Add live wallpaper pipeline diagnostics

A user's live wallpaper reached "started" with zero errors (Cogl and
GStreamer both loading fine, actor added to _backgroundGroup) but
never actually appeared on screen, and no code path here could tell
us why: the bus handler only looked at EOS/ERROR, silently discarding
WARNING and STATE_CHANGED messages where a stalled negotiation or
preroll would actually show up.

Adds: WARNING message logging, PLAYING/PAUSED/etc. state-changed
logging for the playbin itself, a frame counter with first-frame and
periodic debug logs, and a one-shot watchdog that logs an explicit
warning if zero frames arrive within 4s of set_state(PLAYING). Also
switched appsink's pull-sample from an emitted action signal to the
plain pull_sample() method, which is the more directly supported
GstApp.AppSink API and one less variable while debugging.

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

lib/liveWallpaper.js

@@ -48,6 +48,9 @@ export class LiveWallpaperManager {
         this._fullscreenChangedId = 0;
         this._upowerProxy = null;
         this._upowerSignalId = 0;
+
+        this._frameCount = 0;
+        this._noFrameWatchdogId = 0;
     }
 
     get isAvailable() {
@@ -155,7 +158,20 @@ export class LiveWallpaperManager {
             this._busWatchId = bus.connect('message', (_bus, message) => this._onBusMessage(message));
 
             this._applyMute();
-            this._playbin.set_state(Gst.State.PLAYING);
+            const stateChangeResult = this._playbin.set_state(Gst.State.PLAYING);
+            this._logger.debug(`playbin.set_state(PLAYING) returned ${stateChangeResult}`);
+
+            this._frameCount = 0;
+            this._noFrameWatchdogId = GLib.timeout_add_seconds(GLib.PRIORITY_DEFAULT, 4, () => {
+                this._noFrameWatchdogId = 0;
+                if (this._active && this._frameCount === 0) {
+                    this._logger.warn(
+                        'Live wallpaper: no frames received 4s after starting playback. ' +
+                        'The pipeline likely stalled during caps negotiation or preroll without ' +
+                        'raising an error — check for GStreamer WARNING messages logged above.');
+                }
+                return GLib.SOURCE_REMOVE;
+            });
 
             this._setActive(true);
             this._paused = false;
@@ -169,7 +185,7 @@ export class LiveWallpaperManager {
 
     _onNewSample(sink) {
         const Gst = this._Gst;
-        const sample = sink.emit('pull-sample');
+        const sample = sink.pull_sample();
         if (!sample)
             return Gst.FlowReturn.OK;
 
@@ -185,6 +201,11 @@ export class LiveWallpaperManager {
 
             try {
                 this._updateFrame(mapInfo.data, width, height);
+                this._frameCount++;
+                if (this._frameCount === 1)
+                    this._logger.debug(`Live wallpaper: first frame received (${width}x${height})`);
+                else if (this._frameCount % 120 === 0)
+                    this._logger.debug(`Live wallpaper: ${this._frameCount} frames rendered so far`);
             } finally {
                 buffer.unmap(mapInfo);
             }
@@ -221,12 +242,28 @@ export class LiveWallpaperManager {
                 Gst.Format.TIME, Gst.SeekFlags.FLUSH | Gst.SeekFlags.KEY_UNIT, 0);
             break;
         case Gst.MessageType.ERROR: {
-            const [error] = message.parse_error();
+            const [error, debug] = message.parse_error();
-            this._logger.error(error, 'Live wallpaper playback error');
+            this._logger.error(error, `Live wallpaper playback error (${debug ?? 'no debug info'})`);
             this._stop();
             break;
         }
+        case Gst.MessageType.WARNING: {
+            const [warning, debug] = message.parse_warning();
+            this._logger.warn(`Live wallpaper GStreamer warning: ${warning.message} (${debug ?? 'no debug info'})`);
+            break;
         }
+        case Gst.MessageType.STATE_CHANGED:
+            if (message.src === this._playbin) {
+                const [, newState] = message.parse_state_changed();
+                this._logger.debug(`Live wallpaper pipeline state changed to ${this._stateName(newState)}`);
+            }
+            break;
+        }
+    }
+
+    _stateName(state) {
+        const Gst = this._Gst;
+        return Object.keys(Gst.State).find(name => Gst.State[name] === state) ?? String(state);
     }
 
     _applyMute() {
@@ -326,6 +363,11 @@ export class LiveWallpaperManager {
     _stop() {
         this._disconnectPowerWatches();
 
+        if (this._noFrameWatchdogId) {
+            GLib.source_remove(this._noFrameWatchdogId);
+            this._noFrameWatchdogId = 0;
+        }
+
         if (this._playbin) {
             const bus = this._playbin.get_bus();
             if (this._busWatchId) {