nixos: update default cases from KDM/KDE4 to SDDM/KDE5

[grahamc]

Motivation for this change

KDE4 is now extremely out of support, and we should get rid of it.

This PR is completely untested. I'm not even sure how to test it!

cc @domenkozar @globin @vcunat @ttuegel

#15866

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[mention-bot]

@grahamc, thanks for your PR! By analyzing the history of the files in this pull request, we identified @thatdocslady, @edolstra and @matthiasbeyer to be potential reviewers.

[vcunat]

KDE 4 a few lines above.

[grahamc]

Fixed, thank you!

[vcunat]

We might as well fix the "gdb" display manager :-)

[grahamc]

Fixed! :)

[vcunat]

Seems OK to me, at least in principle. Those tests hopefully won't break.

[grahamc]

This test is already broken (not run by hydra, either.)

[grahamc]

This test is already broken (not run by hydra, either.)

[edolstra]

BTW, we should not remove KDE 4 just yet. I switched to KDE 5 for a while but it didn't work very well for me, so I went back to KDE 4...

[grahamc]

That is disappointing that it didn't work well. @Baughn was saying the same in #nixos. I think we need to be moving away from 4, though.

I'd also like to deprecate kde4 / kdm as well. Is that ok with you?

[Baughn]

To be fair, my hardware is unusual (a g-sync monitor). I have long since stopped expecting unusual hardware to work in Linux.

[ttuegel]

BTW, we should not remove KDE 4 just yet. I switched to KDE 5 for a while but it didn't work very well for me, so I went back to KDE 4...

To be fair, my hardware is unusual (a g-sync monitor). I have long since stopped expecting unusual hardware to work in Linux.

@edolstra @Baughn Are there issues filed and have you tagged me in them? To the best of my knowledge, KDE 5 runs absolutely perfectly.

In all seriousness, it should be extremely embarrassing that we even ship KDE 4 at this point. I say this in all seriousness: running KDE 4 on NixOS is about the least-secure thing you could possibly do and if you can't upgrade to KDE 5, you would be better off (from a security standpoint) uninstalling NixOS and installing Windows XP. Windows XP has had security updates more recently than KDE 4, and that's not even considering the fact that NixOS has never had a KDE 4 maintainer.

[Baughn]

Are there issues filed and have you tagged me in them? To the best of my knowledge, KDE 5 runs absolutely perfectly

It isn't a NixOS issue, it's a KDE/nVidia issue. I've attempted to report it, but as the KDE developers don't have g-sync screens I was informed that they had no way of reproducing, and I lack the ability to debug it myself.

[FRidh]

I say this in all seriousness: running KDE 4 on NixOS is about the least-secure thing you could possibly do

Shall we mark KDE4 packages as broken then?

[edolstra]

I'm not really all that concerned about the security of my desktop environment. I don't use it for Internet-facing things. Might be different for people who use Konqueror....

[Baughn]

Meanwhile, I can happily report that KDE5 now works.

I just had to wait twenty minutes for it to launch, then guess that I needed to disable screen flipping in nvidia-settings. (There's a conflict between screen flipping, g-sync and the OpenGL compositor; if all three are enabled it doesn't work. Oddly enough, disabling flipping does not cause tearing.)

[edolstra]

Forget what I said above. If upstream no longer supports KDE 4, neither should we.

[Baughn]

KDE5 also appears to be buggy as hell. I don't think I'll be keeping it.

[ttuegel]

There's a conflict between screen flipping, g-sync and the OpenGL compositor; if all three are enabled it doesn't work. Oddly enough, disabling flipping does not cause tearing.

@Baughn The compositor should be able to prevent tearing on its own. As far as G-Sync, you can set window-specific rules to disable the compositor when certain applications are open (assuming you don't need it enabled all the time).

[Baughn]

Yeah, my current issue is that using KDE5 drops my FPS in all sorts of games by around 50%. I think there's also a more general performance drop, but it's harder to tell.

Disabling the compositor helps, but then the taskbar stops updating.

In any case, this is mostly am excuse for me to get off my back and install xmonad already. No need to hold off on my behalf.

[ttuegel]

Disabling the compositor helps, but then the taskbar stops updating.

@baugh Are you running windowed? IIUC, G-Sync doesn't work anyway in windowed mode.

[Baughn]

@ttuegel Fullscreen, of course. This is really not the place to have this discussion, though.