I've made some shape in Blender 2.8 that roughly resembles a handle one might find on a fridge to learn, following along with Gran Abitt's videos. Where the handle part of the handle meets itself in the mirror, in the "viewport shading" mode, there's an ugly dark band around it. There is no such join on the backplate.
I think my problem is similar to Mirror Modifier making ugly smoothing on mirrored faces, but the proposed solutions don't work for me - there is no option to "clear custom split normals data", and the problem actually goes away when I tick "Auto Smooth".
The real problem is that I just don't understand what I'm doing. Why does ticking "Auto Smooth" make the dark band go away? What is the dark band? I can't see anything untoward in Cycles whether it's on or off - is it actually a problem?