Fixes for non-Nix applications

Flatpak, Distrobox, Appimage and other non-Nix applications can't find system fonts/icons/themes

Flatpak, Distrobox, Appimage and alot of other non-Nix applications sometimes have hardcoded FHS path assumptions for locating fonts, icons and themes. These applications usually expect fonts, icons and themes to be available under /usr/share/fonts, /usr/share/icons and /usr/share/themes, respectively.

For these applications, instead of using a complicated fhsEnv solution, users can choose to simply bindmount the directories from /run/current-system/sw/share to their respective locations inside /usr/share.

Note that the following fix uses bindfs instead of normal bindmounting or symlinking. This is because of edgecases like OnlyOffice that will not follow symlinks while evaluating resources under /usr/share. Thus, the resolve-symlink functionality of bindfs becomes crucial for covering even the most niche corner cases.

# misc-usr-fix.nix
{ pkgs, ... }:

let
  # Mount options for the bind mounts
  mountOptions = [
    "ro"
    "x-gvfs-hide"
    # Resolves symlinks as if they were real files
    # Needed for things like OnlyOffice
    "resolve-symlinks"
  ];
in
{
  # Bind mounts fonts, icons and themes from
  # The /run/current-system/sw/share/* paths
  # to their /usr/share/* equivalents

  # Expose all fonts
  # under /run/current-system/sw/share/X11/fonts
  fonts.fontDir.enable = true;

  # Flatpak: Bind mounts /usr/share/* directories
  system.fsPackages = [ pkgs.bindfs ];
  
  # Fonts
  fileSystems."/usr/share/fonts" = {
    device = "/run/current-system/sw/share/X11/fonts";
    fsType = "fuse.bindfs";
    options = mountOptions;
  };

  # Icons
  fileSystems."/usr/share/icons" = {
    device = "/run/current-system/sw/share/icons";
    fsType = "fuse.bindfs";
    options = mountOptions;
  };

  # Themes
  fileSystems."/usr/share/themes" = {
    device = "/run/current-system/sw/share/themes";
    fsType = "fuse.bindfs";
    options = mountOptions;
  };
}