As I haven’t found any answer to my question, I making this topic.
After a fresh install of Manjaro as a virtual machine, everything works perfectly (no flickering or glitching).
I use 3D hardware acceleration.
After a fresh install, KDE version is 5.19.5 with kernel version 5.8.16-2.
Per habits, I want to update system to the most recent version.
After update, KDE version is 5.20.2 with kernel version 5.8.18-1.
I can’t paste link to a GIF showing the result…
If anyone has an idea without disable 3D hardware acceleration.