Zen won't run: libpangoft2-1.0.so.0: undefined symbol: FcConfigSetDefaultSubstitute

[ebardie]

• [ x] I've updated the flake input to the latest version
• [ x] I've correctly set up my XDG portals

Describe the bug

Zen has been working fine, but after updating fails to run with the following output:

XPCOMGlueLoad error for file /nix/store/r61gswgki5069jbw3j4v8v3jjxbjcvf5-zen-beta-bin-unwrapped-1.17.8b/lib/zen-bin-1.17.8b/libmozgtk.so:
/nix/store/0fm6p8gpz61dns8g6s18wva6q3dhvsa9-pango-1.57.0/lib/libpangoft2-1.0.so.0: undefined symbol: FcConfigSetDefaultSubstitute
Couldn't load XPCOM.

Which looks like a problem with NixOS Unstable's Pango being ahead of whatever version of Pango Zen's built against.

See solution below.

To Reproduce

Steps to reproduce the behavior:

1. Update to latest Zen
2. See error

Expected behavior

Zen to run as usual.

Screenshots

n/a - see text copied from terminal above

System Information

• OS: (e.g. NixOS, Ubuntu, etc)
  NixOS 25.05
• Zen Browser version: (e.g. twilight, beta)
  beta, but same issue with twilight
• Desktop environment / Compositor / WM: (e.g. KDE, GNOME, Hyprland)
  X11 with i3wm

Additional context

"Fixed" by changing the inputs in flake.nix from:

inputs.nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";

to:

inputs.nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-25.05";

This works for me, but I've not done extensive testing.

[luisnquin]

@ebardie try with this overlay:

  (_self: super: {
    pango = super.pango.overrideAttrs (_oldAttrs: rec {
      version = "1.56.4";
      src = super.fetchurl {
        url = "mirror://gnome/sources/pango/${super.lib.versions.majorMinor version}/pango-${version}.tar.xz";
        hash = "sha256-FwZeL8xfWlvb/8iEyVa/x8RRqW6MT7L4rYN8ZBPLWgE=";
      };
    });
  })

Anyway, thanks for reporting this issue!