this is useful if using something like, for example Deskreen to mirror a secondary display on another device when no other physical secondary display exists. original source

• Extensible Virtual Display Interface (EVDI) module: sudo apt install evdi-dkms
• load module: sudo modprobe evdi initial_device_count=4
• now if you do xrandr you'll see some virtual DVI ports available. Choose one to be your enabled port. For this example we'll choose DVI-I-2-1 with format DVI-I-x-y e.g. x=2 y=1
• add resolutions as you need:
  • existing resolution: xrandr --addmode DVI-I-2-1 "1920x1080"
  • create a resolution. Example: 2048×1536:
    • cvt 2048 1536 will return Modeline "2048x1536_60.00" 267.25 2048 2208 2424 2800 1536 1539 1543 1592 -hsync +vsync
    • then do: xrandr --newmode 2048x1536_60.00" 267.25 2048 2208 2424 2800 1536 1539 1543 1592 -hsync +vsync
• force virtual port as connected: sudo sh -c "echo on > /sys/kernel/debug/dri/2/DVI-I-1/force" note x=2 y=1 in this example
• From here use the standard display tools (GUI or otherwise) to set the monitor position etc.

Sunshine can run on the host machine to show the virtual display on another machine

• install with flatpak