a98fc4f8f9
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.