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SUMMARY

Under Wayland and X11, KDE Plasma 6.0.4 does not detect external displays connected to my laptop via USB-C DisplayPort, either when connected before starting the laptop up or during the Plasma session.

STEPS TO REPRODUCE
1. Connect an external display via USB-C DisplayPort, either before starting up the laptop or during the Plasma session.

OBSERVED RESULT
Nothing happens.

EXPECTED RESULT
The external display is detected and becomes visible in display settings.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: openSUSE Tumbleweed 20240418
(available in About System)
KDE Plasma Version: 6.0.4
KDE Frameworks Version: 6.1.0
Qt Version: 6.7.0

ADDITIONAL INFORMATION
The machine has an AMD Radeon 610M internal GPU and an Nvidia GeForce RTX 4090 Laptop discrete GPU and a MUX switch. Whether the MUX switch is on or off makes no difference. The proprietary Nvidia drivers, version 550.67 are installed. Offloading using the environment variables works fine on the internal display and there are no issues with running games or other applications offloaded on the Nvidia GPU using the environment variables "__NV_PRIME_RENDER_OFFLOAD=1 __GLX_VENDOR_LIBRARY_NAME=nvidia __GL_SYNC_TO_VBLANK=". There are no issues with running applications on the internal GPU. KDE Plasma 6.0.3 did detect the external displays under Wayland (I did not check X11 on 6.0.3) and correctly allowed me to select the maximum resolution for them, but this caused them to turn black, leading me to use them at a lower resolution, which worked fine, with the MUX switch set to either discrete GPU only or hybrid graphics. However, since I could not use them at their maximum resolution, so that was not ideal either.