I have a visual artefacts in a KDE popup boxes and KDE menu with a Mesa 18.3.4 + ATI Park (Cedar (RV 810)):
Changing visual renderer in a KDE Settings from OpenGL 2.0 to OpenGL 3.1 wasn’t help.
Previously Mesa (18.0.2 - 18.1 - 18.2 - maybe 18.3.1) works well.
I am using Mesa repository as system: https://download.opensuse.org/repositories/X11:/XOrg/openSUSE_Leap_15.0/x86_64/.
Additional info: it seems to me that Mesa 19.0 will be delayed due to requirements of LLVM:
50s] llvm-config found: NO found ‘6.0.1’ but need ‘>= 7.0.0’
It is better not to update to Mesa 19.0 with a openSUSE 15.0 right now because some files have previous version (18.3.4).
Popup box in a LibreOffice Writer:
Some popup boxes have artefacts, and some don’t.
Does the problem remain if you switch to the modesetting DDX? https://en.opensuse.org/SDB:Nomodeset:Work_Around_Graphic_Upgrade&_Installation_Obstacles describes switching, and why to do so.
I have two 1002:68f9 HD5450 Cedar running on modesetting DDX in TW and haven’t seen what your Plasma images show.
Problem was solved by a updating to a Mesa 19.0.1 with an aforementioned openSUSE Mesa repository.
All Mesa files in a repo was updated to a 19.0.1, another files (such as xf86-video-ati-19.0.1-lp150.91.1.x86_64) was also updated.