Just a quick note than an alternative to getting a dummy HDMI dongle is to use the dummy monitor driver xserver-xorg-video-dummy. I've used it with X11vnc on Ubuntu 20.04 and 22.04. I use a script to switch between dummy mode and HDMI mode depending on whether or not I want to run my Mele PCs headless or not.

If interested just google xserver-xorg-video-dummy. There are articles out there on setting it up.

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