Force enable dynacast if backupCodec is used

.changeset/four-seals-flash.md

@@ -0,0 +1,5 @@
+---
+'livekit-client': patch
+---
+
+Force enable dynacast when backupCodec is enabled

src/options.ts

@@ -31,6 +31,9 @@ export interface InternalRoomOptions {
    * enable Dynacast, off by default. With Dynacast dynamically pauses
    * video layers that are not being consumed by any subscribers, significantly
    * reducing publishing CPU and bandwidth usage.
+   *
+   * Dynacast will be enabled if SVC codecs (VP9/AV1) are used. Multi-codec simulcast
+   * requires dynacast
    */
   dynacast: boolean;
 

src/room/participant/LocalParticipant.ts

@@ -682,6 +682,9 @@ export default class LocalParticipant extends Participant {
 
         // set up backup
         if (opts.videoCodec && opts.backupCodec && opts.videoCodec !== opts.backupCodec.codec) {
+          if (!this.roomOptions.dynacast) {
+            this.roomOptions.dynacast = true;
+          }
           const simOpts = { ...opts };
           simOpts.simulcast = true;
           simEncodings = computeTrackBackupEncodings(track, opts.backupCodec.codec, simOpts);