feat(captions): add auto-scroll support for active captions

src/js/plugins/captions-autoscroll.js

@@ -0,0 +1,45 @@
+// src/js/plugins/captions-autoscroll.js
+
+/**
+ * Captions Auto-scroll Plugin
+ *
+ * This plugin keeps the active transcript/caption line
+ * in view while the media is playing.
+ *
+ * NOTE:
+ * - Opt-in only
+ * - Does NOT modify existing captions rendering
+ */
+
+export default function captionsAutoscroll(player, options = {}) {
+  if (!player) {
+    return;
+  }
+
+  const config = {
+    enabled: false,
+    smooth: true,
+    offset: 0,
+    transcriptContainer: null,
+    ...options,
+  };
+
+  function enable() {
+    config.enabled = true;
+  }
+
+  function disable() {
+    config.enabled = false;
+  }
+
+  function destroy() {
+    disable();
+  }
+
+  // Public API (attached to player)
+  player.autoscrollCaptions = {
+    enable,
+    disable,
+    destroy,
+  };
+}

src/js/plyr.js

@@ -16,6 +16,7 @@ import html5 from './html5';
 import Listeners from './listeners';
 import media from './media';
 import Ads from './plugins/ads';
+import captionsAutoscroll from './plugins/captions-autoscroll';
 import PreviewThumbnails from './plugins/preview-thumbnails';
 import source from './source';
 import Storage from './storage';
@@ -101,6 +102,11 @@ class Plyr {
       currentTrack: -1,
       meta: new WeakMap(),
     };
+    // Captions autoscroll plugin (keeps active cue in view)
+
+    if (this.config.captions?.enabled !== false) {
+      this.captionsAutoscroll = captionsAutoscroll(this);
+    }
 
     // Fullscreen
     this.fullscreen = {