Fix potential login freeze: skip blocking GStreamer registry rescan

Gst.init() has no async variant. If the plugin registry cache is stale
(e.g. right after a system update), it synchronously rescans every
plugin, spawning gst-plugin-scanner per plugin and probing hardware
for codecs. Since gnome-shell is single-threaded and is the
compositor, that can freeze the whole screen for several seconds,
same as the blocking UPower proxy lookup fixed earlier.

Set GST_REGISTRY_UPDATE=no when a cache already exists so Gst.init()
trusts it and returns immediately, then refresh the cache in a
throwaway subprocess afterward so it doesn't go stale forever.

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

lib/gstreamerAvailability.js

@@ -1,3 +1,67 @@
+import Gio from 'gi://Gio';
+import GLib from 'gi://GLib';
+
+/**
+ * True if a GStreamer plugin registry cache already exists for this user.
+ * A plain local directory listing — fast, unlike the scan below.
+ */
+function hasGstRegistryCache() {
+    const cacheDir = Gio.File.new_for_path(
+        GLib.build_filenamev([GLib.get_user_cache_dir(), 'gstreamer-1.0']));
+    try {
+        const enumerator = cacheDir.enumerate_children('standard::name', Gio.FileQueryInfoFlags.NONE, null);
+        const hasEntry = enumerator.next_file(null) !== null;
+        enumerator.close(null);
+        return hasEntry;
+    } catch (e) {
+        return false;
+    }
+}
+
+/**
+ * Gst.init() has no async variant. If its plugin registry cache is stale
+ * (e.g. right after a system update touched GStreamer packages), it
+ * synchronously rescans every plugin — spawning gst-plugin-scanner per
+ * plugin, and probing hardware for codec plugins — which can block
+ * gnome-shell's single main thread (it *is* the compositor) for many
+ * seconds, exactly the conditions right after login. That froze the entire
+ * screen the same way the blocking UPower proxy lookup did.
+ *
+ * GST_REGISTRY_UPDATE=no makes Gst.init() trust the existing cache
+ * unconditionally instead of validating it, so it returns immediately. Only
+ * set when a cache already exists — an extension-lifetime-first-ever
+ * GStreamer run on this machine still needs one real scan to have anything
+ * to trust.
+ */
+function keepGstInitFast() {
+    if (hasGstRegistryCache())
+        GLib.setenv('GST_REGISTRY_UPDATE', 'no', true);
+}
+
+/**
+ * Refreshes the on-disk registry cache in a throwaway subprocess, so a
+ * stale cache (skipped above) doesn't stay stale forever and newly
+ * installed plugins eventually become visible. Runs fully out-of-process:
+ * even if a hardware probe inside it hangs, gnome-shell is unaffected.
+ */
+function refreshGstRegistryInBackground() {
+    try {
+        const launcher = new Gio.SubprocessLauncher(
+            Gio.SubprocessFlags.STDOUT_SILENCE | Gio.SubprocessFlags.STDERR_SILENCE);
+        launcher.setenv('GST_REGISTRY_UPDATE', 'yes', true);
+        const proc = launcher.spawnv(['gst-inspect-1.0', '--exists', 'playbin']);
+        proc.wait_check_async(null, (source, result) => {
+            try {
+                source.wait_check_finish(result);
+            } catch (e) {
+                // Best-effort refresh; a missing binary or nonzero exit just leaves the cache as-is.
+            }
+        });
+    } catch (e) {
+        // Best-effort refresh; a missing gst-inspect-1.0 just leaves the cache as-is.
+    }
+}
+
 /**
  * Loads the GObject-Introspection bindings the live wallpaper feature needs
  * (Gst, GstApp for the appsink signals, Cogl for uploading decoded frames)
@@ -13,6 +77,8 @@
  * checkGstreamerBaseAvailable() there instead.
  */
 export async function loadGstreamerModules() {
+    keepGstInitFast();
+
     const [{default: Gst}, , {default: Cogl}] = await Promise.all([
         import('gi://Gst?version=1.0'),
         import('gi://GstApp?version=1.0'),
@@ -22,6 +88,8 @@ export async function loadGstreamerModules() {
     if (!Gst.is_initialized())
         Gst.init(null);
 
+    refreshGstRegistryInBackground();
+
     return {Gst, Cogl};
 }
 
@@ -33,6 +101,8 @@ export async function loadGstreamerModules() {
  * failure here is a genuine, actionable problem, unlike a Cogl probe.
  */
 export async function checkGstreamerBaseAvailable() {
+    keepGstInitFast();
+
     const [{default: Gst}] = await Promise.all([
         import('gi://Gst?version=1.0'),
         import('gi://GstApp?version=1.0'),
@@ -40,6 +110,8 @@ export async function checkGstreamerBaseAvailable() {
 
     if (!Gst.is_initialized())
         Gst.init(null);
+
+    refreshGstRegistryInBackground();
 }
 
 export const GSTREAMER_INSTALL_HINT =