Update:
Please ignore this post.

I am wrong. The tweak won't solve the problem. The flickering problem is happening all again. So sad.

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I notice an option in System Settings (Hardware->Display and Monitor->Display Configuration->Legacy Applications(X11)), which I believe controls how xwayland applications are scaled.

I would stick to choosing "Apply scaling themselves" so that xwayland windows can look smoother on my hi-dpi screen, but I experienced some display glitches with certain xwayland applications: Virtualbox and WPS Office keep flickering. I thought this may have something to do with non-integer scaling, so I tried switching to "Scaled by the system", and Virtual Box is now functional (WPS untested), however, at the cost of blurring xwayland applications under hi-dpi.

So, I would like to ask, is there a way to specify scaling mode for different xwayland applications, so I can use system-scaling for Virtual Box only?
(Of course, it would be better if the flickering issue can be addressed in a more elegant way, but I fear that is an upstream problem beyond my reach.)

System information:
DIstro: Manjaro
Host: 82JQ Lenovo Legion R9000P2021H
Graphic Card: RTX 3060 Mobile, proprietary driver installed
Plasma Version: 5.26.4
KWin Version: 5.26.4
XWayland Version: 22.1.7
VirtualBox Version: 7.0.4

Monitor 1 (Builtin): 340x220mm, 2560x1600px, scaled to 100%
Monitor 2: 530x290mm, 1080x1080px, scaled to 150%