Stamped element can be overlapped by dragged element

Hello,
I'm keeping a few blocks fixed in position, so that calling layout() or adding/removing elements does not move them, while others shifts around.
It works correctly with the stamp method.

**Adding Draggabilly break this behavior by overlapping.**

Steps to reproduce:
1) init Packery and Draggabilly as outlined in the instructions
2) stamp a Packery element so that it's fixed in position
3) drag an item over it and release

**Test case:** https://codepen.io/anon/pen/arLByQ
(Only added stamping ability to the original demo: https://codepen.io/desandro/pen/aGvIq)

I understand there's no collision detection inside draggabilly, but this seems more a Packery related issue to me.
Thanks

**Edit:** The same problem happens with jquery draggable

Reproduce the overlap using the linked CodePen with Packery stamping and Draggabilly, then compare it with the original Packery demo. Start by tracing the stamp and drag-release integration; done means a dragged item cannot overlap a stamped element, including when using jQuery draggable, with a regression test or reproduction added if the project provides one.

Written by the indexing model from the issue text.