Since installing the latest windows 10 update (June 12, 2018—KB4284835), whenever DisplayFusion is running, the focused window remains illuminated but all other windows, the desktop and taskbar, are dimmed (see attached screenshot). I have the latest version of DisplayFusion installed.

Perhaps there is a setting for this within windows or DisplayFusion, but I cannot find any nor have I changed the settings in either for many months.

Have you checked Settings/Monitor Fading to make sure there is no checkmark in the box?

Ah, that was it! I hadn't used this feature before but can see that I had a HotKey for it that I must of accidentally pressed at some point.

Thank you very much.