Build the live wallpaper sink bin explicitly instead of via string parsing

User testing pinned the bug precisely: our try/catch is airtight now,
and the diagnostics showed "new-sample signal fired 0 times" — the
appsink genuinely never receives a single sample inside gnome-shell's
process, while an equivalent pipeline (videoconvert ! videoscale !
RGBA caps ! appsink) played correctly standalone via gst-launch-1.0.

Replaces Gst.parse_bin_from_description()'s gst-launch mini-language
parsing (with its automatic unlinked-pad ghosting) with explicit
ElementFactory.make() + Bin.add() + element.link() + a manually
created GhostPad. This removes the string-parsing path as a variable
entirely and, importantly, surfaces any link() failure as a thrown
error instead of failing silently, which the old code path had no way
to report even if that patch were the whole cachet of the problem.

Also fixes a related warning ("Trying to dispose element ..., but it
is in PLAYING instead of the NULL state") from _stop() dropping the
playbin reference immediately after requesting the NULL state, before
the (async) transition actually completed — now waits briefly via
get_state() for it to settle first.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019RDqbdjsiisSU7CbQke4g8

lib/liveWallpaper.js

@@ -128,6 +128,61 @@ export class LiveWallpaperManager {
         this._start();
     }
 
+    /**
+     * Builds videoconvert ! videoscale ! capsfilter(RGBA) ! appsink using
+     * explicit element creation, linking, and a ghost pad, rather than
+     * Gst.parse_bin_from_description()'s gst-launch mini-language. Both
+     * are meant to be equivalent, but a user's testing showed the
+     * string-parsed version's appsink never fired 'new-sample' even once
+     * inside gnome-shell's process while an equivalent standalone
+     * gst-launch-1.0 pipeline worked fine — this removes the string
+     * parsing (and its automatic ghost-pad detection) as a variable, and
+     * surfaces link() failures explicitly instead of failing silently.
+     */
+    _buildSinkBin() {
+        const Gst = this._Gst;
+
+        const videoconvert = Gst.ElementFactory.make('videoconvert', 'benthicbloom-convert');
+        const videoscale = Gst.ElementFactory.make('videoscale', 'benthicbloom-scale');
+        const capsfilter = Gst.ElementFactory.make('capsfilter', 'benthicbloom-capsfilter');
+        const appsink = Gst.ElementFactory.make('appsink', 'benthicbloom-appsink');
+
+        if (!videoconvert || !videoscale || !capsfilter || !appsink) {
+            throw new Error(
+                'Failed to create one or more GStreamer elements ' +
+                '(videoconvert/videoscale/capsfilter/appsink) — a required plugin is likely missing');
+        }
+
+        capsfilter.set_property('caps', Gst.Caps.from_string('video/x-raw,format=RGBA'));
+        appsink.set_property('emit-signals', true);
+        appsink.set_property('max-buffers', 2);
+        appsink.set_property('drop', true);
+        appsink.set_property('sync', true);
+
+        const sinkBin = new Gst.Bin({name: 'benthicbloom-sinkbin'});
+        sinkBin.add(videoconvert);
+        sinkBin.add(videoscale);
+        sinkBin.add(capsfilter);
+        sinkBin.add(appsink);
+
+        if (!videoconvert.link(videoscale))
+            throw new Error('Failed to link videoconvert -> videoscale');
+        if (!videoscale.link(capsfilter))
+            throw new Error('Failed to link videoscale -> capsfilter');
+        if (!capsfilter.link(appsink))
+            throw new Error('Failed to link capsfilter -> appsink');
+
+        const sinkPad = videoconvert.get_static_pad('sink');
+        const ghostPad = Gst.GhostPad.new('sink', sinkPad);
+        if (!ghostPad)
+            throw new Error('Failed to create ghost pad for live wallpaper sink bin');
+        ghostPad.set_active(true);
+        sinkBin.add_pad(ghostPad);
+
+        this._appsink = appsink;
+        return sinkBin;
+    }
+
     _start() {
         const path = this._settings.get_string(SettingsKey.LIVE_WALLPAPER_PATH);
         if (!this._available || !path)
@@ -143,11 +198,7 @@ export class LiveWallpaperManager {
             Main.layoutManager._backgroundGroup.add_child(this._actor);
             this._layoutActor();
 
-            const sinkBin = Gst.parse_bin_from_description(
-                'videoconvert ! videoscale ! video/x-raw,format=RGBA ! ' +
-                'appsink name=benthicbloom_sink emit-signals=true max-buffers=2 drop=true sync=true',
-                true);
-            this._appsink = sinkBin.get_by_name('benthicbloom_sink');
+            const sinkBin = this._buildSinkBin();
             this._appsink.connect('new-sample', sink => this._onNewSample(sink));
 
             this._playbin = Gst.ElementFactory.make('playbin', 'benthicbloom-live-wallpaper');
@@ -381,13 +432,22 @@ export class LiveWallpaperManager {
         }
 
         if (this._playbin) {
+            const Gst = this._Gst;
             const bus = this._playbin.get_bus();
             if (this._busWatchId) {
                 bus.disconnect(this._busWatchId);
                 this._busWatchId = 0;
             }
             bus.remove_signal_watch();
-            this._playbin.set_state(this._Gst.State.NULL);
+
+            this._playbin.set_state(Gst.State.NULL);
+            // Block briefly for the (normally fast) transition to actually
+            // finish before dropping our reference — otherwise the element
+            // can get disposed mid-transition, which GStreamer logs as
+            // "Trying to dispose element ..., but it is in PLAYING instead
+            // of the NULL state".
+            this._playbin.get_state(200 * Gst.MSECOND);
+
             this._playbin = null;
             this._appsink = null;
         }