I just discovered that the semantics of moving things between projects by 
dropping them in a project view appears to be a (deep) copy.  In contrast, 
dropping them on flaps looks at first blush like a move.

I haven't fully traced through or cleaned up things, but I thought I'd warn 
others who might be thinking of the same maneuver.

Visually, dropping something into a project creates a copy, since the 
original remains in the current project.  So the hints are all there, but 
in my case hope and expectation triumphed over what was in front of my 
face.  So I had a bunch of windows up, each referring to the same 
underlying data.  After I moved them, I had many copies of the underlying data.

Since this is a warning, not a warranty, I'll say that I'm not sure that 
moving a bunch of morphs to the flap, and then into a different project, 
will avoid this problem.