Can someone explain these split lines?

[Contours]

I'm baffled by what I'm seeing in one of my drawings. I had done a couple of retangle fills using a linear gradient. Then I think I applied a transparency and then took it away. At that point I changed line width from None to 16. That's when this split line color effect showed up. Now, every rectangle I create has the split line color. I've tried to find a setting for it, to no avail. And btw, at no point did I apply a contour or bevel — or paste from another drawing.

What have I enabled? ... and how do I disable it?

Thanks in advance for any help.