GNU/Linux (Fedora 23) Intel and nVidia gfx , intel cpu
Broken: current git
Worked: maybe never
Short description of error
So if you have a parent mesh, with some vertex children
and the parent mesh has a constructive modifier (even a disabled one)
and you select parent + children , then copy it.
the children in the copy will 'jump' as if the vertex order has changed in the parent (copy)aa
Exact steps for others to reproduce the error
First download the .blend file (no python needed)
1- Open the file
2- Note that everything is selected, body_hands (the parent) is active
3- note the position of the fingers and thumbs
4- shift D and drag mouse to place the copy somewhere then click.
5- note the fingers.L/.R and thumb.L/.R have now migrated to new locations.
4b- remove the solidify modifier
5b- copy as in step 4 above
6b- readd the solidify modifier or copy it onto body_hands.001
The solidify modifier is just an example - I've had the bug trigger with Mask modifier and others. It may not be all constructive modifiers (I've gotten away with subsurf) My guess is the vertex order is getting changed, but I haven't really checked it, it's possible that offsets are getting recalculated wrongly - using the derived mesh instead of the original one. (this might need some finesse to make sure things stay working for other modifiers - e.g. armature which currently doesn't trigger the bug)