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Description of problem:
Blue area appears when resize a spice vm window by mouse via virt-viewer 

Version-Release number of selected component (if applicable):
libvirt-6.0.0-28.module+el8.3.0+7827+5e65edd7.x86_64
virt-viewer-9.0-4.el8.x86_64
spice-gtk3-0.38-3.el8.x86_64
qemu-kvm-5.1.0-9.module+el8.3.0+8182+ac9ced32.x86_64
kernel-4.18.0-240.el8.x86_64


How reproducible:
100%

Steps to Reproduce:
1. Prepare a spice RHEL8 vm:  
Display: spice + Video: qxl + spice-vdagentd.service is running)
spice-vdagent-0.20.0-1.el8.x86_64
kernel-4.18.0-240.el8.x86_64

2. Run virt-viewer to connect to the guest.
$ virt-viwer -c qemu:///system xxx

3. Resize the vm's window(dragging the virt-viewer window by mouse).

4. Check the vm's resolution.

Actual results:
The vm's resolution changes as expect, but there are blue areas appears when resizing the window.
Then change to 'Fullscreen' mode, the blue areas disappears, but the blue areas appears when changing back to window mode.

Please see attachments for details, thanks.

Expected results:
Fix it.

Additional info:
Can't reproduce this issue with RHEL7 vm.

Created attachment 1720219 [details]
Screenshot when resizing the vm's window

Created attachment 1720220 [details]
Debug log for the virt-viewer

If I understand your description, and looking at your screenshot, it is just the desktop image not filling the new desktop size.

I have tried and reproduced this behaviour. As you mention, the screen resolution is properly updated, so SPICE is working as expected.
Now in the guest, the background image is set to "scale", which means the image resizes only to the limits of its ratio. If you pick a resolution that is not standard, the scaling cannot fill the screen, and the parts of the background that is not filled with the image is showing the default background color (blue).
When you set the window to fullscreen, it gets a standard resolution (the one from your monitor), and the problem disapears.

This can be verified by issuing the following command:

$ dconf write /org/gnome/desktop/background/picture-options "'zoom'"

This will set the desktop image to "zoom" rather than "scale", and make it fill the desktop whatever its size.

I guess the default has changed since RHEL7, which is why it didn't happen before?