Can specify a minimum to maintain in the playback RingBuffer...

... this may be more than the batch size used in ongoing playback.

It is expected that this larger batch size is used only once when priming
the queue before starting play.  But then FillBuffers() may attempt to
refill up to the minimum in case demand is outpacing supply.

Thus the new number defines a "yellow zone" for the queue.

src/AudioIO.cpp

@@ -2293,6 +2293,10 @@ bool AudioIO::AllocateBuffers(
 #endif
                     Mixer::WarpOptions(mPlaybackSchedule.mTimeTrack);
 
+            mPlaybackQueueMinimum = mPlaybackSamplesToCopy;
+            mPlaybackQueueMinimum =
+               std::min( mPlaybackQueueMinimum, playbackBufferSize );
+
             for (unsigned int i = 0; i < mPlaybackTracks.size(); i++)
             {
                // Bug 1763 - We must fade in from zero to avoid a click on starting.
@@ -2325,7 +2329,8 @@ bool AudioIO::AllocateBuffers(
                   mPlaybackSchedule.mT0,
                   endTime,
                   1,
-                  mPlaybackSamplesToCopy, false,
+                  std::max( mPlaybackSamplesToCopy, mPlaybackQueueMinimum ),
+                  false,
                   mRate, floatSample, false);
                mPlaybackMixers[i]->ApplyTrackGains(false);
             }
@@ -3924,14 +3929,23 @@ void AudioIO::FillBuffers()
       // The exception is if we're at the end of the selected
       // region - then we should just fill the buffer.
       //
+      // May produce a larger amount when initially priming the buffer, or
+      // perhaps again later in play to avoid unerfilling the queue and falling
+      // behind the real-time demand on the consumer side in the callback.
+      auto nReady = GetCommonlyReadyPlayback();
+      auto nNeeded =
+         mPlaybackQueueMinimum - std::min(mPlaybackQueueMinimum, nReady);
+
+      // wxASSERT( nNeeded <= nAvailable );
+
       auto realTimeRemaining = mPlaybackSchedule.RealTimeRemaining();
       if (nAvailable >= mPlaybackSamplesToCopy ||
           (mPlaybackSchedule.PlayingStraight() &&
            nAvailable / mRate >= realTimeRemaining))
       {
          // Limit maximum buffer size (increases performance)
-         auto available =
-            std::min( nAvailable, mPlaybackSamplesToCopy );
+         auto available = std::min( nAvailable,
+            std::max( nNeeded, mPlaybackSamplesToCopy ) );
 
          // msmeyer: When playing a very short selection in looped
          // mode, the selection must be copied to the buffer multiple

src/AudioIO.h

@@ -720,7 +720,12 @@ private:
    double              mSeek;
    double              mPlaybackRingBufferSecs;
    double              mCaptureRingBufferSecs;
+
+   /// Preferred batch size for replenishing the playback RingBuffer
    size_t              mPlaybackSamplesToCopy;
+   /// Occupancy of the queue we try to maintain, with bigger batches if needed
+   size_t              mPlaybackQueueMinimum;
+
    double              mMinCaptureSecsToCopy;
    /// True if audio playback is paused
    bool                mPaused;