Bug 2490995

I just upgraded my system to Linux 7.0 and KDE Plasma 6.7 (https://bodhi.fedoraproject.org/updates/FEDORA-2026-279229780a) and now I'm presented with just a blinking cursor after the Plymouth boot screen.

I found that:
- plasma-login-manager fails to start
- SDDM similarly fails to start
- GDM works fine
- Plasma (Wayland) sessions, from GDM, similarly fail to start
- GNOME sessions, from GDM, work fine

Notable logs from kwin_wayland:
> Failed to open /dev/dri/card0 device (Device or resource busy)
> No suitable DRM devices have been found

Many other KDE components (kded6, powerdevil, kaccess, etc.) are all crashing instantly on startup with Qt platform initialization errors:
> the application failed to start because no Qt platform could be initialized
> Could not load the Qt platform plugin "wayland" even though it was found
> Could not load the Qt platform plugin "xcb" even though it was found

Reinstalling mesa-dri-drivers, qt6-qtwayland and clearing the QML caches (to the best of my abilities) did not solve the problem.

Reverting to Linux 6.9 from the GRUB boot selector did not help either.

Starting Plasma in a X11 session, in contrast, works fine.

Similar case in the wild (reporter + 5 me-too's):
https://www.reddit.com/r/cachyos/comments/1ua1hze/update_broke_my_plasma/
> I updated and then went to sleep last night, I wake up this morning to find that whenever I login I always end up on a black screen with a cursor

Attaching logs of a failed attempt at starting a Plasma session.

Reproducible: Always

Steps to Reproduce:
1. Have plasma-login-manager or SDDM set as the desktop manager
2. Boot system

...or...

1. Have GDM set as the desktop manager
2. Log in with a Plasma session
Actual Results:
Blank screen with cursor blinking

Expected Results:
Login screen or desktop showing up normally

Additional Information:
GPU: Nvidia GeForce GTX 970
Driver: Nouveau
Kernel: 7.0.12-201.fc44.x86_64
Packages:
- qt6-qtbase: 6.11.1-1.fc44
- plasma-desktop: 6.7.0-1.fc44