Ah . . I understand.
Fences has an option to take a snapshot of your fence layout for later restoration. Typically the wallpaper is displayed.
When you have a dream running the background is black. This can make it more difficult to see your fences. The issue is magnified if you are running multiple monitors as everything is reduced to fit into the snapshot dimensions.
The last item can be mitigated by resizing the Restore Snapshot window though.
I'll pass this feedback on to the dev.
