You have a "leftover" old Mac that you want to use as an additional display for your current main Mac.

It is an oldish iMac that as such still has a very nice display but it is underpowered for your actual work, and you have a newer iMac, or MacBook, that you do your work on. You want to use the old iMac as a display for your main Mac. You can use FluffyDisplay for that.

You set up the old iMac so that it is right on either side of your main Mac's display.

You start FluffyDisplay. It appears as an icon in the menu bar. You use it to create a "virtual" monitor on your main Mac.

In System Preferences -> Displays, arrange the displays so that the virtual one is on the side of the built-in display where you put your old iMac.

You then use Screen Sharing app on the other Mac to connect to your main Mac, and specifically select in Screen Sharing's View menu to view the "virtual" monitor. You switch Screen Sharing to observation mode and to full-screen. You then don't need to touch the mouse, trackpad or keyboard on the other Mac until you want to close the connection. (Or until the other Mac starts its screen saver.)

You can also run FluffyDisplay also on the other Mac. In that case the main Mac notices that and automatically adds a menu "New on peer" (suggestions for better but still short name welcome) under which will appear the displays of other Macs running FluffyDisplay. When you choose one of those, FluffyDisplay on that other Mac will automatically start ScreenSharing to connect to your main Mac.

You will still have to switch to observation mode and to full-screen, etc, this is not (and can not be) fully automated. It would be lovely if Screen Sharing could be passed query parameters in the vnc: URL that would tell it which display to show, to switch to observation mode, and to full-screen. But apparently no. Having those settings in a .vncloc file doesn't seem to work either.

The performance of Screen Sharing is probably not good enough to let you view videos in high quality, sorry. If that is what you want to do, just view them on the other Mac locally.

Even if you move the cursor of the main Mac off the virtual display (that is showing in Screen Sharing on the other Mac), there is still a "ghost" cursor moving in the Screen Sharing window. That is highly irritating and misleading.

The use is a bit too complicated, but see the next section.

I suspect that the use of FluffyDisplay can't be made much simpler or more automated while still being able to run FluffyDisplay as a sandboxed app.

Sandboxing and notarization is something I definitely want to keep. End-users should not run random non-sandboxed apps downloaded from the Internet, period. I don't trust such apps, and correspondingly, it would be rude to expect people who download a ready-built FluffyDisplay app to trust it.

As this is open source, I can't prevent a third party from taking this code and producing something similar. That might then be distributed as a non-sandboxed app that works in a much more automated fashion. But end-users should then be aware that such an app could potentially be a very large security risk. The FluffyDisplay.app released here (inside the .zip archive(s) is digitally signed, securely timestamped, notarized, and runs sandboxed.

(In the same way as macOS has Sidecar, for using an iPad as a secondary display.) I would welcome that very much. They would be able to make the user experience much smoother than what this tool offers.

No way. It uses undocumented CoreGraphics APIs and requires the com.apple.security.temporary-exception.mach-lookup.global-name entitlement.

If you find FluffyDisplay useful, and want to thank me in some way, feel free to contact me and I can send an invoice. Or point you to a charity of my choice.