Comment 11 for bug 2000739

Thanks, I think my confusion is now gone.
I was lured by e.g. iso.png that you attached before that the top-bar would be at an offset or something.

Now I get what you describe, you are (correct me if I'm wrong) actually saying: "The output goes full-screen (guest screen now consumes all of the pane that the gtk app draws to) but the window does not go full-screen (the window doesn't consume all of the [host]screen, it is still at the same position it was before)."

Now that I know what to actually check for I've found that it also does in general "not behave", for example maximize does not maximize it to a screen. Nor do the hot keys for left/right half screen align it that way - it just stays "in place" as if all those would not mean anything to it.

Now that I knew that it isn't anything with the actual guest display or virtualization, but essentially just window behavior the first suspect to "behaves different in Jammy" is that the new default is wayland.

And the immediate "try this" worked like a charm.
Prepending GDK_BACKEND=x11 to the command like

$ GDK_BACKEND=x11 qemu-system-x86_64 -boot c -hda /tmp/ubuntu22.04-desktop.qcow2 -m 4096M -machine type=q35,accel=kvm -cpu host -smp 2 -device qxl-vga -display gtk

Makes it behave as expected.
Your view-fullscreen will work as you expect.
But also the minimize, maximize, left-half, right-half hotkeys work as expected for me now.

I need to try asking a few people that understand more of desktop/wayland to make suggestions how to fix this in the default mode.