This seems to be the case with scalemode stretch or letterbox.(did not tried the other ones)
Nobody seems to care but, now I understand the problem better. It’s not just about
clipViewport its about rendering to outside of the camera.
1-) Even without the
clipViewport, If there is nothing to render to outside of the camera,
2-) And you use a post processing shader(just try with
h2d.filter.Nothing() ) with a scalemode.
Playing with the window size will cut the screen into half.