Solution found!! My brother is a high level IT Professional who has been using Linux since early days. He also runs Manjaro i3 on the Raspberry Pi 4. While he does NOT run a dual monitor setup when he ran xrandr he was getting the same output as Mobius1 and I. He was unable to change his screen resolution or rotate his screen because xrandr was not recognizing his display correctly.
He knew it worked on XFCE so he compared the Xorg start logs for i3 and XFCE. While XFCE probed the monitor and got modes back, i3 did not. After comparing the X configs, he found the problem. There is a config file for a non-Pi graphics driver that somehow ended up in i3’s X config. This is at best useless, at worst dangerous. Anyway, if you delete this file, the Pi will use a generic probe on boot and set up your displays correctly. It will also fix xrandr so it will see your displays and allow you to configure your screen resolutions, rotations, etc.
One final ancillary benefit of this fix is that it significantly increases Picom’s efficiancy. Before implementing this solution Picom was using 12 - 15% of my CPU at rest. Enough that I always had to disable Picom. After implementing the fix outlined here, Picom is using a fraction of 1% CPU.
I suggest you try this solution. It is easy, and works immediately:
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Boot into Manjaro i3
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Open a terminal and become root
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Navigate to /etc/X11/xorg.conf.d
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In xorg.conf.d you will find the file 99-fbturbo.conf.d Remove this file by running the command:
rm 99- fbturbo.conf.d -
Reboot
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Enjoy your new dual display, or new ability to adjust your display settings with xrandr!!
I consider this SOLVED (at least for me), thanks to my brilliant brother…
One final thing, there is a config file in xorg.conf.d for a touchpad that is called something like ‘30-touchpad’? You can remove that as well, unless you are running a keyboard with a touchpad, which is unlikely if you are using the Pi 4."