Originally, I thought the shared face bookkeeping could occur each time there is an element update. However, if you look at the call tree for createVoxelAt, observe that as the insertion unwinds from the leaf voxel, elementUpdated gets called from each level of the octree through the notifyHooks method. Doing the shared face inspection here would add another deep recursion of the octree at each level, which would be prohibitively expensive.