Wayland greeter (qt6): no way to configure multi-monitor layout

[klausman]

As per the title. With the X11 greeter, I could use DisplayCommand to run a shell script which sets the right outputs using xrandr. While something similar to arandr/xrandr exists for Wayland (kanshi, which uses the wlr-output-management protocol), there seems to be no way to hook it into the Wayland greeter's startup.

Is something like that planned? Or is there another way to tell sddm the correct screenlayout. Note that I can of course set the right layout (using kanshi) after login, but I'd like the greeter to have the right layout, too. Arguably, the greeter doing this (instead of some script) is probably the more robust approach.

[nagmat84]

What Wayland compositor are you using? IMHO, it is not the responsibility of SDDM to configure the monitor setup, but the Wayland compositor is responsible.

I am using SDDM with kwin_wayland. It that case one needs to put the configuration into ~sddm/.config/kwinoutputconfig.json. I have a triple-monitor setup and with one monitor rotated 90° and it works perfectly. In case of Plasma/KDE the easiest way to create kwinoutputconfig.json is to log into a KDE session, set the monitor layout as desired and then copy the file from the current user to ~sddm/.config/kwinoutputconfig.json. Source: Arch Linux Wiki: SDDM - Match Plasma Display Configuration.

Other Wayland compositors use different configuration files.

This is no different than before. With X11, the X server was the responsible to configure the monitors correctly. The default monitor layout was set in the Monitor section in /etc/X11/xorg.conf.d/*.conf. SDDM had never a part in that. xrandr was not part of SDDM, but part of the X11 ecosystem and requested the X11 server to change the monitor layout on-the-fly. Depending on your Wayland compositor and its Xwayland support, xrandr may even still work today, if the Xwayland compatibility layer correctly translates the legacy X11 requests from xrandr into what your compositor understands. But I assume that most Wayland compositors only allow xrandr to show the current configuration, but not to change it.

However, you can also use the native command line tool of your Wayland compositor to change the monitor layout on-the-fly. For kwin_wayland the command is called kscreen-doctor.

kscreen-doctor --help
Usage: kscreen-doctor [options] [output.<name>.<setting> output.<name>.setting [...]]
kscreen-doctor allows to change the screen setup from the command-line.

Setting the output configuration is done in an atomic fashion, all settings
are applied in a single command.
kscreen-doctor can be used to enable and disable outputs, to position screens,
change resolution (mode setting), etc.. You should put all your options into 
a single invocation of kscreen-doctor, so they can all be applied at once.

Usage examples:
...

I don't know how other Wayland compositors work, because I don't use them.

[klausman]

So the problem isn't that kanshi would not work, I just have no way to hook its startup into sddm's (or the compositor it uses, which AIUI, is different from the compositor I use for my desktop (which is labwc)). As I mentioned, for X11, one can use the DisplayCommand config option to tell sddm to run something for me, but for Wayland mode, no such thing exists, as far as I can tell.

Somewhat unrelatedly, the machine I was using SDDM on is currently out of comission (broken mainboard), so I can't really try more things until I get a replacement. I will get back to this once the replacement MB arrives.