The issue was actually affecting all data types that are not regular CDLayer ones, since by default
DerivedMesh references mesh data (verts, edges, etc. - modifying custom normals often implies
modifying edges' sarpness too).
Modfying edge/face sharpness etc. could directly affect the mesh in this case, **bad**!
So we detect whether we are copying data types that may affect non-CDlayers data, and whether
verts array of org dm is same as mesh one - in which case we copy the DM. This avoids
useless copying in most cases.
Note Edit Normals was quite obviously suffering the same issue.
Error in custom split normals work, non-autosmooth normals != vertex normals!
Loops from flat faces shall take normal of their face, not their vertex.
Tsst...
Nothing much to say here, basic tool to make normals point toward a target,
or to make them point 'outward' as if object was a spheroid (useful for game bushes etc.).
Also, forgot a big thank you to Campbell for the extensive review work he has done on this project!
