Retina mode does not report the native display resolution to a virtual machine

[lesiw]

Describe the issue

I'm running my M3 Air (native res: 2560x1664) at a display resolution of 1710x1112.

When I install spice-vdagent, fullscreen the VM, and run xrandr --output Virtual-1 --auto, xrandr reports a resolution of 1710x1069.

If I check the "retina" checkbox and perform the same actions, I get a resolution of exactly 2x, or 3420x2138.

Neither of these are the native resolution of my M3 Air.

This impacts integer scaling: it seems as though UTM is operating on the fake, 1710x1112 resolution when performing integer scaling, as it results in uneven pixelation at 1280x768, which should be nearest-neighbor upscaled cleanly (2x) to a width of 2560.

Configuration

• UTM Version: 4.5.3 (99)
• macOS Version: 14.6.1
• Mac Chip (Intel, M1, ...): M3 Air

[osy]

So I am learning about macOS rendering but my understanding is the following:

1. backingScaleFactor returns the scale of the framebuffer NOT the physical scale factor. See: https://stackoverflow.com/a/54487166 This seems to always be 1.0 or 2.0.
2. The way macOS rendering works is that in any retina mode, the backing store is always 2.0 and then the OS will re-scale it to the correct physical dimensions.
3. It seems like it is impossible to avoid macOS's scaler: https://stackoverflow.com/questions/53551692/how-to-render-each-pixel-of-a-bitmap-texture-to-each-native-physical-pixel-of-th#53563363 even if you force the drawable to be the right resolution, it will end up getting re-scaled to 2x and back down to 1.5x

So unless someone has a better way, I don't think it is possible to fix this from UTM...