I think I'm also having the same issue (under testing, version 4.0-1); as mentioned by Gerasimos Melissaratos virt-viewer is working when in fullscreen.
I remember noticing that the Gtk-WARNING about negative dimensions was also there while virt-viewer used to work, but I've found two GSpice-CRITICAL. This is a full log of the output I had while creating a test virtual machine (I think it's short enough that it's worth pasting in full) using virt-install, which launches virt-viewer: $ virt-install --name wtf --memory 1024 --cpu host --cdrom debian-8.5.0-i386-DVD-1.iso --disk size=4,format=qcow --network user --virt-type kvm Starting install... Allocating 'wtf.qcow' | 4.0 GB 00:00 Creating domain... | 0 B 00:00 ** (virt-viewer:25901): WARNING **: Error retrieving accessibility bus address: org.freedesktop.DBus.Error.ServiceUnknown: The name org.a11y.Bus was not provided by any .service files (virt-viewer:25901): Gtk-WARNING **: Failed to get the GNOME session proxy: The name org.gnome.SessionManager is not owned (virt-viewer:25901): Gtk-WARNING **: Failed to get the Xfce session proxy: The name org.xfce.SessionManager is not owned (virt-viewer:25901): Gtk-WARNING **: Failed to get an inhibit portal proxy: The name org.freedesktop.portal.Desktop is not owned (virt-viewer:25901): GSpice-WARNING **: PulseAudio context failed Connection refused (virt-viewer:25901): GSpice-WARNING **: pa_context_connect() failed: Connection refused (virt-viewer:25901): GSpice-CRITICAL **: egl init failed: cannot create EGL context (virt-viewer:25901): GSpice-CRITICAL **: egl realize failed: failed to activate context (virt-viewer:25901): Gtk-WARNING **: Allocating size to SpiceDisplay 0xd3e400 without calling gtk_widget_get_preferred_width/height(). How does the code know the size to allocate? (virt-viewer:25901): Gtk-WARNING **: gtk_widget_size_allocate(): attempt to underallocate VirtViewerDisplaySpice's child SpiceDisplay 0xd3e400. Allocation is 720x400, but minimum required size is 640x480. Domain installation still in progress. You can reconnect to the console to complete the installation process. -- Elena ``of Valhalla''
