Flatpak: make bindfs an optional parameter

hosts/Khanda/default.nix

@@ -79,6 +79,8 @@ in
           "org.keepassxc.KeePassXC"
           "org.mozilla.firefox"
         ];
+
+        useBindFS = true;
       };
     };
 

modules/ui/flatpak.nix

@@ -10,11 +10,10 @@
 let
   cfg = config.aux.system.ui.flatpak;
 in
-with lib;
 {
   options = {
     aux.system.ui.flatpak = {
-      enable = mkEnableOption (mdDoc "Enables Flatpak support.");
+      enable = lib.mkEnableOption { description = "Enables Flatpak support."; };
       packages = lib.mkOption {
         description = "Flatpak packages to install.";
         type = lib.types.listOf lib.types.str;
@@ -31,10 +30,14 @@ with lib;
           }
         ];
       };
+      useBindFS = lib.mkEnableOption {
+        description = "Whether to use a BindFS mount to support custom themes and cursors. May cause performance issues.";
+      };
     };
   };
 
-  config = mkIf cfg.enable {
+  config = lib.mkMerge [
+    (lib.mkIf cfg.enable {
       # Enable Flatpak
       services.flatpak = {
         enable = true;
@@ -51,5 +54,44 @@ with lib;
         # Install base Flatpaks. For details, see https://github.com/gmodena/nix-flatpak
         packages = cfg.packages;
       };
+    })
+    (lib.mkIf cfg.useBindFS {
+      # Workaround for getting Flatpak apps to use system fonts, icons, and cursors
+      # For details (and source), see https://github.com/NixOS/nixpkgs/issues/119433#issuecomment-1767513263
+      # NOTE: If fonts in Flatpaks appear incorrect (like squares), run this command to regenerate the font cache:
+      #  flatpak list --columns=application | xargs -I %s -- flatpak run --command=fc-cache %s -f -v
+      system.fsPackages = [ pkgs.bindfs ];
+      fileSystems =
+        let
+          mkRoSymBind = path: {
+            device = path;
+            fsType = "fuse.bindfs";
+            options = [
+              "ro"
+              "resolve-symlinks"
+              "x-gvfs-hide"
+            ];
           };
+          aggregatedIcons = pkgs.buildEnv {
+            name = "system-icons";
+            paths = with pkgs; [
+              (lib.mkIf config.aux.system.ui.desktops.gnome.enable gnome-themes-extra)
+              (lib.mkIf config.aux.system.ui.desktops.kde.enable kdePackages.breeze-icons)
+              papirus-icon-theme
+              qogir-icon-theme
+            ];
+            pathsToLink = [ "/share/icons" ];
+          };
+          aggregatedFonts = pkgs.buildEnv {
+            name = "system-fonts";
+            paths = config.fonts.packages;
+            pathsToLink = [ "/share/fonts" ];
+          };
+        in
+        {
+          "/usr/share/icons" = mkRoSymBind "${aggregatedIcons}/share/icons";
+          "/usr/local/share/fonts" = mkRoSymBind "${aggregatedFonts}/share/fonts";
+        };
+    })
+  ];
 }