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.
Not much to add, modifier uses same code as operator basically, only key difference
is that modifier will never create data layers itself, you have to use dedicated operator
for that.
