my elements, using hitTest to figure out which is being touched by the dragging element, and I added inset borders with box shadow and some custom logic to figure out which side to snap to....