SDDM manages (two Wayland) sessions incorrectly

Support Software & Applications
1

Hi,

I’ve got two session types, Plasma (Wayland) and Niri for the same and only user. SDDM is offering the choice of sessions, as it should. If I just log into Niri, again and again, everything’s fine. When I try to log into Plasma, I get a black screen. As soon as I press Ctrl+Alt+F3, I see SDDM screen again. If I keep on trying to log into Plasma, I can’t get beyond the black screen.

The weird thing happens when I change the required session back to Niri. The login takes a lot of time, and in the end I have both Plasma and Niri on my monitor. Both Plasma Panel and Niri (DMS) are functional, though Niri takes over the shortcuts and window management :slight_smile: . Logging out and again into Niri takes me back to normal.

I suspect that something around virtual terminals, or whatever they are in Wayland, is configured incorrectly. Unfortunately, I don’t know where to start.

On another box, a very similar installation works fine. The session configuration seems identical.

Niri installed via GitHub - AvengeMedia/DankMaterialShell: Desktop shell for wayland compositors built with Quickshell & GO, optimized for niri, hyprland, sway, MangoWC, and labwc.

2

I cannot say I know what is head and tail in this - but I suspect there is some bleed-in from Plasma onto Niri - perhaps back as well - through sddm.

I can make Niri behave absolute strange in some instances.

I have found that using greetd instead of sddm eliminates some of the issues.

Example

  • select Niri from the sddm greeter
  • connect from my workstation using konsole and waypipe
    waypipe ssh user@host startplasma-wayland
  • closing the konsole session make the the plasma session bleed into the Niri session in an unpredictable manner
  • I had to restart user space to get back systemctl soft-reboot

I theorise that because of the wayland protocol some mix-in can happen through shared memory buffers - understanding it is way above my paygrade … :slight_smile:

On the laptop I can also start a labwc session - which I have setup to autostart dms.

Because of this topic - I got the idea to sign onto a labwc - simply to see if I could reproduce the behaviour - and hey-ho - I cannot.

Which could mean the issue relates to the Niri session somehow - still way beyond me… :slight_smile:

EDIT:
I almost forgot - I am running Plasma 6.6 beta with plasmalogin as greeter (fork of sddm so possibly very alike)

1 Like
3

and how to solve the Authentication Error ?

4

This is expected.

Two concurrent sessions are explicitly unsupported by KDE.

The ReuseSession= setting in SDDM configuration should allow a second session, but it doesn’t. Instead, it causes Plasma to display a black screen. KDE have no intention of fixing this.

However, KDE do intend replacing SDDM with the new Plasma Login Manager, which does not have this limitation.

So, that’s great news over the horizon, but meanwhile there is little option than to wait – Fedora will apparently be the first Linux distribution to adopt the Plasma Login Manager, but that’s not scheduled until April 2027.

1 Like
5

Sorry, then it shouldn’t work at all, ever. Meanwhile, it works on another box, with about the same present configuration, but a very different story of killing/reinstalling desktops :slight_smile:

I don’t really know what are “concurrent sessions”, but I never meant anything like that. Pure unintended consequences. I’ve been trying merely to log out of one session, then log in into another.

6

Apart from the polkit issue, which - for what I know - only manifest on kernels earlier then 6.6 - I have no issues, authentication and otherwise.

That is a bit strange :thinking: technically it could be anything - I have a habit of pushing my test systems to the limits - and do things that is highly discouraged by the experienced members of this forum :slight_smile: - but when you learned the flow, it is also a piece of cake to rollback.