Background changes when dragging file to right screen edge.
The desktop background changes when I drag any icon to the right screen edge.
It changes from the Mojave Night image ... one I've never selected.
The dock or desktop icons don't change.
I have to drag an icon to the left screen edge to return to 'home' screen.
This is not the Dynamic Desktop feature.
Mission Control is off.
There are no custom Pref Panels.
This has happened since High Sierra.
MacBook Pro 15", macOS 10.14