Mamarok - finally the right clue. I switched the cables to the two monitors, and the artifact did not move, supporting your thought it's something in the monitor itself. So I started playing with the on screen monitor settings, and changing the "ClearView Display" removed the artifact. Oddly, putting it back to its original setting did not bring the artifact back. So, I still have no idea what the actual problem was, but it was something in the monitor, not in the software. I suppose I can mark as solved.

I don't think it could be the background picture, because I don't think that could affect the rendering of any windows above that spot, but who knows. Also, it's the default background, but I don't know if it's KDE's default, or Gentoo's. I certainly didn't make any explicit choice. Doesn't matter now, fortunately.

I was searching here for a solution for the exact same problem...



Something that I noticed is that when I change the composite from OpenGL3 to Xrender, the issue disappeared .. IDK but this option may not accelerate the composite by hardware...

I checked and have installed OpenGL... I don't know what else could be causing this...

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~$ glxinfo | grep "OpenGL version"

OpenGL version string: 4.5 (Compatibility Profile) Mesa 21.2.1


glxgears works fine tho...

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$ glxgears
Running synchronized to the vertical refresh.  The framerate should be
approximately the same as the monitor refresh rate.
X connection to :0 broken (explicit kill or server shutdown).