Observation

I have two external monitors, the first one is a 2.5k Samsung monitor, and athe second one is a Asus monitor.

If I use the built-in monitor in laptop, everything works perfect.
If I connect my laptop to an external monitor, strange things happens:

• In KDE, if the global scale is 100%, then qt widgets renders normally.
• In KDE, if the global scale is 150% or 200%, then qt widgets renders corruptly until a re-paint.
• For both Samsung monitor and Asus monitor, the external monitor screen becomes black, only with a movable mouse cursor. (The desktop environment is running normally, but renders black with only a mouse cursor)
• For both Samsung monitor and Asus monitor, awake from a sleep makes the external screen becomes black.
• For both Samsung monitor and Asus monitor, there is a possibility to reboot into black screen in external monitor (The desktop environment is running normally, but renders black with only a mouse cursor).
• Only for the Asus monitor, move the windows will cause a compisition flickers on top-left of the moved window, and you can see the things below the moved window.
• In Tray Panel, the Plasma Renderer says Software rendering in use.

Solution

Add nvidia_drm.fbdev=1 into your kernel parameters, to let KWin runs with hardware rendering mode.