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Use ScreenCaptureKit (macOS 13.0+) to capture system audio and mix it into the microphone audio stream when screen sharing with audio: true. - ScreenAudioCapturer: wraps SCStream to capture system audio into a thread-safe ring buffer (mono float32, 48kHz) - ScreenAudioMixer: ExternalAudioProcessingDelegate that reads from the ring buffer and mixes system audio into the mic's RTCAudioBuffer - FlutterRTCDesktopCapturer: parses audio constraint in getDisplayMedia, starts/stops screen audio capture lifecycle - Weakly links ScreenCaptureKit so the plugin still works on macOS 10.15+ - Excludes current process audio by default to prevent echo
Use consistent flags (kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment) in both the sizing and retrieval calls to CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer. The alignment flag increases the required buffer size, so using flags=0 for sizing and the alignment flag for retrieval produced an undersized allocation.
Temporary debug logs at 4 checkpoints to identify where audio is lost: 1. SCStream audio callback firing (format, frame count) 2. Ring buffer state after write (available frames) 3. Mixer invocation (called? capturer ready?) 4. Frames read + sample values (system audio vs mic scale) Rate-limited to every 100th callback to avoid log flooding.
The screen audio capture feature is working correctly; remove the debug checkpoints that were added to trace the audio flow.
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