Black Plasma desktop while application windows render correctly (X11)

[gregl]

Hello,

I would like to report what appears to be a rendering issue in NoMachine.

Environment
NoMachine: 9.7.3

Distribution: openSUSE Tumbleweed

Desktop: KDE Plasma 6.7.0

Session type: X11

GPU: NVIDIA GeForce RTX 5060 Ti

NVIDIA Driver: 580.159.03 (Open Kernel Driver G06)

Resolution: 1920×1200

Connection type: LAN

Client: Windows 11

Server: Linux

Problem description

The remote connection is established successfully. Authentication works correctly. The local Linux desktop works perfectly. However, when connected through NoMachine:

• application windows (Firefox, Konsole, etc.) are rendered correctly,
• mouse and keyboard work correctly,
• video acceleration (NVENC/H.264) is active,
• the Plasma desktop itself is black,
• the bottom Plasma panel is not rendered.

Interestingly, the connection preview thumbnail inside NoMachine already shows the same issue, suggesting that the problem originates on the server side before the image reaches the Windows client.

What has already been verified

• X11 session (not Wayland)
• PlasmaShell is running normally
• Physical monitor works correctly
• NVIDIA driver works correctly
• EGL Capture disabled (no change)
• NoMachine services are running correctly
• Authentication works correctly
• Hardware encoding works correctly
• Applications are displayed correctly
• The issue only affects the Plasma desktop (wallpaper + panel)

NoMachine configuration

Relevant configuration:

• EnableEGLCapture 0
  EnableDisplayServerVideoCodec 1
  DisplayServerVideoFrameRate 60
  DisplayServerUseVideoFrameRate 1

Changing EnableEGLCapture from 1 to 0 did not change the behavior.

Expected behavior

The complete KDE Plasma desktop (wallpaper, panel and widgets) should be rendered exactly as it appears on the physical monitor.

Actual behavior

Only application windows are visible.

The desktop background and Plasma panel remain black or are not rendered.

Could you please confirm whether this is a known issue with KDE Plasma 6.7 on X11?

If needed, I can provide additional logs or perform further testing.

Thank you.

 

[Britgirl]

Hi,

is this a headless machine?
Are you connecting to the physical display?
When you connect, is the desktop you see in the NoMachine Player the same as the local Linux desktop?

In the meantime, you could try disabling hardware decoding on the Windows (client) side – connect, open the NoMachine menu, go to Display > Change display > Advanced Option (Click Modify) > Tick the box for Disable client side hardware decoding.

[gregl]

Thank you for your reply.

To clarify the setup:
Both the Windows 11 client and the openSUSE Tumbleweed server are connected to a single physical monitor through a hardware KVM switch (Unitek V1310AGY01‑EU, DisplayPort). The Linux machine is not headless — I am connecting to the physical X11 display.

Yes, I am connecting to the physical desktop.
Locally, the KDE Plasma desktop renders correctly.
In NoMachine, only application windows are visible, while the Plasma background and panel appear black.

The desktop shown in NoMachine is therefore not the same as the one displayed on the monitor.

I also tested disabling client‑side hardware decoding as suggested, but this did not change the behavior.

If helpful, I can provide logs or EDID information from both KVM ports.

Thank you for your assistance.

[bucu]

Hi,
Could you please run this command on the server:
xwd -root -out ~/root-window.xwd
Then open the file and check if the desktop background and panel are visible, or if parts are missing/black.
Also, does the issue still happen if you connect without the KVM switch (direct cable)?
Let us know the result of the xwd test and whether bypassing the KVM makes any difference.

[gregl]

Hello,

I have now found a solution.

The issue was related to the monitor being connected through the hardware KVM switch. When the KVM was switched to the other computer, the Linux machine apparently lost or changed the monitor EDID, even though the X11 session itself remained active. In that state, NoMachine displayed application windows correctly, but the Plasma desktop background and panel were missing or black.

I connected a DisplayPort dummy plug / EDID emulator to the second DisplayPort output of the NVIDIA GPU and left it enabled as a separate 1920×1080 display, without screen mirroring.

Current setup:

• DisplayPort 1 → KVM switch and physical monitor
• DisplayPort 2 → dummy DisplayPort EDID emulator

After switching the physical monitor to the Windows computer, NoMachine now continues to show the complete Plasma desktop, including the wallpaper and bottom panel.

I also tested connections from another Windows 11 computer and the full desktop is rendered correctly. Disabling client-side hardware decoding and changing EnableEGLCapture were not necessary for the final solution.

Therefore, the problem appears to have been caused by loss of EDID / active display information when the KVM switched away from the Linux computer, rather than by Plasma rendering or NoMachine video encoding.

Regarding the requested xwd test: the issue no longer occurs with the dummy plug connected. I can still perform the test without the dummy plug if it would be useful for further diagnosis.

Thank you for your help.

[Author]

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