Hello, all. I reported an issue to the gitlab (https://gitlab.freedesktop.org/spice/spice-gtk/-/issues/183), but I see the readme wants issues sent to the mailing list. I will summarize here:
VM viewers that use spice-gtk do not render properly when running under wayland with GNOME/Mutter's experimental fractional scaling enabled. Wayland windows are scaled by rendering at a higher-than-native resolution (I think 2 x (native res / scaling factor)) and then downscaled to native res. Spice-gtk reports the higher-than-native resolution to the VM client, which renders at said resolution. My screen width is 2256, but with 125% fractional scaling, VMs report and render with width (2256 / 1.25) * 2 ~ 3606.
Have a nice day.