You have only one choice, SPACEDESK for now.
When I got started working-at-home because of covid-19, I thought about how I could use my iMac as a subscreen of my company-provided windows note PC. I surfed many sites, and all of them say, "You can use the target-display-mode. Beware that Retina displays won't support this function but Mid 2014 or the older model will." My iMac is a Retina model. Oh, my. But finally, I found SPACEDESK will do for my purpose. This information is not reported widely for some reason, so I will write here.
Setup the primary machine (Windows PC)
SPACEDESK DRIVER SOFTWARE FOR WINDOWS PRIMARY PC (SERVER)
There are four installers for Windows10/8.1 x 64/32bit. Download a suitable installer for your PC. If conditions in Attention! or Important apply to your PC, update the graphic drivers before installing SPACEDESK. The install method is described at
SETUP PRIMARY MACHINE. Probably you do not need to read "Verify if Setup was Successful" if no error occurs during the install.
Download the installer and right-click it.
The administrator authority is required to perform the installation. You have to give up if you do not have the authority because of the security policy of your company.
Once the installation is completed, there will be an icon of SPACEDESK in the taskbar. You can right-click the icon and open the window of SPACEDESK. See the status is ON. Check the IP address of your PC. It is a private address in most cases.
You can also change the Status OFF.
Display Settings... moves you display settings of Windows. You can change the screen layout there.
View SPACEDESK from iMac
There are client viewers for Windows10/7, iOS, Android, and Amazon at
SPACEDESK VIEWER SOFTWARE FOR SECONDARY MACHINE (CLIENT).
There is none for macOS, but you do not have to give up. Open Go To HTML5 Viewer Page by Safari or Chrome on your iMac.
Input the IP address of your Windows PC then connect. Once you connect, iMac turns into the subscreen of Windows PC.
Push esc key and exit fullscreen, click the hamburger icon, open the settings menu.
You can control the Windows PC by the magic trackpad of iMac, however, I do not recommend doing it because the mouse cursor on iMac becomes difficult to use. I recommend this function in case of using a tablet as a subscreen.
The SPACEDESK server (Windows) sends compressed pictures and the SPACEDESK viewer (iMac) decompresses them, I think. When I move the cursor, some blur appear on fine details on the edge.
Stopping the cursor, details become clear. You can find the difference between upper and lower snapshots, can't you?
You are able to tune the performance, but you should take the network speed into consideration.
Display settings of Windows
Please layout multi displays as you wish. I love large screens!
SPACEDESK is great, but a dedicated display and cable are best as expected.
If you don't have any external display but iMac, I'll highly recommend SPACEDESK. It was the biggest help for me.
Unfortunately, this app is a bit unstable than hardware displays and cables. There is a small but certain time lag. It disconnects every few hours. I have three screens now, so I don't use SPACEDESK these days.
- Target Display Mode
Retina won't support target display mode.
Sidecar won't work on Windows.
- Duet Display
Only tablets with iOS or Android can be clients.
- Luna Display
Luna display for Windows has kickstarted, but it can turn only iPad into a second display.
- Windows remote desktop for mac
It can control Windows from iMac but have a poor response.
Key layouts of Windows and iMac are a little too different.
You can also an android or an iOS tablet turn into a subscreen by SPACEDESK. Touchscreen function on the tablet remains available.
Install the SPACEDESK viewer for each OS. It works smoothly and finely than the HTML5 viewer.
You can feel like you use a Windows tablet even though using an Android when you enabled Touchpad at the settings of SPACEDESK.
I bought this tablet 3 years ago before Huawei was banned from the U.S.