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.
  • on trixie (Debian 13.3) standard display tools didn't work. Instead: xrandr --output DVI-I-2-2 --mode 1920x1080 --pos 1920x0 (or similar)

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

• install with flatpak

• run Moonlight on the machine which will be the secondard monitor