[Wayland][multi-screen][NVIDIA] Black screen with 3 monitors on KWin 6.6.6; X11 works

[Mikael Larsson]

Hello KWin developers,

I am reporting a reproducible multi-monitor Wayland failure on an Ubuntu
26.04/Kubuntu workstation. The symptoms closely match previously fixed KWin
multi-screen issues, but I am reproducing the problem on KWin 6.6.6.

Please let me know whether this should be filed as a new Bugzilla
regression or attached to an existing report.

SUMMARY

With three monitors connected to an NVIDIA GeForce RTX 4070, Plasma Wayland
produces black screens. The same three monitors work simultaneously in a
Linux TTY and in Plasma X11.

Switching to X11 immediately restores a working three-monitor desktop.

SYSTEM

   - OS: Ubuntu 26.04 LTS (Resolute Raccoon), Kubuntu/KDE environment
   - KWin: 6.6.6
   - Qt: 6.10.2
   - Kernel: 7.0.0-29-generic
   - GPU: NVIDIA GeForce RTX 4070
   - NVIDIA driver: 595.84, open kernel modules
   - nvidia_drm modeset=Y
   - nvidia_drm fbdev=Y
   - simpledrm is not loaded
   - The system also has an Intel iGPU, but all three displays in this test
   are connected to the RTX 4070.

MONITORS

   - 3840×2160 display via DisplayPort
   - 3840×2160 display via DisplayPort
   - 1920×1080 display via HDMI, portrait orientation

REPRODUCTION

   1. Connect all three monitors to the RTX 4070.
   2. Start Plasma Wayland.
   3. KWin detects the outputs, but the graphical session becomes black on
   all displays / applying the output configuration fails.
   4. Switching to a TTY gives visible output.
   5. Starting Plasma X11 with exactly the same physical monitor
   configuration gives a normal desktop on all three displays.

I also tested a deliberately conservative Wayland configuration:

   - Both 4K displays: 3840×2160 @ 60 Hz
   - 1080p display: 1920×1080 @ 60 Hz
   - HDR disabled
   - Wide color gamut disabled
   - VRR disabled
   - 8 bpc
   - No display replication/cloning

The Wayland failure remained unchanged.

RELEVANT WAYLAND LOG OUTPUT

kwin_scene_opengl: Invalid framebuffer status:
“GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT”

kwin_scene_opengl: 0x502: GL_INVALID_OPERATION error generated. and are
incompatible

kwin_core: Applying output configuration failed!

During one failed Wayland session I also saw:

kwin_wayland_drm: Atomic modeset test failed! Permission denied

kwin_core: Applying output configuration failed!

HARDWARE / DRIVER CHECKS

There are:

   - no NVIDIA Xid errors
   - no PCIe/AER errors
   - no NVIDIA hardware power-brake events
   - normal RTX 4070 200 W power limit

All three displays work in both TTY and Plasma X11, making a physical GPU,
cable, port or PSU limitation unlikely.

X11 CONTROL CASE

Plasma X11 currently works correctly with all three displays:

   - portrait display: 1920×1080 @ 60 Hz
   - 4K display 1: 3840×2160 @ 240 Hz
   - 4K display 2: 3840×2160 @ 120 Hz

No new GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT errors occur under kwin_x11.

POSSIBLY RELATED KDE BUGS

   - KDE Bug 500819 — SDDM / Desktop is not reachable since 6.3.2 with
   multiple monitors configuration
   - KDE Bug 491751 — Wayland freeze with
   GL_FRAMEBUFFER_INCOMPLETE_ATTACHMENT, multiscreen/Wayland-only
   - KDE Bug 509635 — black screen / output configuration failure in a 4K
   multi-screen Wayland setup; X11 works

The important difference is that these reports are marked fixed, while
closely related behaviour is reproducible here on KWin 6.6.6.

I can reproduce the problem and can provide full journalctl output, DRM
state, EDIDs, kwinoutputconfig.json, or test specific
configurations/patches if useful.

Best regards,

Mikael
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