what is the black gizmo on the scene?

I use 3 spot lights. The monkey rests in a box with 3 faces deleted and normal flipped.
I donno what is the black thingy in the corner. I have tried subdivided the box, but all is the same. Any hints? Thank you.

Set your clipend to the end of the cube.


You are right!
I have to extend the clipEnd, although I rotate the scene and find that the line runs across the wall already.