Prevent audio thread from starving UI thread from CSoundGen::m_csAPULock

Source/SoundGen.h

@@ -371,6 +371,17 @@ class CSoundGen : IAudioCallback
 	// Thread synchronization
 private:
 	mutable std::mutex m_csAPULock;		// // //
+	/// Acquired whenever waiting for m_csAPULock.
+	/// 
+	/// This ensures that if the audio thread holds m_csAPULock,
+	/// and the UI thread is waiting for m_csAPULock (while holding m_waitingForApuLock),
+	/// the audio thread cannot release and reacquire m_csAPULock (starving the UI thread of access).
+	/// 
+	/// Instead the audio thread blocks on m_waitingForApuLock
+	/// until the UI thread acquires m_csAPULock and releases m_waitingForApuLock.
+	/// Then the audio thread must wait for the UI thread to release m_csAPULock.
+	mutable std::mutex m_waitingForApuLock;
+
 	mutable std::mutex m_csVisualizerWndLock;
 
 	// Handles
@@ -515,10 +526,18 @@ class CSoundGen : IAudioCallback
 
 public:
 	std::unique_lock<std::mutex> Lock() {
+		auto waiting = std::unique_lock(m_waitingForApuLock);
+
+		// Unlocks `waiting`.
 		return std::unique_lock<std::mutex>(m_csAPULock);
 	}
 
 	std::optional<std::unique_lock<std::mutex>> TryLock() {
+		auto waiting = std::unique_lock(m_waitingForApuLock, std::defer_lock);
+		if (!waiting.try_lock()) {
+			return {};
+		}
+
 		std::optional out = std::unique_lock<std::mutex>(m_csAPULock, std::defer_lock);
 		if (!out->try_lock()) {
 			out = {};