This reverts commit aaabb44ac7.
I figured out how to trigger this: it's when scaling is enabled.
To maintain VM aspect ratio, we have to constrain the size of the
viewer widget. When the widget is smaller than the VM window, the
viewport widget is visible. This change made the viewport the default
theme color, rather than black.
Problem is, in the non-scaling case, we don't shrink the viewer
widget but let it expand to fill the whole area. This is necessary
to get the 'Autoresize VM with window' option to work, but is also
simpler in the cdoe. The viewport widget is not visible, and gtk-vnc
and spice-gtk paint the non VM owned areas as black. AFAICT that's not
configurable in any way.
So after this change we have differing behavior for scaled and
non-scaled cases, which is confusing and visually kinda comes off as
a bug.
So this reverts back to the old behavior and explains all this in
a comment.
87e0ed5de0