The former. A tiling WM covers the entire desktop with whatever windows are visible in the workspace. One window starts out full screen, creating a second window halves the first one and tiles them side-by-side, and so on. I would only see the desktop when I switch to a new empty workspace, but the reason I switched to a new empty workspace in the first place is because I wanted to start a new window there, so that glimpse of the desktop would be short-lived.