fix(audio): don't read a blocked sample queue as silent sources on macOS

electron/native/screencapturekit/Sources/OpenScreenCaptureCore/AudioTrackMixer.swift

@@ -86,6 +86,10 @@ public final class AudioTrackMixer {
 		/// jitter never trips it, and the file is written offline, so the latency it costs at a
 		/// transition into silence costs the recording nothing.
 		static let emissionGraceFrames = sampleRate / 4
+		/// A gap between two passes of the sample queue longer than this means the queue itself
+		/// was blocked, not that the sources went quiet. The ticker fires every 10 ms, so ten
+		/// times that is never ordinary scheduling jitter. See `discountQueueStall(until:)`.
+		static let queueStallFrames = sampleRate / 10
 		/// Longest hole a source may silence-pad across. Past this the source has been dead long
 		/// enough that padding would materialize however long the outage lasted — the cursor
 		/// carries the track across the gap instead.
@@ -121,6 +125,8 @@ public final class AudioTrackMixer {
 	private var deliveredThroughFrame = [Int64?](repeating: nil, count: Source.allCases.count)
 	private var undeliveredFrames = [Int64](repeating: 0, count: Source.allCases.count)
 	private var longestHoleFrames = [Int64](repeating: 0, count: Source.allCases.count)
+	/// Where the clock stood at the previous pass of the sample queue, in absolute frames.
+	private var lastPassFrame: Int64?
 	private var pending: [CMSampleBuffer] = []
 	private var didWarnAboutBacklog = false
 	private var didWarnAboutDecode: Set<Int> = []
@@ -178,6 +184,7 @@ public final class AudioTrackMixer {
 		}
 
 		let now = clock()
+		discountQueueStall(until: now)
 		let startFrame = frameIndex(of: presentationTime, from: anchor)
 		noteDelivery(source, from: startFrame, frameCount: Int64(frames.count / MixFormat.channelCount))
 		sources[source.rawValue].ingest(frames, atFrame: startFrame)
@@ -194,7 +201,33 @@ public final class AudioTrackMixer {
 		guard anchor != nil else {
 			return
 		}
-		drain(upTo: clock(), grace: Int64(MixFormat.emissionGraceFrames))
+		let now = clock()
+		discountQueueStall(until: now)
+		drain(upTo: now, grace: Int64(MixFormat.emissionGraceFrames))
+	}
+
+	/// Keeps a blocked sample queue from reading as silent sources (#1117).
+	///
+	/// The grace is measured on the clock, but the clock keeps running while the queue cannot:
+	/// a writer or a busy queue that holds it past `emissionGraceFrames` would otherwise make
+	/// the first pass after it — a tick, or the first of the buffers waiting behind it — see
+	/// every source as quiet for the whole stall, emit silence up to now, and then drop the
+	/// audio already queued for that span as `droppedFrames`. The sources were delivering the
+	/// whole time; it is the mixer that was not listening. So a pass that comes much later than
+	/// the ticker allows moves each source's last delivery forward by the gap, which measures
+	/// the grace against the time the queue was actually running.
+	private func discountQueueStall(until now: CMTime) {
+		guard let anchor, now.isValid, now.isNumeric else {
+			return
+		}
+		let nowFrame = frameIndex(of: now, from: anchor)
+		if let lastPassFrame, nowFrame - lastPassFrame > Int64(MixFormat.queueStallFrames) {
+			let gap = nowFrame - lastPassFrame
+			for index in sources.indices {
+				sources[index].lastDeliveryFrame += gap
+			}
+		}
+		lastPassFrame = nowFrame
 	}
 
 	/// Carries the track out to the end of the take and writes out everything still buffered.

electron/native/screencapturekit/Tests/OpenScreenCaptureCoreTests/AudioTrackMixerTests.swift

@@ -237,17 +237,15 @@ final class AudioTrackMixerTests: XCTestCase {
 			makeSourceBuffer(burst(seconds: 0.2, left: 0, right: 0.5), at: clock.now, nonInterleaved: false),
 			from: .microphone
 		)
-		clock.advance(seconds: 0.2)
-		mixer.tick()
+		runTicker(mixer, clock, seconds: 0.2)
 
 		for _ in 0..<10 {
 			// Each buffer covers the 200 ms about to elapse, the way a live source delivers.
 			mixer.ingest(
 				makeSourceBuffer(burst(seconds: 0.2, left: 0.5, right: 0), at: clock.now, nonInterleaved: true),
 				from: .system
 			)
-			clock.advance(seconds: 0.2)
-			mixer.tick()
+			runTicker(mixer, clock, seconds: 0.2)
 		}
 		clock.advance(seconds: 0.2)
 		mixer.finish(atSourceTime: clock.now)
@@ -370,9 +368,9 @@ final class AudioTrackMixerTests: XCTestCase {
 			makeSourceBuffer(burst(seconds: 0.5, left: 0.5, right: 0.5), at: .zero, nonInterleaved: true),
 			from: .system
 		)
-		// Long enough past the grace that the source is written off and the cursor runs on.
-		clock.advance(seconds: 2)
-		mixer.tick()
+		// Long enough past the grace that the source is written off and the cursor runs on —
+		// with the queue running, so this is a quiet source and not a blocked queue.
+		runTicker(mixer, clock, seconds: 2)
 
 		// …and only now does it hand over the span it went quiet for, which is already behind
 		// the cursor and cannot be placed.
@@ -393,6 +391,67 @@ final class AudioTrackMixerTests: XCTestCase {
 		XCTAssertEqual(report.trimmedSeconds, 0, accuracy: 0.011)
 	}
 
+	/// A blocked sample queue is not a silent source (#1117).
+	///
+	/// Both sources keep delivering while the queue is held for a second — by the writer, or by
+	/// anything else on it — so their buffers for that second wait behind the block. When the
+	/// queue frees up, the ticker gets there first, which is the worst order: measured on the
+	/// clock alone, both sources would look a second quiet, the cursor would run to now in
+	/// silence, and the queued second would then arrive behind it and be dropped.
+	func testABlockedQueueDoesNotTurnQueuedAudioIntoSilence() {
+		let sink = RecordingSink()
+		let clock = TestClock(.zero)
+		let mixer = makeMixer(sink: sink, clock: clock, includesMicrophone: true, microphoneGain: 1)
+		mixer.beginTimeline(at: clock.now)
+
+		var delivered = CMTime.zero
+		func deliverBoth(seconds: Double) {
+			mixer.ingest(
+				makeSourceBuffer(burst(seconds: seconds, left: 0.5, right: 0), at: delivered, nonInterleaved: true),
+				from: .system
+			)
+			mixer.ingest(
+				makeSourceBuffer(burst(seconds: seconds, left: 0, right: 0.5), at: delivered, nonInterleaved: false),
+				from: .microphone
+			)
+			delivered = CMTimeAdd(delivered, CMTime(seconds: seconds, preferredTimescale: 48_000))
+		}
+
+		for _ in 0..<50 {
+			deliverBoth(seconds: 0.01)
+			runTicker(mixer, clock, seconds: 0.01)
+		}
+
+		// The queue is blocked for a second: no tick, no buffer, while the clock runs on.
+		clock.advance(seconds: 1)
+		mixer.tick()
+		for _ in 0..<100 {
+			deliverBoth(seconds: 0.01)
+		}
+
+		for _ in 0..<50 {
+			deliverBoth(seconds: 0.01)
+			runTicker(mixer, clock, seconds: 0.01)
+		}
+		mixer.finish(atSourceTime: clock.now)
+
+		let frames = mixedFrames(sink)
+		assertContiguous(sink)
+		XCTAssertEqual(seconds(ofFrames: frames), 2.0, accuracy: 0.011)
+		XCTAssertEqual(mixer.deliveryReport(for: .system).droppedSeconds, 0, accuracy: 0.0001)
+		XCTAssertEqual(mixer.deliveryReport(for: .microphone).droppedSeconds, 0, accuracy: 0.0001)
+		for step in 0..<20 {
+			let from = Double(step) * 0.1
+			for channel in 0..<channelCount {
+				XCTAssertGreaterThan(
+					peak(frames, from: from, to: from + 0.1, channel: channel),
+					audibleThreshold,
+					"silence on channel \(channel) at \(from)s means queued audio was dropped"
+				)
+			}
+		}
+	}
+
 	// MARK: - Pause
 
 	/// A pause stops the clock instead of resetting it. `ScreenCaptureRecorder.timelineNow()`
@@ -763,6 +822,14 @@ final class AudioTrackMixerTests: XCTestCase {
 		}
 	}
 
+	/// Moves the clock the way the recorder's 10 ms ticker sees it, with the queue running.
+	private func runTicker(_ mixer: AudioTrackMixer, _ clock: TestClock, seconds: Double) {
+		for _ in 0..<Int((seconds * 100).rounded()) {
+			clock.advance(seconds: 0.01)
+			mixer.tick()
+		}
+	}
+
 	private func peak(_ samples: [Int16], from: Double, to: Double, channel: Int? = nil) -> Int16 {
 		let firstFrame = Int(from * Double(sampleRate))
 		let lastFrame = min(Int(to * Double(sampleRate)), samples.count / channelCount)