Address comments (libMesh#3385)

src/mesh/mesh_smoother_laplace.C

@@ -137,22 +137,30 @@ void LaplaceMeshSmoother::smooth(unsigned int n_iterations)
               return true;
             }
 
-          // 2D - collinear - check cross product of first edge with all other edges
+          // 2D - collinear - check cross product of first edge with all other edges.
+          // curvature is only zero if the second connected edge spans a zero area
+          // parallelogram with teh first edge. There shouldn't be a third edge, but if there
+          // ever is, we enforce it to be collinear as well.
           if (_mesh.mesh_dimension() == 2)
             return (base.cross(vec).norm_sq() < libMesh::TOLERANCE * libMesh::TOLERANCE);
 
-          // 3D
+          // 3D - we compute the cross product of the first and second edge...
           if (n_edges == 2)
             {
               const auto cross = base.cross(vec);
               const auto cross_norm_sq = cross.norm_sq();
+              // if the second edge is collinear to the first we simply drop it. This does not violate
+              // coplanarity, but it is insufficient to construct a normal for the tangent plane to check the
+              // remaining edges against.
               if (cross_norm_sq < libMesh::TOLERANCE * libMesh::TOLERANCE)
                 n_edges--;
               else
                 base = cross / std::sqrt(cross_norm_sq);
               return true;
             }
 
+          // edges 3 and up are coplanar if they are orthogonal to the normal vector of the tangent
+          // plane calculated above.
           return (base * vec < libMesh::TOLERANCE);
         };
 

src/mesh/mesh_tools.C

@@ -537,7 +537,7 @@ MeshTools::find_block_boundary_nodes(const MeshBase & mesh)
   // mark them as true in on_boundary.
   for (const auto & elem : mesh.active_element_ptr_range())
     for (auto s : elem->side_index_range())
-      if (elem->neighbor_ptr(s) && elem->neighbor_ptr(s)->subdomain_id() != elem->subdomain_id())
+      if (elem->neighbor_ptr(s) && (elem->neighbor_ptr(s)->subdomain_id() != elem->subdomain_id()))
         {
           auto nodes_on_side = elem->nodes_on_side(s);
 
@@ -569,7 +569,7 @@ MeshTools::build_subdomain_boundary_node_map(const MeshBase & mesh)
                 auto nodes_on_side = elem->nodes_on_side(s);
 
                 for (auto & local_id : nodes_on_side)
-                  block_boundary_node_map[elem->node_ptr(local_id)->id()].insert(std::make_pair(std::min(id1, id2), std::max(id1, id2)));
+                  block_boundary_node_map[elem->node_ptr(local_id)->id()].insert(std::minmax(id1, id2));
               }
           }
       }