Could you link the change? At least a few days ago virt-manager still seemed to have scaling issues with guest displays, on nixos-unstable. I had viewer scaling on though as a workaround, so maybe I just didn’t notice.
It's not perfect. On older versions, it'd sometimes add a black border the size of your scaling factor. It still reports your window size times your scaling factor as internal resolution with guest additions though.
asmor|2 years ago