zen: hot-reload the theme via autoconfig sheet watcher

Gecko has no userChrome.css reload, so wrap zen with a mozilla.cfg script
that registers chrome/zen-theme.css through nsIStyleSheetService and
re-registers it under a cache-busting URI when its mtime moves.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

apps/zen.nix

@@ -1,9 +1,65 @@
 #./apps/zen.nix
 { inputs, pkgs, lib, config, ... }:
+let
+  # Autoconfig (mozilla.cfg) runs privileged JS in the parent process at
+  # startup — the only scriptable way into chrome, since Gecko has no reload
+  # for userChrome.css and Zen exposes no command for one.
+  #
+  # So don't use userChrome.css at all for the wallpaper theme. Register
+  # <profile>/chrome/zen-theme.css as a style-sheet-service USER_SHEET here
+  # and re-register it under a cache-busting URI whenever its mtime moves —
+  # that is what makes a `theme-apply` switch land in a running Zen
+  # (settings/theming.nix writes that file on every switch). A separate file,
+  # not userChrome.css: the legacy loader would hold its own copy of the same
+  # sheet and the two would race on cascade order.
+  themeHotReload = ''
+    try {
+      const { classes: kCc, interfaces: kCi } = Components;
+      const sss = kCc["@mozilla.org/content/style-sheet-service;1"]
+                    .getService(kCi.nsIStyleSheetService);
+
+      const file = Services.dirsvc.get("ProfD", kCi.nsIFile);
+      file.append("chrome");
+      file.append("zen-theme.css");
+      const base = Services.io.newFileURI(file).spec;
+      let live = null;
+
+      const apply = () => {
+        if (!file.exists()) return;
+        // install(1) stamps the copy with the current time, so mtime alone
+        // distinguishes one wallpaper's sheet from the next.
+        const stamp = String(file.lastModifiedTime);
+        if (stamp === live) return;
+        // A fresh query string dodges the CSS loader's per-URI cache; without
+        // it a re-register would hand back the sheet parsed at startup.
+        sss.loadAndRegisterSheet(Services.io.newURI(base + "?" + stamp),
+                                 sss.USER_SHEET);
+        if (live !== null) {
+          const old = Services.io.newURI(base + "?" + live);
+          if (sss.sheetRegistered(old, sss.USER_SHEET))
+            sss.unregisterSheet(old, sss.USER_SHEET);
+        }
+        live = stamp;
+      };
+
+      const timer = kCc["@mozilla.org/timer;1"].createInstance(kCi.nsITimer);
+      // The observer service holds the only strong reference that keeps the
+      // timer (and this closure) off the GC's list for the session.
+      Services.obs.addObserver({ timer, observe() {} }, "quit-application");
+      timer.initWithCallback({ notify: apply }, 2000,
+                             kCi.nsITimer.TYPE_REPEATING_SLACK);
+      apply();
+    } catch (e) {
+      Components.utils.reportError(e);
+    }
+  '';
+in
 {
   config = lib.mkIf (lib.elem config.networking.hostName [ "FredOS-Gaming" "FredOS-Macbook" ]) {
     environment.systemPackages = [
-      inputs.zen-browser.packages.${pkgs.stdenv.hostPlatform.system}.default
+      (inputs.zen-browser.packages.${pkgs.stdenv.hostPlatform.system}.default.override {
+        extraPrefs = themeHotReload;
+      })
     ];
   };
 }

settings/theming.nix

@@ -7,9 +7,10 @@
 # every app that can reload without restarting.
 #
 # What follows a switch immediately: quickshell, hyprland, ghostty (D-Bus
-# reload-config), GTK3 apps such as nemo (each wallpaper installs
-# its own named GTK theme, switched over gsettings) and the folder icon tint.
-# On next launch only: GTK4/libadwaita apps, btop, zen.
+# reload-config), zen (autoconfig sheet watcher, see apps/zen.nix), GTK3 apps
+# such as nemo (each wallpaper installs its own named GTK theme, switched over
+# gsettings) and the folder icon tint.
+# On next launch only: GTK4/libadwaita apps, btop.
 #
 # stylix stays the source of truth for build-time defaults and for every
 # target not listed here — walls/wallpaper.png remains its image.
@@ -518,7 +519,12 @@ let
     # the inode keeps the watch, and vencord hot-swaps quickCss with no reload.
     mkdir -p "$HOME/.config/vesktop/settings"
     cat "$theme/vesktop.css" > "$HOME/.config/vesktop/settings/quickCss.css"
-    install -Dm644 "$theme/zen.css"     "$HOME/${zenChrome}/userChrome.css"
+    # zen-theme.css, not userChrome.css: Zen's autoconfig script watches this
+    # file's mtime and re-registers it as a style sheet, so a switch lands in
+    # the running window (see apps/zen.nix). A userChrome.css copy would keep
+    # loading the wallpaper that was live at startup and fight it.
+    install -Dm644 "$theme/zen.css"     "$HOME/${zenChrome}/zen-theme.css"
+    rm -f "$HOME/${zenChrome}/userChrome.css"
 
     # Icon theme: replace the directory contents (cheap — the tree is
     # symlinks into the papirus store path) so GTK's icon-theme monitor sees