fix: make graphical VMs usable with a mouse

internal/qemu/args.go

@@ -113,6 +113,14 @@ func Args(v *config.VM, graphical bool) []string {
 		a = append(a, "-display", "none", "-vnc", "unix:"+v.VNCPath())
 	}
 
+	// Absolute pointer for the framebuffer above. qemu's default is a relative
+	// PS/2 mouse; a desktop guest cannot align its cursor to the host cursor
+	// from relative deltas, so the pointer drifts and clicks land off target.
+	// usb-tablet reports absolute coordinates, so the guest cursor tracks the
+	// host cursor in the gtk window and in a VNC client alike. qemu-xhci gives
+	// it a USB bus, which the default pc machine has none of.
+	a = append(a, "-device", "qemu-xhci,id=xhci", "-device", "usb-tablet")
+
 	// Two exports (core-api.md §10.2): the user's dir read-only, stoat's
 	// per-VM scratch writable.
 	//

internal/qemu/args_test.go

@@ -29,11 +29,16 @@ func TestArgsLive(t *testing.T) {
 		"-virtfs local,path=/home/u/vms,mount_tag=host,security_model=mapped-xattr,readonly=on",
 		"-cdrom /data/isos/alpine.iso",
 		"-drive file=fat:rw:/data/live1/ovl,format=raw,if=virtio",
+		"-device qemu-xhci,id=xhci", "-device usb-tablet",
 	} {
 		if !strings.Contains(got, want) {
 			t.Errorf("missing %q in:\n%s", want, got)
 		}
 	}
+	// The tablet must ride the VNC path too, or a VNC client's clicks miss.
+	if vnc := joined(Args(v, false)); !strings.Contains(vnc, "-device usb-tablet") {
+		t.Errorf("headless VM lacks usb-tablet, so VNC clicks would miss:\n%s", vnc)
+	}
 	if strings.Contains(got, "disk.qcow2") {
 		t.Error("live mode must not attach a disk")
 	}

internal/recipes/bundled/xfce/install-alpine.sh

@@ -12,6 +12,19 @@ setup-apkrepos -c -1
 setup-xorg-base
 apk add xfce4 xfce4-terminal dbus-x11
 
+# Xorg reads its input devices from udev. Alpine boots mdev by default, which
+# never feeds Xorg, so libinput finds no mouse or keyboard and the desktop
+# ignores every click. setup-devd switches the device manager to eudev and
+# adds udev, udev-trigger, and udev-settle to sysinit for the next boot.
+# Ubuntu, Debian, and Arch already run udev under systemd, so only Alpine
+# needs this.
+setup-devd udev
+# Cold-plug the devices that already exist so Xorg finds the tablet in this
+# boot, not only after a reboot. The live case below starts X now with a HUP
+# to init, before udev-trigger would run at the next sysinit.
+udevadm trigger 2>/dev/null || true
+udevadm settle 2>/dev/null || true
+
 rc-update add dbus
 rc-service dbus start || true