@fraseyboo as you say, I expect that the issue is pretty deep. Probably the conversion of a thickened line to a polygonal approximation in OpenGL code. On mac with m1, my understanding is that apple does not use native OpenGL anymore, but Metal with OpenGL emulation through MoltenVK. I’d rather not hack a fix by rendering my own tris because it would be difficult to make the line-thickness the same (i.e. a constant pixel dimension) at all zoom levels. This is the problem with other approaches to edge highlighting such as vtkTubeFilter.