Unity on Ubuntu 16.04 has a nice feature. When switching among workspaces, the switcher preview dims the other workspaces to highlight the current workspace.
However on Ubuntu 18.04 this feature does not work for the background.
Both screenshots above are taken on fresh installations after manually installing
ubuntu-unity-desktop (only for Ubuntu 18.04) and
compizconfig-settings-manager and after increasing the virtual desktop size to 4x4.
A dist-upgrade from 16.04 to 18.04 yields the same fault.
How can I activate this helpful effect?