Improve scrubbing speed control (2 finger swipe) on Mac

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

@@ -5626,6 +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/tracks/ui/Scrubbing.cpp

@@ -381,9 +381,7 @@ bool Scrubber::MaybeStartScrubbing(wxCoord xx)
          mOptions.startClockTimeMillis = ::wxGetLocalTimeMillis();
 
       if (IsScrubbing()) {
-         using Mode = AudacityProject::PlaybackScroller::Mode;
-         mProject->GetPlaybackScroller().Activate
-            (mSmoothScrollingScrub ? Mode::Centered : Mode::Off);
+         ActivateScroller();
          mPaused = false;
          mLastScrubPosition = xx;
 
@@ -561,6 +559,9 @@ 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);
@@ -775,6 +776,24 @@ 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();
@@ -793,9 +812,7 @@ void Scrubber::DoScrub(bool scroll, bool seek)
    }
    else if(!match) {
       mSmoothScrollingScrub = scroll;
-      using Mode = AudacityProject::PlaybackScroller::Mode;
-      mProject->GetPlaybackScroller().Activate
-         (scroll ? Mode::Centered : Mode::Off);
+      ActivateScroller();
       mAlwaysSeeking = seek;
       UncheckAllMenuItems();
       CheckMenuItem();

src/tracks/ui/Scrubbing.h

@@ -128,6 +128,7 @@ public:
    bool IsPaused() const;
 
 private:
+   void ActivateScroller();
    void DoScrub(bool scroll, bool seek);
    void OnActivateOrDeactivateApp(wxActivateEvent & event);
    void UncheckAllMenuItems();