hello 
When my computer’s screen goes to sleep I am noticing from time to time horizontal white line appear. It is happening on KDE and silverblue since fedora 38. Adding images captured on a phone camera. I wonder what would be causing that?
When you get these lines with different OS’s, I don’t think it is caused by them, but something might be wrong with the screen itself. Is this a built-in laptop screen or an external one?
What does the monitor do when the computer is either off or producing a totally black screen like with alt-F1 or even in a terminal in maximum screen size mode?
Is it a fairly new monitor or is so old already that failures are to be expected?
Does the line show when you go into the UEFI BIOS setup?
As @jandemus suggests this may be a propblem with the display hardware.
It is a built-in OLED laptop screen - Dell/Samsung SDCA029 on 2020 razor laptop. It took very long for Kernel developers to implement screen dimming and it still does not work very well nor on KDE, nor on silverblue - it forgets brighness settings after screen sleep.
This horizontal line do not appear on an external lcd display connected thru HDMI.
I do not think it appears because screen is old or damaged.
Does the line show when you go into the UEFI BIOS setup?
No, it does not.
Okay, so we know that. Would you try to boot into a live environment and put that to sleep? Start with the same version you have installed as well, and try a completely different one. Just to see if it is software related.
I just tried to boot fedora 39 live from USB and the white line when screen goes to sleep still appearing. Now I am downloading fedora 36. I will try it tomorrow.
I was wrong - with fedora 36 white lines appearing as well.
Can you also download a completely different distro, for example Ubuntu or Mint? Just to see if it is Fedora related.
With Ubuntu as well 
Hello @jandemus ,
Just to check things like your hardware you could run the Phoronix test suite on your graphics subsystems and see if there truly is a hardware issue or not. I believe they are in the fedora repos, but … yes the are
jakfrost ~]$ dnf5 search phoronix
Updating and loading repositories:
Repositories loaded.
Matched fields: name
phoronix-test-suite.noarch: An Automated, Open-Source Testing Framework
I do not think that it is hardware related problem, because on windows when screen goes to sleep no white lines appear and dimming working as it should. Most probably there are no proper drivers for this display in Linux kernel.
I installed phoronix, but there are so many tests.
Witch one to run to test display?
Well, sorry I didn’t notice your previous comment about Windows not having the issue. I would agree with your assessment and testing your hardware would only be a time sink. It could be related more to the handling of the power management at the device level, like are artifacts being left in the video memory, or does the video not get to the desired suspend state?
Amateurish workaround for the problem. I installed “Gtk4 Desktop Icons NG (DING)” from “Gnome Extensions” added black jpg image equivalent to desktop resolution and when I leave computer for some time before going I open that jpg with an image viewer in full screen mode. It is an OLDE screen so most probably it will be using the same amount of electricity as if it is off with screensaver and no white lines. 