Thanks for explanation. Makes sense now.
But I ran into another problem.
See this pg:
As long as the cylinder lines (41, 42, 47) are commented it works as expected:
There is a disc cut out of the box.
As soon as the cylinder is added, which exactly covers the disc at exact same height, the cylinder top is flickering or does not show a complete planar plane.
It seems as if it is interfering with the not-drawn disc.
Another interesting point is: If I reduce the discMesh.visibility to e.g. 0.01, this effect is (almost) gone.
Is there still something drawn even if disableColorWrite=true is set?