Ugh. Looks like it is worse nowadays. Previously, it was just the NIX_GSETTINGS_OVERRIDES_DIR variable, which I have on my to-do list to eventually remove in favour of using the new dconf module. Now it is also defaultSession, which DE modules should not really set and ssh-askpass.
As the error says, you can use lib.mkForce the resolve the conflict. You can choose a value from one of the mentioned modules.
For example, to use GSettings settings (NIX_GSETTINGS_OVERRIDES_DIR) from GNOME, copy the following and wrap the value with lib.mkForce:
In this case, it refers to some variables so you will need to copy them as well:
environment.sessionVariables.NIX_GSETTINGS_OVERRIDES_DIR =
let
cfg = config.services.xserver.desktopManager.gnome;
nixos-background-light = pkgs.nixos-artwork.wallpapers.simple-blue;
nixos-background-dark = pkgs.nixos-artwork.wallpapers.simple-dark-gray;
flashbackEnabled = cfg.flashback.enableMetacity || lib.length cfg.flashback.customSessions > 0;
nixos-gsettings-desktop-schemas = pkgs.gnome.nixos-gsettings-overrides.override {
inherit (cfg) extraGSettingsOverrides extraGSettingsOverridePackages favoriteAppsOverride;
inherit flashbackEnabled nixos-background-dark nixos-background-light;
};
in
lib.mkForce (pkgs.glib.getSchemaPath nixos-gsettings-desktop-schemas);
You will need the same for ssh.askPassword, Have gnome & plasma at the same time? - #2 by BatteredBunny
If you do not use auto-login, you can also set defaultSession to null, which will make DM pre-select the last used session:
services.displayManager.defaultSession = lib.mkForce null;