Play pre-generated next chapter audio directly without page navigation to avoid autoplay block on locked screen

scraper/internal/server/ui.go

@@ -2503,6 +2503,7 @@ const chapterTmpl = `
 
   // ── next-chapter prefetch at 80% ─────────────────────────────────────────────
   var prefetchFired = false;
+  var prefetchedUrl = null; // resolved proxy URL for the next chapter, if ready
 
   // ── audio event handlers (named so they can be removed on next swap) ──────────
   function onTimeUpdate() {
@@ -2518,7 +2519,12 @@ const chapterTmpl = `
         body: JSON.stringify({ voice: voiceSel.value, speed: parseFloat(speedSlider.value) })
       })
       .then(function (res) { return res.ok ? res.json() : Promise.reject(res.status); })
-      .then(function () { if (!stale()) setNextState('ready'); })
+      .then(function (data) {
+        if (!stale()) {
+          prefetchedUrl = (data && data.url) ? data.url : null;
+          setNextState('ready');
+        }
+      })
       .catch(function () { if (!stale()) setNextState('error'); });
     }
     // paragraph highlight
@@ -2564,6 +2570,25 @@ const chapterTmpl = `
   // ── auto-next ────────────────────────────────────────────────────────────────
   function goNextChapter() {
     if (!NEXT_N) return;
+
+    // If audio was already pre-generated, play it directly without a page
+    // navigation.  This keeps the existing audio context alive so the browser
+    // doesn't block autoplay on a locked screen where a user interaction was
+    // already granted for the current chapter.
+    if (prefetchedUrl) {
+      var url = prefetchedUrl;
+      prefetchedUrl = null;
+      audio.src = url;
+      audio.load();
+      // Update Media Session so lock-screen shows the next chapter.
+      if ('mediaSession' in navigator && navigator.mediaSession.metadata) {
+        navigator.mediaSession.metadata.title = 'Ch.\u00a0' + NEXT_N;
+      }
+      // audio.play() is called by onCanPlay once the element is ready.
+      return;
+    }
+
+    // Fall back to HTMX navigation (audio not ready yet).
     var nextURL = '/books/' + SLUG + '/chapters/' + NEXT_N;
     try { localStorage.setItem('tts_autostart', '1'); } catch(_) {}
     htmx.ajax('GET', nextURL, {
@@ -2579,6 +2604,7 @@ const chapterTmpl = `
     audio.pause();
     audio.src = '';
     prefetchFired = false;
+    prefetchedUrl = null;
     setStopped();
   }