As of virt-viewer 4.0 (or spice-gtk 0.32) the spice window just displays "connected to graphic server" but then doesn't update the window. When one resizes the window (e.g. makes it slightly larger) the image is displayed correctly from there on. Bringing it back to "normal size" makes the updating stop again. This looks somewhat related to #1330824. Users also reported it for virt-manager but I'm not able to reproduce it there.
Can confirm this for a related package (virt-viewer) on Arch Linux. Initially there is a very tiny black bar with the console (for an Arch Linux guest, no GUI) in the bottom center. Resizing restores the output. Output also contains: (remote-viewer:25890): Gtk-WARNING **: Allocating size to SpiceDisplay 0x24aa330 without calling gtk_widget_get_preferred_width/height(). How does the code know the size to allocate? Package versions: virtviewer 4.0-1 spice-gtk3 0.32-2 spice-glib 0.32-1 gtk3 3.22.1-1 spicy (from spice-gtk3) has no issues with the display or screen updates.
Should be fixed in spice gtk v0.33
Confirmed that 0.33 fixes the issue for remote-viewer. Thanks!