External monitor not showing in Display settings on Mac

deep_ion wrote:

Lately my monitor is NOT showing in the Display settings, although I am still able to share any window of choice over to the monitor using the Green icon on the window and clicking "Mirror this window".

My understanding is that mirroring windows is not an option for directly-attached displays ("hardware displays"), but is an option for AirPlay displays.

You might have created multiple Spaces (virtual displays) on your Mac, and somehow created a Space that has a single application window, but if the display connection is a hardware one, the external display would be showing everything on the virtual screen (including space around the window if the window was not full-screen).

So are we dealing with having switched from a hardware video connection to AirPlay, somehow (which might be possible with smart TVs that have both HDMI inputs and AirPlay support)? Or is this a Spaces setup thing?

Work in multiple spaces on Mac - Apple Support

View open windows and spaces in Mission Control on Mac - Apple Support

what make&model external display?

what cables, adapters, and docks are used to make the connection?