External monitor detected but shows no video output in Sway / Wayland

Question

I have an external monitor connected to a thunderbolt dock which I would like to utilize. For whatever reason, the monitor appears to be recognized by the system but fails to display anything.

• I can confirm that the cable and monitor are fine, as it works perfectly when booted into Windows
• The thunderbolt dock seems to not be the issue, as USB peripherals are working fine
• Running swaymsg -t get_outputs shows the built in monitor as eDP-1 and the external monitor as DP-2. There is no indication here that the second monitor would not be working.
• Running wdisplays shows a preview of what the second display should look like, so evidently the system is rendering what is meant to be sent to the monitor
• When I reboot the machine, the tty output is mirrored onto the second monitor. So evidently it is recognized.
• There are no errors in sway --debug
• There are no entries in dmesg which appear related to the issue
• Running lspci -k | grep -A 4 VGA shows that the video driver is loaded

00:02.0 VGA compatible controller: Intel Corporation Iris Plus Graphics G7 (rev 07)
    DeviceName:  Onboard IGD
    Subsystem: Hewlett-Packard Company Device 86f9
    Kernel driver in use: i915
    Kernel modules: i915

• I have set the environment variable WLR_DRM_NO_MODIFIERS=1 as has been recommended to solve related issues
• I have attempted to install other versions of wlroots to no avail, though I honestly don't know whether this would be expected to fix the problem.
• While the refresh rate is very bad during normal use (hence why I am attempting this method), I can successfully use this monitor with an external Nvidia RTX 2060 GPU, connected via thunderbolt 3, with the the noveau driver.

I don't really have a sense of how to best approach troubleshooting this problem, so any advice would be appreciated! If there is any additional information that would be relevant, I am more than happy to provide it.

System information:

• OS: Manjaro 21.1.6 Pahvo
• WM: sway 1.6.1
• wlroots 0.14.1-2

Answer 1

I had a similar problem, where my external displays were recognised and shown as usable in the settings, but whenever I tried to activate them and actually extend the displays, nothing happened and they remained black. I nice tooling from POP!_OS helped me out. I wrote a small post about it, hope it helps here too

Extended Displays on Manjaro with AMD CPU and Nvidia GPU