That's a very recent change to virt-manager, so recent it's on nixos-unstable, but not on nixos-23.11. So it might be in Fedora 39 and some rolling-release distros... and nowhere else.
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.
It’s basically the opposite for both. I use virt-manager because the GUI is simpler (and setting up virtualbox is a nightmare anyway).
Regardless, this feature is a step in the right direction. I’m wondering if distributions will pick it up or if it will ever be integrated upstream.