Incorrect lighting on OpenGL2 for wireframe representation.
OpenGL1 on the left, OpenGL2 on the right. The OpenGL2 backend is not applying the default headlight effects.
These are generated by the (soon to be merged; see !1820 (merged)) Rendering/Core TestHiddenLineRemovalPass test. To reproduce, remove the actor->GetProperty()->LightingOff()
line from the test.
Note that running the data through vtkPolyDataNormals to explicitly generate the normals fixes the issue, so it appears to be a problem with the fragment shader normal calculation for line renderings.