Plasma6 on nixOS 25.11: KDE Applications and plasmashell don't launch

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1

hiya!

I am running nixOS 25.11, and am struggling to get KDE Plasma 6 running properly as per the wiki (wiki.nixos.org). Having put the relevant options in I do get SDDM and Plasma itself, but most KDE applications including krunner, spectacle, and plasmashell itself exit with an error similar to this:

$ plasmashell --replace
kf.plasma.quick: Applet preload policy set to 1
QQmlComponent: Component is not ready
kf.plasma.quick: Error loading the wallpaper QList(file:///nix/store/vrfi5n3hma1x09hn4sasv6j4738z8ab7-plasma-workspace-6.5.6/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml:56:5: Type ImageStackView unavailable
        ImageStackView {
        ^, file:///nix/store/vrfi5n3hma1x09hn4sasv6j4738z8ab7-plasma-workspace-6.5.6/share/plasma/wallpapers/org.kde.image/contents/ui/ImageStackView.qml: module "breeze" is not installed)
kf.plasma.quick: "file:///nix/store/vrfi5n3hma1x09hn4sasv6j4738z8ab7-plasma-workspace-6.5.6/share/plasma/wallpapers/org.kde.image/contents/ui/main.qml" 
 "Error loading QML file.\n56: Type ImageStackView unavailable\nmodule \"breeze\" is not installed\n"

I’ve made sure to update nixpkgs, my full configuration is here: tasiaiso/nixos-config

The relevant files (AFAIK) would be:
common/components/de/plasma6.nix
hosts/e490/configuration.nix

nixpkgs is handled by nix-flake, which I’ve locally cloned and ran nix flake update nixpkgs on to update that input, and subsequently pointed my local copy of nixos-config at on my filesystem. I then also ran nix flake update nix-flake there and rebuilt.

I am on commit 7e495b747b51f95ae15e74377c5ce1fe69c1765f of nixpkgs.

2

(do note I didn’t push the flake.nix change in which I made the nix-flake input point at a local directory.)

3

I did, finally, figure it out.

To quote myself:

Apparently, there is no “breeze” theme that works across QtQuick and
QtWidgets. Force-setting “breeze” here breaks things as apps start
looking for things they don’t have.

Simply unsetting it (qt nixos options) fixes the issue.