No "On Drag Complete"?

Okay, so you need to use this on the “draggable” so the blue square that’s being dragged around in your analogy (not the original not-yet-dragged version):

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And a return node:
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So THEN you have your “OnDrop”. So this would be on the widget with the canvas you’re operating on- so like the base level here. So as you were saying, the receiver of the drop- but through this method you can kind of “mirror” the “OnDrop”, if that makes sense.
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