bug: `qt.platformTheme == "kde"` breaks KDE Plasma 6 System Settings

[NovaViper]

Are you following the right branch?

• [X] My Nixpkgs and Home Manager versions are in sync

Is there an existing issue for this?

• [X] I have searched the existing issues

Issue description

If I set qt.platformTheme to "kde", it installs all of the packages meant for KDE Plasma 5; which works fine on Plasma 5 but it completely breaks Plasma 6 (it makes many core system settings disappear from the System Settings app, like the Display Configuration and more). there should be an option for KDE Plasma 6 so it can install the packages under kdePackages (which is where all of the Plasma6/QT6 packages are coming from) instead of libsForQt5.

Maintainer CC

@rycee @thiagokokada

System information

❯ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 6.7.7, NixOS, 24.05 (Uakari), 24.05.20240303.768bb87`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.18.1`
 - nixpkgs: `/nix/store/l5xja2y73b785kppa8glplaw3r584gvh-source`

[muggenhor]

I experienced the same issue after just upgrading to nixos-24.05 (and going from services.xserver.desktopManager.plasma5.enable to services.desktopManager.plasma6.enable in the process).

For now I've set qt.enable = false in my Home Manager config and the System Settings app has become usable again.

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[Rayzeq]

This issue is still present. For those searching for a workaround, here is my config before the issue:

qt = {
    enable = true;
    platformTheme.name = "kde";
    style.name = "Breeze";
}

and after:

# Technically not related to this issue, but still useful
xdg.configFile."menus/applications.menu".source = "${pkgs.kdePackages.plasma-workspace}/etc/xdg/menus/plasma-applications.menu";

qt = {
    enable = true;
    platformTheme.package = with pkgs.kdePackages; [
        plasma-integration
        # I don't remember why I put this is here, maybe it fixes the theme of the system setttings
        systemsettings
    ];
    style = {
        package = pkgs.kdePackages.breeze;
        name = "Breeze";
    };
};
systemd.user.sessionVariables = { QT_QPA_PLATFORMTHEME = "kde"; };