Comment 3 for bug 184428

I believe I've found how to make it work: if you open System Settings, and click "Desktop", make sure you have the desktop effects enabled, and click "Advanced Options". Once the window opens, you'll see an unchecked option for "Keep thumbnails of hidden windows up to date"; check and apply the setting, and the Thumbnail Asside plugin seems to work from any desktop (as well as previews for minimized windows).