This was a TODO in the code. Previously the Face Set datalayer was deleted and recreated with a constant ID of 0. Now the datalayer is preserved and set to the SculptSession after slicing the mask and a new ID is calculated for the new faces that the slicing operation produced, so they can be easily isolated for further tweaking. Reviewed By: sergey Differential Revision: https://developer.blender.org/D8583