SOLUTION that worked for this project: edge fading to 0 AND mark overscan
I have used settings as set out here: Does transparency work in eevee
however, when I render in eevee the transparent material is getting darker near the camera borders. (not the white background)
I've tried making the transparent layer wider, and did some changes in lighting, unchecked passepartout in camera(although that stated outside the image area). but that did not resolve it. Turning the sun off did make a change.
This is not what i want, so is it possible to turn that feature off?
i've made an example file(is the picture above):