Weird Artifact on Custom Screenshot Resolutions

[sallliisa]

If custom screenshot resolution is set to other than the default values, the resulting screenshot will be incorrect and distorted.

Input image: 1080x1080

Running the image through this shader: https://www.shadertoy.com/view/4sySzw

Outputting it to a 480x360 image:

Expected behaviour: the same as if I change the canvas size in the web version of shadertoy