nixos/qt: `qt.platformTheme = "gtk2";` breaks qt6 programs (`pcmanfm-qt`, `telegram-desktop` and `keepassxc`) in a pure wayland environment

[Schweber]

Describe the bug

If i set

  qt = {
    enable = true;
    style = "gtk2";
    platformTheme = "gtk2";
  };

then pcmanfm-qt, telegram-desktop and keepassxc fail with Gtk-WARNING **: 20:09:24.876: cannot open display: in a pure wayland environment. They do work in xwayland though. Other (maybe older/qt5) programs are not affected.

If i delete the settings or set e.g.

  qt = {
    enable = true;
    platformTheme = "qt5ct";
    style = "adwaita-dark";
  };

then the programs work.

Steps To Reproduce

Steps to reproduce the behavior:

1. see above

Expected behavior

I would like the gtk theme to be adopted by qt in a wayland environment as it is in an X11 environment.

Additional context

lxqt/pcmanfm-qt#1939
#333964

Notify maintainers

@NixOS/qt-kde

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 6.10.10, NixOS, 24.11 (Vicuna), 24.11.20240913.345c263`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Lix, like Nix) 2.91.0
System type: x86_64-linux
Additional system types: i686-linux, x86_64-v1-linux, x86_64-v2-linux, x86_64-v3-linux, x86_64-v4-linux
Features: gc, signed-caches
System configuration file: /etc/nix/nix.conf
User configuration files: /home/sperber/.config/nix/nix.conf:/etc/xdg/nix/nix.conf:/home/sperber/.nix-profile/etc/xdg/nix/nix.conf:/nix/profile/etc/xdg/nix/nix.conf:/home/sperber/.local/state/nix/profile/etc/xdg/nix/nix.conf:/etc/profiles/per-user/sperber/etc/xdg/nix/nix.conf:/nix/var/nix/profiles/default/etc/xdg/nix/nix.conf:/run/current-system/sw/etc/xdg/nix/nix.conf
Store directory: /nix/store
State directory: /nix/var/nix
Data directory: /nix/store/rgpx7kwskibyk067x8bp8a6ihk17fizm-lix-2.91.0/share`
 - nixpkgs: `/nix/store/l3amk5lsakpc93him5kry24kax23sn4h-source`

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[Schweber]

@Misterio77 I suspect that qt6gtk2 might play a role in that.

Also pinging @thiagokokada @Scrumplex @linsui because you commited to the module recently

[Scrumplex]

Perhaps a symptom of trialuser02/qt6gtk2#2

[Schweber]

Thank you, the thread showed me the solution:

I have to disable the qt module in Nixos and set QT_QPA_PLATFORMTHEME=gtk3. Then at least the QT 6 programs work as intended. If i understand it correctly, then QT 6 doesn't need compatibility packages to adopt a gtk theme but QT 5 still does.

However, now my QT 5 programs have the default bright skin, which is annoying. There should be a way to enable the compatibility packages for QT 5 without breaking QT 6.

[Atemo-C]

Encountering the same issue as I am currently trying out niri on my laptop… How very annoying. I either get my QT apps half-working with broken theming for half of them, or nothing at all. Theming is a pain, QT theming is a pain, QT theming that matches GTK theme is a pain, under Wayland it's a pain, and with so much pain, I might as well open a bakery 🥖… Terrible "jokes" aside, is there any actual solution/hack/workaround to make things just consistent between GTK and QT apps on a pure-wayland system?

[peterhoeg]

I'm just using breeze everywhere. Seems to work fine in gtk and non-kde qt apps.

[Atemo-C]

I'm just using breeze everywhere. Seems to work fine in gtk and non-kde qt apps.

Yes, I suppose it does, though I do really want my darker Flat-Remix-GTK-Blue-Darkest theme. I suppose one could just make a colorscheme for Breeze in KDE Plasma and transfer it over, but this is a bit of a mess. Perhaps the community could use of a simple python/similar program to let us customize Breeze's colors in any environment we want…and then have this implemented in NixOS or Home Manager to make this even better integrated into NixOS. Hey, a man can dream.

[peterhoeg]

So for the actual request here - gtk2 apps in a pure wayland environment, that's never going to work. gtk2 apps will only run via xwayland (or X11) so the behavior here is expected and has nothing to do with nix(os).

[mobsenpai]

Anyone got any fix yet? nixos 26.05?
If platform theme set to gtk3 then qt6 apps correctly theme to gtk3 theme but qt5 apps remain the white themed.

[Atemo-C]

Still an issue as of 26.11 as well so far. Though, for me, only mediawriter fails to launch with configuring qt = {};. I can bear the pain of using dd and keep my other QT apps themed consistently with my system (because to me, the GTK3 does not configure everything properly either).

I hope there will be a fix sometimes in the not too distant future, but seeing how long this has been an issue for, I really have little hope. I wish I knew how to fix it so that I could make a pull request, but I have no clue.

[Atemo-C]

Greetings again! I am here to inform that I have temporarily worked around the only package affecting me, being Fedora's Media Writer utility (pkgs.mediawriter), by wrapping it with environment variables that force it to set QT_QPA_PLATFORMTHEME to gtk3 and to unset QT_STYLE_OVERRIDE.

I am unsure if my way is a good way to do it, but the program now launches without a fuss, and for me, that is sufficient.

https://github.com/Atemo-C/NixOS-configuration/blob/main/programs/accessories.nix#L17

The compilation time for it is mere seconds on my Ryzen 7 9850X3D, but it may take a bit longer on other systems. The compilation time, obviously, depends on just how big the software is, too.

Anyway, this is not ideal, but it works. I will stay subscribed to this issue in case any new developments occur. Cheers!