I've been fighting with KVM and virt-viewer 11.0-3build2 all day trying to get the displays to behave in some logical fashion. I'm on Ubuntu MATE 24.04.1 LTS, virt-manager 4.1.0-3 using QXL and spice-vdagent is running. No matter what I do, changing the VM window with the mouse causes the display to simply scale. I find ironic that Windows 11 in a VM under the same instance of KVM works seamlessly for resizing the VM window with the mouse; the displayed desktop of the VM window just grows and shrinks as you would expect, with the open windows keeping a constant size. Argh! If they can figure this out for Windows, why can't they get it right for Linux!?

For others who may face the same problem, the solution I've adopted is to just turn off Auto-resize, and inside the VM used Prefences-> Displays to adjust the window to the size I want. My two monitors have different resolutions, so the font is not the same size on both monitors, but good enough.