More responsive scrub engine; improved speed control on Mac

* scrubbing:
  Don't let seek make a stutter at less than unit speed
  Further simplified argument passing
  Improve scrubbing speed control (2 finger swipe) on Mac
  Improve scrub responsiveness: a secondary thread polls the mouse
  Don't let the consumers discard too much from the scrub queue...
  Reduce scrub lag yet more, at expense of possible skips in play...
  Scrub lag: lock mutex not more than once per call to FillBuffers
  Reorganize logic of initializing queue entries
  One second countdown now based on the correct timer interval

src/AudioIO.cpp

@@ -372,32 +372,35 @@ So a small, fixed queue size should be adequate.
 struct AudioIO::ScrubQueue
 {
    ScrubQueue(double t0, double t1, wxLongLong startClockMillis,
-              double rate, double maxSpeed,
+              double rate, long maxDebt,
               const ScrubbingOptions &options)
       : mTrailingIdx(0)
       , mMiddleIdx(1)
-      , mLeadingIdx(2)
+      , mLeadingIdx(1)
       , mRate(rate)
       , mLastScrubTimeMillis(startClockMillis)
       , mUpdating()
+      , mMaxDebt { maxDebt }
    {
-      // Ignore options.adjustStart, pass false.
+      const long s0 = std::max(options.minSample, std::min(options.maxSample,
-
+         lrint(t0 * mRate)
-      bool success = InitEntry(mEntries[mMiddleIdx], nullptr,
+      ));
-         t0, t1, maxSpeed, false, false, options);
+      const long s1 = lrint(t1 * mRate);
-      if (!success)
+      Duration dd { *this };
-      {
+      long actualDuration = std::max(1L, dd.duration);
-         // StartClock equals now?  Really?
+      auto success = mEntries[mMiddleIdx].Init(nullptr,
-         --mLastScrubTimeMillis;
+         s0, s1, actualDuration, options);
-         success = InitEntry(mEntries[mMiddleIdx], nullptr,
+      if (success)
-            t0, t1, maxSpeed, false, false, options);
+         ++mLeadingIdx;
+      else {
+         // If not, we can wait to enqueue again later
+         dd.Cancel();
       }
-      wxASSERT(success);
 
       // So the play indicator starts out unconfused:
       {
          Entry &entry = mEntries[mTrailingIdx];
-         entry.mS0 = entry.mS1 = mEntries[mMiddleIdx].mS0;
+         entry.mS0 = entry.mS1 = s0;
          entry.mPlayed = entry.mDuration = 1;
       }
    }
@@ -420,30 +423,57 @@ struct AudioIO::ScrubQueue
       mAvailable.Signal();
    }
 
-   bool Producer(double end, double maxSpeed, const ScrubbingOptions &options)
+   bool Producer(double end, const ScrubbingOptions &options)
    {
       // Main thread indicates a scrubbing interval
 
       // MAY ADVANCE mLeadingIdx, BUT IT NEVER CATCHES UP TO mTrailingIdx.
 
       wxMutexLocker locker(mUpdating);
-      const unsigned next = (mLeadingIdx + 1) % Size;
+      bool result = true;
+      unsigned next = (mLeadingIdx + 1) % Size;
       if (next != mTrailingIdx)
       {
-         Entry &previous = mEntries[(mLeadingIdx + Size - 1) % Size];
+         auto current = &mEntries[mLeadingIdx];
+         auto previous = &mEntries[(mLeadingIdx + Size - 1) % Size];
 
          // Use the previous end as NEW start.
-         const double startTime = previous.mS1 / mRate;
+         const long s0 = previous->mS1;
-         // Might reject the request because of zero duration,
+         Duration dd { *this };
-         // or a too-short "stutter"
+         const auto &origDuration = dd.duration;
-         const bool success =
+         if (origDuration <= 0)
-            (InitEntry(mEntries[mLeadingIdx], &previous, startTime, end, maxSpeed,
+            return false;
-                       options.enqueueBySpeed, options.adjustStart, options));
+
-         if (success) {
+         auto actualDuration = origDuration;
+         const long s1 = options.enqueueBySpeed
+            ? s0 + lrint(origDuration * end) // end is a speed
+            : lrint(end * mRate);            // end is a time
+         auto success =
+            current->Init(previous, s0, s1, actualDuration, options);
+         if (success)
             mLeadingIdx = next;
-            mAvailable.Signal();
+         else {
+            dd.Cancel();
+            return false;
          }
-         return success;
+
+         // Fill up the queue with some silence if there was trimming
+         wxASSERT(actualDuration <= origDuration);
+         if (actualDuration < origDuration) {
+            next = (mLeadingIdx + 1) % Size;
+            if (next != mTrailingIdx) {
+               previous = &mEntries[(mLeadingIdx + Size - 1) % Size];
+               current = &mEntries[mLeadingIdx];
+               current->InitSilent(*previous, origDuration - actualDuration);
+               mLeadingIdx = next;
+            }
+            else
+               // Oops, can't enqueue the silence -- so do what?
+               ;
+         }
+
+         mAvailable.Signal();
+         return result;
       }
       else
       {
@@ -456,33 +486,85 @@ struct AudioIO::ScrubQueue
       }
    }
 
-   void Transformer(long &startSample, long &endSample, long &duration)
+   void Transformer(long &startSample, long &endSample, long &duration,
+                    Maybe<wxMutexLocker> &cleanup)
    {
       // Audio thread is ready for the next interval.
 
       // MAY ADVANCE mMiddleIdx, WHICH MAY EQUAL mLeadingIdx, BUT DOES NOT PASS IT.
 
-      wxMutexLocker locker(mUpdating);
+      bool checkDebt = false;
+      if (!cleanup) {
+         cleanup.create(mUpdating);
+
+         // Check for cancellation of work only when re-enetering the cricial section
+         checkDebt = true;
+      }
       while(!mNudged && mMiddleIdx == mLeadingIdx)
          mAvailable.Wait();
 
       mNudged = false;
 
-      if (mMiddleIdx != mLeadingIdx)
+      auto now = ::wxGetLocalTimeMillis();
-      {
+
-         // There is work in the queue
+      if (checkDebt &&
+          mLastTransformerTimeMillis >= 0 && // Not the first time for this scrub
+          mMiddleIdx != mLeadingIdx) {
+         // There is work in the queue, but if Producer is outrunning us, discard some,
+         // which may make a skip yet keep playback better synchronized with user gestures.
+         const auto interval = (now - mLastTransformerTimeMillis).ToDouble() / 1000.0;
+         const Entry &previous = mEntries[(mMiddleIdx + Size - 1) % Size];
+         const auto deficit =
+            static_cast<long>(interval * mRate) - // Samples needed in the last time interval
+            mCredit;                              // Samples done in the last time interval
+         mCredit = 0;
+         mDebt += deficit;
+         auto toDiscard = mDebt - mMaxDebt;
+         while (toDiscard > 0 && mMiddleIdx != mLeadingIdx) {
+            // Cancel some debt (discard some new work)
+            auto &entry = mEntries[mMiddleIdx];
+            auto &dur = entry.mDuration;
+            if (toDiscard >= dur) {
+               // Discard entire queue entry
+               mDebt -= dur;
+               toDiscard -= dur;
+               dur = 0; // So Consumer() will handle abandoned entry correctly
+               mMiddleIdx = (mMiddleIdx + 1) % Size;
+            }
+            else {
+               // Adjust the start time
+               auto &start = entry.mS0;
+               const auto end = entry.mS1;
+               const auto ratio = static_cast<double>(toDiscard) / static_cast<double>(dur);
+               const auto adjustment = static_cast<long>(std::abs(end - start) * ratio);
+               if (start <= end)
+                  start += adjustment;
+               else
+                  start -= adjustment;
+
+               mDebt -= toDiscard;
+               dur -= toDiscard;
+               toDiscard = 0;
+            }
+         }
+      }
+
+      if (mMiddleIdx != mLeadingIdx) {
+         // There is still work in the queue, after cancelling debt
          Entry &entry = mEntries[mMiddleIdx];
          startSample = entry.mS0;
          endSample = entry.mS1;
          duration = entry.mDuration;
-         const unsigned next = (mMiddleIdx + 1) % Size;
+         mMiddleIdx = (mMiddleIdx + 1) % Size;
-         mMiddleIdx = next;
+         mCredit += duration;
       }
-      else
+      else {
-      {
+         // We got the shut-down signal, or we got nudged, or we discarded all the work.
-         // We got the shut-down signal, or we got nudged
          startSample = endSample = duration = -1L;
       }
+
+      if (checkDebt)
+         mLastTransformerTimeMillis = now;
    }
 
    double Consumer(unsigned long frames)
@@ -531,21 +613,26 @@ private:
          , mPlayed(0)
       {}
 
-      bool Init(Entry *previous, long s0, long s1, long duration,
+      bool Init(Entry *previous, long s0, long s1,
-         double maxSpeed, bool adjustStart,
+         long &duration /* in/out */,
          const ScrubbingOptions &options)
       {
-         if (duration <= 0)
+         const bool &adjustStart = options.adjustStart;
-            return false;
-         double speed = double(abs(s1 - s0)) / duration;
-         bool maxed = false;
 
-         // May change the requested speed (or reject)
+         wxASSERT(duration > 0);
-         if (!adjustStart && speed > maxSpeed)
+         double speed = static_cast<double>(std::abs(s1 - s0)) / duration;
+         bool adjustedSpeed = false;
+
+         auto minSpeed = std::min(options.minSpeed, options.maxSpeed);
+         wxASSERT(minSpeed == options.minSpeed);
+
+         // May change the requested speed and duration
+         if (!adjustStart && speed > options.maxSpeed)
          {
             // Reduce speed to the maximum selected in the user interface.
-            speed = maxSpeed;
+            speed = options.maxSpeed;
-            maxed = true;
+            mGoal = s1;
+            adjustedSpeed = true;
          }
          else if (!adjustStart &&
             previous &&
@@ -557,86 +644,76 @@ private:
             // continue at no less than maximum.  (Without this
             // the final catch-up can make a slow scrub interval
             // that drops the pitch and sounds wrong.)
-            duration = lrint(speed * duration / maxSpeed);
+            minSpeed = options.maxSpeed;
-            if (duration <= 0)
+            mGoal = s1;
-            {
+            adjustedSpeed = true;
-               previous->mGoal = -1;
-               return false;
-            }
-            speed = maxSpeed;
-            maxed = true;
-         }
-
-        if (speed < ScrubbingOptions::MinAllowedScrubSpeed())
-            // Mixers were set up to go only so slowly, not slower.
-            // This will put a request for some silence in the work queue.
-            speed = 0.0;
-
-         // Before we change s1:
-         mGoal = maxed ? s1 : -1;
-
-         // May change s1 or s0 to match speed change:
-         if (adjustStart)
-         {
-            bool silent = false;
-
-            // Adjust s1 first, and duration, if s1 is out of bounds.
-            // (Assume s0 is in bounds, because it is the last scrub's s1 which was checked.)
-            if (s1 != s0)
-            {
-               const long newS1 = std::max(options.minSample, std::min(options.maxSample, s1));
-               if (s1 != newS1)
-               {
-                  long newDuration = long(duration * double(newS1 - s0) / (s1 - s0));
-                  s1 = newS1;
-                  if (newDuration == 0)
-                     // Enqueue a silent scrub with s0 == s1
-                     silent = true;
-                  else
-                     // Shorten
-                     duration = newDuration;
-               }
-            }
-
-            if (!silent)
-            {
-               // When playback follows a fast mouse movement by "stuttering"
-               // at maximum playback, don't make stutters too short to be useful.
-               if (duration < options.minStutter)
-                  return false;
-               // Limit diff because this is seeking.
-               const long diff = lrint(std::min(1.0, speed) * duration);
-               if (s0 < s1)
-                  s0 = s1 - diff;
-               else
-                  s0 = s1 + diff;
-            }
          }
          else
+            mGoal = -1;
+
+         if (speed < minSpeed) {
+            // Trim the duration.
+            duration = std::max(0L, lrint(speed * duration / minSpeed));
+            speed = minSpeed;
+            adjustedSpeed = true;
+         }
+
+         if (speed < ScrubbingOptions::MinAllowedScrubSpeed()) {
+            // Mixers were set up to go only so slowly, not slower.
+            // This will put a request for some silence in the work queue.
+            adjustedSpeed = true;
+            speed = 0.0;
+         }
+
+         // May change s1 or s0 to match speed change or stay in bounds of the project
+
+         if (adjustedSpeed && !adjustStart)
          {
-            // adjust end
+            // adjust s1
             const long diff = lrint(speed * duration);
             if (s0 < s1)
                s1 = s0 + diff;
             else
                s1 = s0 - diff;
+         }
 
-            // Adjust s1 again, and duration, if s1 is out of bounds.  (Assume s0 is in bounds.)
+         bool silent = false;
-            if (s1 != s0)
+
-            {
+         // Adjust s1 (again), and duration, if s1 is out of bounds,
-               const long newS1 = std::max(options.minSample, std::min(options.maxSample, s1));
+         // or abandon if a stutter is too short.
-               if (s1 != newS1)
+         // (Assume s0 is in bounds, because it equals the last scrub's s1 which was checked.)
-               {
+         if (s1 != s0)
-                  long newDuration = long(duration * double(newS1 - s0) / (s1 - s0));
+         {
-                  s1 = newS1;
+            long newDuration = duration;
-                  if (newDuration == 0)
+            const long newS1 = std::max(options.minSample, std::min(options.maxSample, s1));
-                     // Enqueue a silent scrub with s0 == s1
+            if(s1 != newS1)
-                     ;
+               newDuration = std::max(0L,
-                  else
+                  static_cast<long>(duration * static_cast<double>(newS1 - s0) / (s1 - s0))
-                     // Shorten
+               );
-                     duration = newDuration;
+            // When playback follows a fast mouse movement by "stuttering"
-               }
+            // at maximum playback, don't make stutters too short to be useful.
+            if (options.adjustStart && newDuration < options.minStutter)
+               return false;
+            else if (newDuration == 0) {
+               // Enqueue a silent scrub with s0 == s1
+               silent = true;
+               s1 = s0;
             }
+            else if (s1 != newS1) {
+               // Shorten
+               duration = newDuration;
+               s1 = newS1;
+            }
+         }
+
+         if (adjustStart && !silent)
+         {
+            // Limit diff because this is seeking.
+            const long diff = lrint(std::min(options.maxSpeed, speed) * duration);
+            if (s0 < s1)
+               s0 = s1 - diff;
+            else
+               s0 = s1 + diff;
          }
 
          mS0 = s0;
@@ -646,9 +723,20 @@ private:
          return true;
       }
 
+      void InitSilent(const Entry &previous, long duration)
+      {
+         mGoal = previous.mGoal;
+         mS0 = mS1 = previous.mS1;
+         mPlayed = 0;
+         mDuration = duration;
+      }
+
       double GetTime(double rate) const
       {
-         return (mS0 + ((mS1 - mS0) * mPlayed) / double(mDuration)) / rate;
+         return
+            (mS0 +
+             (mS1 - mS0) * static_cast<double>(mPlayed) / static_cast<double>(mDuration))
+            / rate;
       }
 
       // These sample counts are initialized in the UI, producer, thread:
@@ -668,23 +756,23 @@ private:
       long mPlayed;
    };
 
-   bool InitEntry(Entry &entry, Entry *previous, double t0, double end, double maxSpeed,
+   struct Duration {
-      bool bySpeed, bool adjustStart,
+      Duration (ScrubQueue &queue_) : queue(queue_) {}
-      const ScrubbingOptions &options)
+      ~Duration ()
-   {
+      {
-      const wxLongLong clockTime(::wxGetLocalTimeMillis());
+         if(!cancelled)
-      const long duration =
+            queue.mLastScrubTimeMillis = clockTime;
-         mRate * (clockTime - mLastScrubTimeMillis).ToDouble() / 1000.0;
+      }
-      const long s0 = t0 * mRate;
+
-      const long s1 = bySpeed
+      void Cancel() { cancelled = true; }
-         ? s0 + lrint(duration * end) // end is a speed
+
-         : lrint(end * mRate);        // end is a time
+      ScrubQueue &queue;
-      const bool success =
+      const wxLongLong clockTime { ::wxGetLocalTimeMillis() };
-         entry.Init(previous, s0, s1, duration, maxSpeed, adjustStart, options);
+      const long duration { static_cast<long>
-      if (success)
+         (queue.mRate * (clockTime - queue.mLastScrubTimeMillis).ToDouble() / 1000.0)
-         mLastScrubTimeMillis = clockTime;
+      };
-      return success;
+      bool cancelled { false };
-   }
+   };
 
    enum { Size = 10 };
    Entry mEntries[Size];
@@ -693,6 +781,12 @@ private:
    unsigned mLeadingIdx;
    const double mRate;
    wxLongLong mLastScrubTimeMillis;
+
+   wxLongLong mLastTransformerTimeMillis { -1LL };
+   long mCredit { 0L };
+   long mDebt { 0L };
+   const long mMaxDebt;
+
    mutable wxMutex mUpdating;
    mutable wxCondition mAvailable { mUpdating };
    bool mNudged { false };
@@ -1863,8 +1957,8 @@ int AudioIO::StartStream(const WaveTrackArray &playbackTracks,
       const auto &scrubOptions = *options.pScrubbingOptions;
       mScrubQueue =
          new ScrubQueue(mT0, mT1, scrubOptions.startClockTimeMillis,
-            sampleRate, scrubOptions.maxSpeed,
+            sampleRate, 2 * scrubOptions.minStutter,
-            *options.pScrubbingOptions);
+            scrubOptions);
       mScrubDuration = 0;
       mSilentScrub = false;
    }
@@ -2462,10 +2556,10 @@ bool AudioIO::IsPaused()
 
 #ifdef EXPERIMENTAL_SCRUBBING_SUPPORT
 bool AudioIO::EnqueueScrub
-   (double endTimeOrSpeed, double maxSpeed, const ScrubbingOptions &options)
+   (double endTimeOrSpeed, const ScrubbingOptions &options)
 {
    if (mScrubQueue)
-      return mScrubQueue->Producer(endTimeOrSpeed, maxSpeed, options);
+      return mScrubQueue->Producer(endTimeOrSpeed, options);
    else
       return false;
 }
@@ -3367,6 +3461,7 @@ void AudioIO::FillBuffers()
          // PRL: or, when scrubbing, we may get work repeatedly from the
          // scrub queue.
          bool done = false;
+         Maybe<wxMutexLocker> cleanup;
          do {
             // How many samples to produce for each channel.
             long frames = available;
@@ -3442,7 +3537,7 @@ void AudioIO::FillBuffers()
                if (!done && mScrubDuration <= 0)
                {
                   long startSample, endSample;
-                  mScrubQueue->Transformer(startSample, endSample, mScrubDuration);
+                  mScrubQueue->Transformer(startSample, endSample, mScrubDuration, cleanup);
                   if (mScrubDuration < 0)
                   {
                      // Can't play anything
@@ -3458,7 +3553,7 @@ void AudioIO::FillBuffers()
                         double startTime, endTime, speed;
                         startTime = startSample / mRate;
                         endTime = endSample / mRate;
-                        speed = double(abs(endSample - startSample)) / mScrubDuration;
+                        speed = double(std::abs(endSample - startSample)) / mScrubDuration;
                         for (i = 0; i < mPlaybackTracks->size(); i++)
                            mPlaybackMixers[i]->SetTimesAndSpeed(startTime, endTime, speed);
                      }
@@ -4179,7 +4274,7 @@ int audacityAudioCallback(const void *inputBuffer, void *outputBuffer,
                      (gAudioIO->mT0, gAudioIO->mTime);
             else
                gAudioIO->mWarpedTime = gAudioIO->mTime - gAudioIO->mT0;
-            gAudioIO->mWarpedTime = abs(gAudioIO->mWarpedTime);
+            gAudioIO->mWarpedTime = std::abs(gAudioIO->mWarpedTime);
 
             // Reset mixer positions and flush buffers for all tracks
             for (i = 0; i < (unsigned int)numPlaybackTracks; i++)

src/AudioIO.h

@@ -169,11 +169,10 @@ class AUDACITY_DLL_API AudioIO final {
    * If options.adjustStart is true, then when mouse movement exceeds maximum scrub speed,
    * adjust the beginning of the scrub interval rather than the end, so that
    * the scrub skips or "stutters" to stay near the cursor.
-   * But if the "stutter" is too short for the minimum, then there is no effect
+   * Return true if some sound was really enqueued.
-   * on the work queue.
+   * But if the "stutter" is too short for the minimum, enqueue nothing and return false.
-   * Return true if some work was really enqueued.
    */
-   bool EnqueueScrub(double endTimeOrSpeed, double maxSpeed, const ScrubbingOptions &options);
+   bool EnqueueScrub(double endTimeOrSpeed, const ScrubbingOptions &options);
 
    /** \brief return the ending time of the last enqueued scrub interval.
    */

src/Project.cpp

@@ -5378,8 +5378,18 @@ void AudacityProject::PlaybackScroller::OnTimer(wxCommandEvent &event)
    // Let other listeners get the notification
    event.Skip();
 
-   if (mMode != Mode::Off && mProject->IsAudioActive())
+   if(!mProject->IsAudioActive())
-   {
+      return;
+   else if (mMode == Mode::Refresh) {
+      // PRL:  see comments in Scrubbing.cpp for why this is sometimes needed.
+      // These unnecessary refreshes cause wheel rotation events to be delivered more uniformly
+      // to the application, so scrub speed control is smoother.
+      // (So I see at least with OS 10.10 and wxWidgets 3.0.2.)
+      // Is there another way to ensure that than by refreshing?
+      const auto trackPanel = mProject->GetTrackPanel();
+      trackPanel->Refresh(false);
+   }
+   else if (mMode != Mode::Off) {
       // Pan the view, so that we center the play indicator.
 
       ViewInfo &viewInfo = mProject->GetViewInfo();

src/Project.h

@@ -730,6 +730,7 @@ public:
 
       enum class Mode {
          Off,
+         Refresh,
          Centered,
          Right,
       };

src/TrackPanel.cpp

@@ -2774,8 +2774,11 @@ void TrackPanel::SelectionHandleDrag(wxMouseEvent & event, Track *clickedTrack)
 #endif
 
    ExtendSelection(x, rect.x, clickedTrack);
-   // Don't do this at every mouse event, because it slows down seek-scrub.
+   // If scrubbing does not use the helper poller thread, then
+   // don't do this at every mouse event, because it slows down seek-scrub.
    // Instead, let OnTimer do it, which is often enough.
+   // And even if scrubbing does use the thread, then skipping this does not
+   // bring that advantage, but it is probably still a good idea anyway.
    // UpdateSelectionDisplay();
 }
 
@@ -5623,6 +5626,7 @@ void TrackPanel::HandleWheelRotation(wxMouseEvent & event)
 #ifdef EXPERIMENTAL_SCRUBBING_SCROLL_WHEEL
       if (GetProject()->GetScrubber().IsScrubbing()) {
          GetProject()->GetScrubber().HandleScrollWheel(steps);
+         event.Skip(false);
       }
       else
 #endif

src/TrackPanel.h

@@ -73,7 +73,6 @@ DECLARE_EXPORTED_EVENT_TYPE(AUDACITY_DLL_API, EVT_TRACK_PANEL_TIMER, -1);
 
 enum {
    kTimerInterval = 50, // milliseconds
-   kOneSecondCountdown = 1000 / kTimerInterval,
 };
 
 class AUDACITY_DLL_API TrackInfo

src/tracks/ui/Scrubbing.cpp

@@ -46,6 +46,8 @@ enum {
 #endif
 
    ScrubPollInterval_ms = 50,
+
+   kOneSecondCountdown = 1000 / ScrubPollInterval_ms,
 };
 
 static const double MinStutter = 0.2;
@@ -123,6 +125,32 @@ namespace {
    }
 }
 
+#ifdef USE_SCRUB_THREAD
+
+class Scrubber::ScrubPollerThread final : public wxThread {
+public:
+   ScrubPollerThread(Scrubber &scrubber)
+      : wxThread { }
+      , mScrubber(scrubber)
+   {}
+   ExitCode Entry() override;
+
+private:
+   Scrubber &mScrubber;
+};
+
+auto Scrubber::ScrubPollerThread::Entry() -> ExitCode
+{
+   while( !TestDestroy() )
+   {
+      wxThread::Sleep(ScrubPollInterval_ms);
+      mScrubber.ContinueScrubbingPoll();
+   }
+   return 0;
+}
+
+#endif
+
 class Scrubber::ScrubPoller : public wxTimer
 {
 public:
@@ -140,7 +168,13 @@ void Scrubber::ScrubPoller::Notify()
    // rather than in SelectionHandleDrag()
    // so that even without drag events, we can instruct the play head to
    // keep approaching the mouse cursor, when its maximum speed is limited.
-   mScrubber.ContinueScrubbing();
+
+#ifndef USE_SCRUB_THREAD
+   // If there is no helper thread, this main thread timer is responsible
+   // for playback and for UI
+   mScrubber.ContinueScrubbingPoll();
+#endif
+   mScrubber.ContinueScrubbingUI();
 }
 
 Scrubber::Scrubber(AudacityProject *project)
@@ -167,6 +201,11 @@ Scrubber::Scrubber(AudacityProject *project)
 
 Scrubber::~Scrubber()
 {
+#ifdef USE_SCRUB_THREAD
+   if (mpThread)
+      mpThread->Delete();
+#endif
+
    mProject->PopEventHandler();
    if (wxTheApp)
       wxTheApp->Disconnect
@@ -301,18 +340,19 @@ bool Scrubber::MaybeStartScrubbing(wxCoord xx)
             AudioIOStartStreamOptions options(mProject->GetDefaultPlayOptions());
             options.pScrubbingOptions = &mOptions;
             options.timeTrack = NULL;
-            mOptions.delay = (ScrubPollInterval_ms / 1000.0);
+            mOptions.delay = (ScrubPollInterval_ms * 0.9 / 1000.0);
+            mOptions.minSpeed = 0.0;
 #ifdef USE_TRANSCRIPTION_TOOLBAR
             if (!mAlwaysSeeking) {
                // Take the starting speed limit from the transcription toolbar,
                // but it may be varied during the scrub.
-               mOptions.maxSpeed =
+               mMaxSpeed = mOptions.maxSpeed =
                   mProject->GetTranscriptionToolBar()->GetPlaySpeed();
             }
 #else
             // That idea seems unpopular... just make it one for move-scrub,
             // but big for drag-scrub
-            mOptions.maxSpeed = mDragging ? MaxDragSpeed : 1.0;
+            mMaxSpeed = mOptions.maxSpeed = mDragging ? MaxDragSpeed : 1.0;
 #endif
             mOptions.minSample = 0;
             mOptions.maxSample =
@@ -328,7 +368,7 @@ bool Scrubber::MaybeStartScrubbing(wxCoord xx)
             static const double maxScrubSpeedBase =
                pow(2.0, 1.0 / ScrubSpeedStepsPerOctave);
             mLogMaxScrubSpeed = floor(0.5 +
-               log(mOptions.maxSpeed) / log(maxScrubSpeedBase)
+               log(mMaxSpeed) / log(maxScrubSpeedBase)
             );
 #endif
             mScrubSpeedDisplayCountdown = 0;
@@ -342,12 +382,17 @@ bool Scrubber::MaybeStartScrubbing(wxCoord xx)
          mOptions.startClockTimeMillis = ::wxGetLocalTimeMillis();
 
       if (IsScrubbing()) {
-         using Mode = AudacityProject::PlaybackScroller::Mode;
+         ActivateScroller();
-         mProject->GetPlaybackScroller().Activate
-            (mSmoothScrollingScrub ? Mode::Centered : Mode::Off);
          mPaused = false;
          mLastScrubPosition = xx;
 
+#ifdef USE_SCRUB_THREAD
+         // Detached thread is self-deleting, after it receives the Delete() message
+         mpThread = safenew ScrubPollerThread{ *this };
+         mpThread->Create(4096);
+         mpThread->Run();
+#endif
+
          mPoller->Start(ScrubPollInterval_ms);
       }
 
@@ -356,7 +401,67 @@ bool Scrubber::MaybeStartScrubbing(wxCoord xx)
    }
 }
 
-void Scrubber::ContinueScrubbing()
+void Scrubber::ContinueScrubbingPoll()
+{
+   // Thus scrubbing relies mostly on periodic polling of mouse and keys,
+   // not event notifications.  But there are a few event handlers that
+   // leave messages for this routine, in mScrubSeekPress and in mPaused.
+
+   // Decide whether to skip play, because either mouse is down now,
+   // or there was a left click event.  (This is then a delayed reaction, in a
+   // timer callback, to a left click event detected elsewhere.)
+   const bool seek = PollIsSeeking() || mScrubSeekPress;
+
+   bool result = false;
+   if (mPaused) {
+      // When paused, enqueue silent scrubs.
+      mOptions.minSpeed = 0.0;
+      mOptions.maxSpeed = mMaxSpeed;
+      mOptions.adjustStart = false;
+      mOptions.enqueueBySpeed = true;
+      result = gAudioIO->EnqueueScrub(0, mOptions);
+   }
+   else {
+      const wxMouseState state(::wxGetMouseState());
+      const auto trackPanel = mProject->GetTrackPanel();
+      const wxPoint position = trackPanel->ScreenToClient(state.GetPosition());
+      const auto &viewInfo = mProject->GetViewInfo();
+      if (mDragging && mSmoothScrollingScrub) {
+         const auto lastTime = gAudioIO->GetLastTimeInScrubQueue();
+         const auto delta = mLastScrubPosition - position.x;
+         const double time = viewInfo.OffsetTimeByPixels(lastTime, delta);
+         mOptions.minSpeed = 0.0;
+         mOptions.maxSpeed = mMaxSpeed;
+         mOptions.adjustStart = true;
+         mOptions.enqueueBySpeed = false;
+         result = gAudioIO->EnqueueScrub(time, mOptions);
+         mLastScrubPosition = position.x;
+      }
+      else {
+         const double time = viewInfo.PositionToTime(position.x, trackPanel->GetLeftOffset());
+         mOptions.adjustStart = seek;
+         mOptions.minSpeed = (mDragging || !seek) ? 0.0 : 1.0;
+         mOptions.maxSpeed = (mDragging || !seek) ? mMaxSpeed : 1.0;
+
+         if (mSmoothScrollingScrub) {
+            const double speed = FindScrubSpeed(seek, time);
+            mOptions.enqueueBySpeed = true;
+            result = gAudioIO->EnqueueScrub(speed, mOptions);
+         }
+         else {
+            mOptions.enqueueBySpeed = false;
+            result = gAudioIO->EnqueueScrub(time, mOptions);
+         }
+      }
+   }
+
+   if (result)
+      mScrubSeekPress = false;
+   // else, if seek requested, try again at a later time when we might
+   // enqueue a long enough stutter
+}
+
+void Scrubber::ContinueScrubbingUI()
 {
    const wxMouseState state(::wxGetMouseState());
 
@@ -366,70 +471,20 @@ void Scrubber::ContinueScrubbing()
       return;
    }
 
-   // Thus scrubbing relies mostly on periodic polling of mouse and keys,
+   const bool seek = PollIsSeeking();
-   // not event notifications.  But there are a few event handlers that
-   // leave messages for this routine, in mScrubSeekPress and in mPaused.
-
-   // Seek only when the pointer is in the panel.  Else, scrub.
-   TrackPanel *const trackPanel = mProject->GetTrackPanel();
-
-   // Decide whether to skip play, because either mouse is down now,
-   // or there was a left click event.  (This is then a delayed reaction, in a
-   // timer callback, to a left click event detected elsewhere.)
-   const bool seek = PollIsSeeking() || mScrubSeekPress;
 
    {
       // Show the correct status for seeking.
       bool backup = mAlwaysSeeking;
       mAlwaysSeeking = seek;
       const auto ctb = mProject->GetControlToolBar();
-      ctb->UpdateStatusBar(mProject);
+      if (ctb)
+         ctb->UpdateStatusBar(mProject);
       mAlwaysSeeking = backup;
    }
 
-   const wxPoint position = trackPanel->ScreenToClient(state.GetPosition());
+   if (seek)
-   const auto &viewInfo = mProject->GetViewInfo();
+      mScrubSpeedDisplayCountdown = 0;
-
-   bool result = false;
-   if (mPaused) {
-      // When paused, enqueue silent scrubs.
-      mOptions.adjustStart = false;
-      mOptions.enqueueBySpeed = true;
-      result = gAudioIO->EnqueueScrub(0, mOptions.maxSpeed, mOptions);
-   }
-   else if (mDragging && mSmoothScrollingScrub) {
-      const auto lastTime = gAudioIO->GetLastTimeInScrubQueue();
-      const auto delta = mLastScrubPosition - position.x;
-      const double time = viewInfo.OffsetTimeByPixels(lastTime, delta);
-      mOptions.adjustStart = true;
-      mOptions.enqueueBySpeed = false;
-      result = gAudioIO->EnqueueScrub(time, mOptions.maxSpeed, mOptions);
-      mLastScrubPosition = position.x;
-   }
-   else {
-      const double time = viewInfo.PositionToTime(position.x, trackPanel->GetLeftOffset());
-      mOptions.adjustStart = seek;
-      if (seek)
-         // Cause OnTimer() to suppress the speed display
-         mScrubSpeedDisplayCountdown = 1;
-
-      if (mSmoothScrollingScrub) {
-         const double speed = FindScrubSpeed(seek, time);
-         mOptions.enqueueBySpeed = true;
-         result = gAudioIO->EnqueueScrub(speed, mOptions.maxSpeed, mOptions);
-      }
-      else {
-         mOptions.enqueueBySpeed = false;
-         auto maxSpeed =
-            (mDragging || !seek) ? mOptions.maxSpeed : 1.0;
-         result = gAudioIO->EnqueueScrub(time, maxSpeed, mOptions);
-      }
-   }
-
-   if (result)
-      mScrubSeekPress = false;
-   // else, if seek requested, try again at a later time when we might
-   // enqueue a long enough stutter
 
    if (mSmoothScrollingScrub)
       ;
@@ -441,6 +496,13 @@ void Scrubber::ContinueScrubbing()
 
 void Scrubber::StopScrubbing()
 {
+#ifdef USE_SCRUB_THREAD
+   if (mpThread) {
+      mpThread->Delete();
+      mpThread = nullptr;
+   }
+#endif
+
    mPoller->Stop();
 
    UncheckAllMenuItems();
@@ -494,7 +556,7 @@ double Scrubber::FindScrubSpeed(bool seeking, double time) const
    ViewInfo &viewInfo = mProject->GetViewInfo();
    const double screen = mProject->GetScreenEndTime() - viewInfo.h;
    return (seeking ? FindSeekSpeed : FindScrubbingSpeed)
-      (viewInfo, mOptions.maxSpeed, screen, time);
+      (viewInfo, mMaxSpeed, screen, time);
 }
 
 void Scrubber::HandleScrollWheel(int steps)
@@ -503,6 +565,9 @@ void Scrubber::HandleScrollWheel(int steps)
       // Not likely you would spin it with the left button down, but...
       return;
 
+   if (steps == 0)
+      return;
+
    const int newLogMaxScrubSpeed = mLogMaxScrubSpeed + steps;
    static const double maxScrubSpeedBase =
       pow(2.0, 1.0 / ScrubSpeedStepsPerOctave);
@@ -510,7 +575,7 @@ void Scrubber::HandleScrollWheel(int steps)
    if (newSpeed >= ScrubbingOptions::MinAllowedScrubSpeed() &&
        newSpeed <= ScrubbingOptions::MaxAllowedScrubSpeed()) {
       mLogMaxScrubSpeed = newLogMaxScrubSpeed;
-      mOptions.maxSpeed = newSpeed;
+      mMaxSpeed = newSpeed;
       if (!mSmoothScrollingScrub)
          // Show the speed for one second
          mScrubSpeedDisplayCountdown = kOneSecondCountdown + 1;
@@ -717,6 +782,24 @@ bool Scrubber::PollIsSeeking()
    return mDragging || (mAlwaysSeeking || ::wxGetMouseState().LeftIsDown());
 }
 
+void Scrubber::ActivateScroller()
+{
+   using Mode = AudacityProject::PlaybackScroller::Mode;
+   mProject->GetPlaybackScroller().Activate(mSmoothScrollingScrub
+      ? Mode::Centered
+      :
+#ifdef __WXMAC__
+        // PRL:  cause many "unnecessary" refreshes.  For reasons I don't understand,
+        // doing this causes wheel rotation events (mapped from the double finger vertical
+        // swipe) to be delivered more uniformly to the application, so that spped control
+        // works better.
+        Mode::Refresh
+#else
+        Mode::Off
+#endif
+   );
+}
+
 void Scrubber::DoScrub(bool scroll, bool seek)
 {
    const bool wasScrubbing = IsScrubbing();
@@ -735,9 +818,7 @@ void Scrubber::DoScrub(bool scroll, bool seek)
    }
    else if(!match) {
       mSmoothScrollingScrub = scroll;
-      using Mode = AudacityProject::PlaybackScroller::Mode;
+      ActivateScroller();
-      mProject->GetPlaybackScroller().Activate
-         (scroll ? Mode::Centered : Mode::Off);
       mAlwaysSeeking = seek;
       UncheckAllMenuItems();
       CheckMenuItem();

src/tracks/ui/Scrubbing.h

@@ -21,6 +21,11 @@ Paul Licameli split from TrackPanel.cpp
 
 class AudacityProject;
 
+// Conditionally compile either a separate thead, or else use a timer in the main
+// thread, to poll the mouse and update scrubbing speed and direction.  The advantage of
+// a thread may be immunity to choppy scrubbing in case redrawing takes too much time.
+#define USE_SCRUB_THREAD
+
 // For putting an increment of work in the scrubbing queue
 struct ScrubbingOptions {
    ScrubbingOptions() {}
@@ -35,7 +40,8 @@ struct ScrubbingOptions {
 
    double delay {};
 
-   // A limiting value for the speed of a scrub interval:
+   // Limiting values for the speed of a scrub interval:
+   double minSpeed { 0.0 };
    double maxSpeed { 1.0 };
 
 
@@ -71,7 +77,8 @@ public:
    // Assume xx is relative to the left edge of TrackPanel!
    bool MaybeStartScrubbing(wxCoord xx);
 
-   void ContinueScrubbing();
+   void ContinueScrubbingUI();
+   void ContinueScrubbingPoll();
 
    // This is meant to be called only from ControlToolBar
    void StopScrubbing();
@@ -122,6 +129,7 @@ public:
    bool IsPaused() const;
 
 private:
+   void ActivateScroller();
    void DoScrub(bool scroll, bool seek);
    void OnActivateOrDeactivateApp(wxActivateEvent & event);
    void UncheckAllMenuItems();
@@ -158,9 +166,20 @@ private:
 
    DECLARE_EVENT_TABLE()
 
+#ifdef USE_SCRUB_THREAD
+   // Course corrections in playback are done in a helper thread, unhindered by
+   // the complications of the main event dispatch loop
+   class ScrubPollerThread;
+   ScrubPollerThread *mpThread {};
+#endif
+
+   // Other periodic update of the UI must be done in the main thread,
+   // by this object which is driven by timer events.
    class ScrubPoller;
    std::unique_ptr<ScrubPoller> mPoller;
+
    ScrubbingOptions mOptions;
+   double mMaxSpeed { 1.0 };
 };
 
 // Specialist in drawing the scrub speed, and listening for certain events