qt6ct and qt6Packages.qtstyleplugins-kvantum are not working together

[thiagokokada]

Describe the bug

Setting QT_QPA_PLATFORMTHEME=qt5ct or QT_QPA_PLATFORMTHEME=qt6ct and QT_STYLE_OVERRIDE=kvantum is not working. This is what happens when I run the qt6ct application via command-line:

$ qt6ct
QApplication: invalid style override 'kvantum' passed, ignoring it.
    Available styles: qt6ct-style, Windows, Fusion
Configuration path: "/home/thiagoko/.config/qt6ct"
# ...

However doing the same and running qt5ct works as expected. I am assuming we are missing a wrap or something here.

Steps To Reproduce

Steps to reproduce the behavior:

1. nix-shell -p qt6Packages.qtstyleplugin-kvantum qt6ct
2. Run export QT_QPA_PLATFORMTHEME=qt6ct export QT_STYLE_OVERRIDE=kvantum
3. Run qt6ct

Expected behavior

qt6ct should work correctly and you should see kvantum styles in the Style dropdown.

Screenshots

• qt5ct

• qt6ct

Ignore the The application is not configured correctly. This happens because of the QT_STYLE_OVERRIDE=kvantum, and while this is not recommended by qt6ct it is recommended by kvantum.

Additional context

I am using the qt module in Home-Manager, but in case of issues in the module I am happily to port any fixes to it.

Notify maintainers

• kvantum maintainers: @romildo @Scrumplex

• qt6ct maintainers: @Flakebi @Scrumplex

Metadata

Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

[user@system:~]$ nix-shell -p nix-info --run "nix-info -m"
 - system: `"x86_64-linux"`
 - host os: `Linux 6.3.4-zen1, NixOS, 23.11 (Tapir), 23.11.20230623.3ae20aa`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.15.1`
 - nixpkgs: `/nix/store/h26619k1aq8xj025905p5qqsxj8xdcmm-source`

[Flakebi]

I can reproduce an issue with inconsistent looking themes, but qt6ct works with kvantum for me. I have some dark theming in qt5 and qt6, but the style is inconsistent.

In addition to your settings, I also have this in .config/qt6ct/qt6ct.conf:

[Appearance]
style=kvantum
…

My .config/Kvantum/kvantum.config:

[General]
theme=KvArcDark

Config, that I have that is related to theming:

  environment.systemPackages = with pkgs; [
      arc-kde-theme
      libsForQt5.frameworkintegration
      kde-gtk-config
      kwayland-integration
      qt5.qtwayland
      libsForQt5.qtstyleplugin-kvantum
      qt6.qtwayland
      kvantumQt6
      qt6ct
      qtstyleplugin-kvantum-qt4
  ];

    environment.variables = {
      QT_STYLE_OVERRIDE = "kvantum";
      QT_QPA_PLATFORMTHEME = lib.mkForce "qt6ct";
  };

    qt = {
      enable = true;
      platformTheme = "qt5ct";
    };

[Scrumplex]

If you set QT_QPA_PLATFORMTHEME to qt6ct, Qt 5 apps will not be themed using either qt5ct or qt6ct. If you set it to qt5ct both Qt 5 and Qt 6 app should use qt5ct and qt6ct respectively.

You shouldn't need to add qt6ct to environment.systemPackages either as that's already done by qt.platformTheme = "qt5ct"

[thiagokokada]

Ok, so if I set:

# NixOS
qt = {
  enable = true;
  platformTheme = "qt5ct";
};

It works for both qt5ct and qt6ct, however:

# Home-Manager
qt = {
  enable = true;
  platformTheme = "qtct";
};

Only works in qt5ct, even if it basically does the same (export QT_QPA_PLATFORMTHEME=qt5ct). If you set both in NixOS and Home-Manager it also works.

So I assume the issue here is either that qt5ct only works when added to environment.systemPackages (but doesn't work with the Home-Manager equivalent, home.packages) or is that the NixOS module is adding extra packages that are missing in the Home-Manager module.

[thiagokokada]

So I assume the issue here is either that qt5ct only works when added to environment.systemPackages (but doesn't work with the Home-Manager equivalent, home.packages)...

This also explain why it also doesn't work with nix-shell -p qt6Packages.qtstyleplugin-kvantum qt6ct, because the issue seems to be at user profile level.

[thiagokokada]

I double checked the packages that the NixOS modules adds vs Home-Manager and it doesn't seem they're doing anything different (both add libsForQt5.qtstyleplugin-kvantum and qt6Packages.qtstyleplugin-kvantum when qt.platformTheme = "qt{,5}ct" is set). So I assume it is something that environment.systemPackages is doing that makes it work.

I am also curious why home.packages works in qt5ct and not in qt6ct. Maybe whatever qt5ct is doing could be backported to qt6ct so this could be fixed for every use case.

[Scrumplex]

Maybe this? https://github.com/NixOS/nixpkgs/blob/094681cd1e14261333f503bcbfacbb62362b686a/nixos/modules/config/qt.nix#L111-L116

It's very important that both Qt 5 and Qt 6 plugin paths are linked

[thiagokokada]

Maybe this?

https://github.com/NixOS/nixpkgs/blob/094681cd1e14261333f503bcbfacbb62362b686a/nixos/modules/config/qt.nix#L111-L116

It's very important that both Qt 5 and Qt 6 plugin paths are linked

Interesting that the Qt modules in Home-Manager does neither of them, and qt5ct still works.

But I will try this later.

[thiagokokada]

Maybe this?

https://github.com/NixOS/nixpkgs/blob/094681cd1e14261333f503bcbfacbb62362b686a/nixos/modules/config/qt.nix#L111-L116

It's very important that both Qt 5 and Qt 6 plugin paths are linked

Just setting those 2 environment variables in my config did the trick. So I think this is actually an bug with Home-Manager module where we need to setup those 2 variables too.

[thiagokokada]

Going to close this issue since I opened a PR in Home-Manager to fix this issue: https://github.com/nix-community/home-manager/pull/4579

[diniamo]

I have this exact issue, with the only difference being that it happens even with both the NixOS and the HM module enabled, I have also tried enabling either one. My configuration has been working for a while, and I didn't change anything, meaning it broke because of updating nixpkgs. Any ideas?

[eljamm]

This happened to me lately as well. Bisecting nixpkgs, I traced the issue back to qt6: 6.7.3 -> 6.8.0 · NixOS/nixpkgs@d4f7b84

[diniamo]

@K900 any idea what's going on?

[K900]

Fixed in staging-next, please don't necropost.

[Scrumplex]

@diniamo @eljamm see #350514

[diniamo]

I couldn't find that issue when I last looked.