Thanks again @sebavan!
That explanation is indeed illuminating.
So for a small width:
box.outlineWidth = 0.08;
Then the gap in the corners is becoming indeed less noticable:
And when I apply the smooth function (‘borrowed’ from here) to my box, then the edges can indeed be thicker in my new playground:
But with both workaround the edges in the middle don’t become red. I assume that is due to the same root cause, because those edges are entirely between two surfaces? But it is not clear to me how I can solve this…
I have never realized that a simple outline color could be so challenging, but I can imagine that the maths behind the scenes are quite heavy…