fix(sdk): scale simulcast layers explicitly for Chrome 154+ (OPTI-4760)

[bcostdolby]

Summary

Fixes OPTI-4760. On Chrome 154 and later, simulcast: true publishes three layers that are all at full capture resolution (e.g. 3x 1280x720 at about 0.8 / 0.8 / 3.8 Mbps). Earlier Chrome versions send 1/4, 1/2 and full resolution. We have seen this in production.

Cause: WebRTC 341526e2ca "[Interop 2026] Apply simulcast scaling defaults" (chromium:344943229) makes addTransceiver write scaleResolutionDownBy: 1 on the single encoding. SdpParser.setSimulcast() then adds two more layers by munging the SDP (a=ssrc-group:SIM), and those layers get no scale. After setLocalDescription, getParameters().encodings reports scale [1, undefined, undefined], and Chrome encodes every layer at full size.

Fix: add PeerConnection.setSimulcastScaling(). Once the munged local description is set, it sets the video sender's encodings to scaleResolutionDownBy = 2 ** (n - 1 - i), i.e. [4, 2, 1] for 3 layers:

// Publish.connect(), after publish + setLocalDescription
if (!this.options.disableVideo && this.options.simulcast) await webRTCPeerInstance.setSimulcastScaling()
// getRTCLocalSDP({ setSDPToPeer: true, simulcast: true }) calls it as well

• It does nothing if the sender has no getParameters or fewer than 2 encodings. This covers non-Chromium browsers and codecs where setSimulcast() leaves the SDP unchanged.
• A setParameters failure is logged as a warning and does not break publishing.
• Before this change, this path set no scaling at all, so the values match what Chrome ≤153 applied by default.

Testing

• New unit scenarios:
  • SetLocalDescription.feature: 3 encodings are set to [4, 2, 1]; a single encoding is left unchanged; getRTCLocalSDP({simulcast:true}) calls setSimulcastScaling.
  • Publish.feature: connect({simulcast:true}) calls setSimulcastScaling once, and connect() without simulcast does not call it.
  • The full unit suite passes (177 tests) and eslint is clean.
• Browser check with a local loopback harness. It runs the SDK's real getRTCLocalSDP({simulcast:true}) and setLocalDescription steps, with a 1280x720 fake camera in Chrome for Testing. Layer sizes are read from outbound-rtp stats:

Chrome	Without fix	With fix
153 (VP8, H.264)	320x180 / 640x360 / 1280x720	same, scales [4,2,1]
154 (VP8, H.264)	3x 1280x720	320x180 / 640x360 / 1280x720
157 canary (VP8, H.264)	3x 1280x720	320x180 / 640x360 / 1280x720

The scaling also still applies after a later updateBitrate() renegotiation, checked on 153, 154 and 157.

Link to Devin session: https://dolby.devinenterprise.com/sessions/00476be9da3b47819e7c35883c75e293
Open in Devin Desktop: https://dolby.devinenterprise.com/desktop/session/00476be9da3b47819e7c35883c75e293?variant=devin
Requested by: @bcostdolby

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🦋 Changeset detected

Latest commit: 90327f9

The changes in this PR will be included in the next version bump.

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[devin-ai-integration]

✅ Devin resolved the finding on 90327f9

Fixed by Devin (1)

• Scaling tests bypass publishing integration

View all findings in Devin Review