ui/src/lib/components/AudioPlayer.svelte

@@ -15,9 +15,18 @@
 	 *   4. If 404, POST /api/audio/:slug/:n to generate. Drive pseudo progress bar.
 	 *      On success, presign again and set audioUrl.
 	 *
-	 * ── Pre-fetch at 90% ─────────────────────────────────────────────────────
-	 * A $derived watches currentTime/duration.  When >= 90% and autoNext is on
-	 * and there IS a next chapter, it kicks off prefetchNext():
+	 * ── Pre-fetch (immediate + 90% fallback) ────────────────────────────────
+	 * When autoNext is on, prefetchNext() is called as soon as the current
+	 * chapter starts playing (via maybeStartPrefetch() at the end of
+	 * startPlayback()).  This gives the maximum lead time for Kokoro to
+	 * generate the next chapter so the transition is seamless.
+	 *
+	 * A $effect also watches currentTime/duration and fires prefetchNext() at
+	 * the 90% mark as a fallback — covering the case where autoNext was toggled
+	 * on mid-playback after startPlayback() had already returned.
+	 * The nextStatus !== 'none' guard prevents double-runs in all cases.
+	 *
+	 * prefetchNext():
 	 *   • Calls POST /api/audio for next chapter (sets nextStatus='prefetching')
 	 *   • On success, presigns and stores URL in audioStore.nextAudioUrl
 	 *     (sets nextStatus='prefetched')
@@ -268,6 +277,8 @@
 				audioStore.audioUrl = url;
 				audioStore.status = 'ready';
 				// Don't restore saved time for auto-next; position is 0
+				// Immediately start pre-generating the chapter after this one.
+				maybeStartPrefetch();
 				return;
 			}
 
@@ -278,6 +289,8 @@
 				audioStore.status = 'ready';
 				// Restore last saved position after the audio element loads
 				restoreSavedAudioTime();
+				// Immediately start pre-generating the next chapter in background.
+				maybeStartPrefetch();
 				return;
 			}
 
@@ -299,6 +312,8 @@
 			audioStore.audioUrl = url2;
 			audioStore.status = 'ready';
 			// Don't restore time for freshly generated audio — position is 0
+			// Immediately start pre-generating the next chapter in background.
+			maybeStartPrefetch();
 		} catch (e) {
 			stopProgress();
 			audioStore.progress = 0;
@@ -307,6 +322,25 @@
 		}
 	}
 
+	/**
+	 * Start pre-fetching the next chapter if autoNext is on, there is a next
+	 * chapter, and no prefetch is already running or completed.
+	 * Called as soon as current-chapter playback begins so that the next
+	 * chapter's audio is ready before we need it (seamless transition).
+	 * The 90%-mark $effect acts as a fallback for cases where autoNext is
+	 * toggled on mid-playback.
+	 */
+	function maybeStartPrefetch() {
+		if (
+			audioStore.autoNext &&
+			nextChapter !== null &&
+			nextChapter !== undefined &&
+			audioStore.nextStatus === 'none'
+		) {
+			prefetchNext();
+		}
+	}
+
 	/**
 	 * Fetch the saved audio time for this chapter and seek to it after a short
 	 * delay (to allow the audio element to load the source).