When I hit the switch window (not application, but windows of a same application, Alt+~ by default) over the desktop... it switches to.. "another desktop" with its own wallpaper but unable to hold plasmoids (if I try to add a widget, I'm switched back to the "original desktop window").

It's not switching to a different virtual desktop (u can't set different wallpapers to vd anyway) neither to a different activity since I have only 1 (and this "second desktop" won't hold widgets, while activities can).

Any idea what this alternative desktop reality is?