In that case, weightpainting would still be done threaded, but current mirroring code cannot be run in parallel. Caused by rB5502517c3c12 [which was checking mesh 'flag', needs to be 'editflag' -- thus test was always true]. Reviewers: pablodp606 Maniphest Tasks: T81461 Differential Revision: https://developer.blender.org/D9126