Yeah this 4K monitor is capable of 138Hz through overclocking or whatever, briefly tried that on X11, but the option didn’t appear in the display settings, so I just went with 100Hz. I also saw in those links that people were having issues with 120Hz specifically, and not 144Hz or some other common frequency ranges. I expected 100Hz to work. 
And no, still on “sddm-git” - so I can try going back to that and then try 100Hz too, but I reckon that will not likely change anything?
No not likely but sddm is the standard package.
Edit: At least it’s working. On my systems i set desktop effects to run translucency, wobbly windows with setting on 1st notch and magic lamp. I also go int appearance, cursors, configure lauch feedback and set none 2 secs. You’ll find the plasma desktop is much smoother and more fluid both on X11 and Wayland.
I’m not sure if it was frequency related. I know that deleting the kscreens directory like you did at least allowed me to login though without black screen on 5.27.
As for installing plasma 6 from the testing repos. You just have to edit /etc/pacman.conf and uncomment extra-testing lines. However, since it isn’t officially released there may still be problems related to packaging. I am documenting such issues now while I wait for the official release to the stable branch. I will then report bugs for these issues. However, for the most part it works. I have a documented list of 21 minor issues that I may need to potentially file as bugs.
Plasma 6 will be out very shortly so you’ll be able to try it soon enough. At least it’s working. I haven’t had any issues with my GTX 1060 and 60 hz.
Do you use wayland or x11? I’m testing out wayland. I have a whole slew of issues currently. Nothing major, but still minor bugs here and there. Just found a few more since my last post in this thread.
Edit: Again not sure if these are packaging issues yet, so I’m waiting to report.
I’m using Wayland on nvidia and amdgpu. I don’t really have any significant issues but then again I’m not doing things like a lot of linux users tend to do. I just use the computer. No gaming or video streaming stuff whatever. I mean i do use youtube for playing music videos and i also use music players but nothing fancy here. I don’t get into changing my kde desktop settings much just minor things.
Edit: I stick to what i know works.
Yeah, that certainly simplifies things, but some of the issues I have are related to built in features. For example one issue I an currently tracking is the command palette in dolphin. I believe the default shortcut is ctrl + shift + i. If search for create folder and select that option. It will create the folder in the correct directory, but if I open a new tab to a different folder and use the command palette to create the folder again. The folder will be created in the other tab and not the current one.
Edit: the shortcut is ctrl + alt + i by default. My mistake.
Ya see i don’t do may of those things. I know Kde has a lot of features that i just don’t bother with unless i know about them and i try them and they work. Lots i don’t know or have tried.
Understandable, I stumbled on that one by accident because I rarely use the command palette for that anyways, but while I am looking for issues. I might as well report everything that I find.
I say’s create new folder F10?
Edit: I tried it just out of curiosity but haven’t tried a new tab.
Are you talking about the command palette? It’s called find action if you are looking through dolphins shortcuts. It list the action and the shortcut.
OK I switched back to “sddm” and as fully expected, no changes - broke at 100Hz again.
Anyway, now giving KDE Plasma 6 an experimental try out of curiosity. I intend to do a clean wipe of the drive afterward and then use KDE Plasma 5.27 until a stable release of the new attire - just want to experiment with this higher refresh rate bug.
I don’t think you need to do a clean wipe of the drive unless you want to but if you do I’m telling you btrfs with btrfs-assistant, snapper-support and btrfsmaintence is awesome.
Edit: Of course i use grub and i think with systemdboot you can’t boot snapshots or something? Not exactly sure? But i use it and it never fails me with grub.
Oh I’m just being extra, haha - messed around enough with stuff I don’t want anything weird lingering that causes problems for me down the road that I didn’t cause. In KDE Plasma 6 now, it’s shiny! About to reboot and see what happens. Very exciting to see the HDR toggle there too.
Do you recommend grub over systemd-boot? I’m entirely uninformed on this matter.
Well that was interesting - still broken, black screen - except this time there was an unblinking cursor at the top left of the display. No button combinations worked or anything. But…progress? LOL.
Oh Nvidia…
I’m a grub user… i can’t really say much about which is better or why. I’m just more comfortable with grub because I’m used to it. Not to say when you run into problems with booting that it is easy. I’m not an expert just a user with some knowledge and some limited skills. 
@d-air1
The folder creation works properly for me with another dolphin tab open. I assume you are creating the folders in the first tab but each in different locations. I created one in my home folder then opened another tab and then created another folder inside documents.
Edit: Is this what you are doing?
Edit: I’ll leave it at that since we are getting off topic here. Sorry.
Is there anyway for me to share a short video?
@pr0927
Hopefully this answers your black screen Wayland issue at least to some degree of resolution.
Haha, to some degree, I think. I wonder if in another day or so of this sitting out, someone who might know more on this subject might be able to swoop in with some other secret special answer.
Unfortunately, it doesn’t really fix the issue - since I have a 120Hz monitor, I’d like to use it at that refresh rate, especially when gaming. 
“I paid for the whole monitor, I wanna’ use the whole monitor.”
Of course it works on X11 - but I understand that Wayland will be replacing it as the default on KDE, and that it is meant to replace X11 outright (also not sure if any HDR capability is available on X11 in KDE Plasma 6 either - something that’s also important to me).
Maybe I’ll share this info in the subreddit and get more attention to this issue.
Appreciate your very involved assistance and that link to that thread!