Since D14143 has lowered the thresholds for heuristics in constraints thickness mode, spikes started to appear when the faces around a vertex are non planar (T95963). It was also noted that upon triangulation of the mesh, the spikes would dissappear, as the faces would then be planar.

With this insight, I came up with the idea to calculate local normals per mesh loop. They have a threshold for how much they are allowed to deviate from the total face normal. When using these local normals, most spikes dissapeared. Some new ones are created as well, but they are smaller and only there for very bad non planar cases. Users of solidify should be able to easily avoid the remaining spike issues by changing the topology of the mesh.

This patch should not change the result for geometry without non planar faces.