Default video degradation preference by track source, including the backup codec

[xianshijing-lk]

Aligns Flutter with the behavior landed in client-sdk-android (livekit/client-sdk-android#991). Two related changes.

Source-based defaults

Previously every video track fell back to maintainResolution, and the preference was only applied to camera and screen share tracks at all:

if ([TrackSource.camera, TrackSource.screenShareVideo].contains(track.source)) {
  final degradationPreference = options.degradationPreference ?? DegradationPreference.maintainResolution;
  await track.setDegradationPreference(degradationPreference);
}

Now getDefaultDegradationPreference(source) resolves camera → maintainFramerate (smoother video for real-time communication), screen share → maintainResolution (clarity matters for text/UI), other → balanced, and it is applied to every video sender. Custom sources previously got whatever WebRTC derived implicitly from the native source; balanced is the preference the WebRTC spec mandates as the default and is the honest choice when the application declined to declare a motion-vs-detail intent.

An explicitly set degradationPreference still wins in all cases — the default only fills a null.

Backup codec sender

Degradation preference is a property of the sender, not of the track — a top-level field on RtpParameters, not per-encoding. publishAdditionalCodecForPublication adds a second transceiver and therefore a second sender, which was never configured, so the backup encoder resolved a preference implicitly and could adapt along a different axis than the primary.

Both senders sink from the same video source, so a diverging backup does not just degrade itself — its restriction is merged onto the shared source and affects the primary too.

setDegradationPreference now stores the resolved preference and fans out to every sender, and publishAdditionalCodecForPublication applies it to the backup sender once created. Using the track's stored resolved value means the two encoders cannot disagree.

Note simulcast is unaffected — all simulcast encodings live under one sender and already share its preference. Only the backup codec is a separate sender.

Tests

test/options/degradation_preference_test.dart covers the three source mappings. Full suite passes (379 tests), flutter analyze lib/ test/ clean, dart format clean at the repo's 120-column width.

Cross-SDK

client-sdk-js gets the backup-sender half in livekit/client-sdk-js#2040 (its source-based defaults already matched). The Rust SDK already resolves the same defaults and has no backup-codec publish path. Swift follows separately.

🤖 Generated with Claude Code