Implement scrolling for the note track vertical ruler

Command zooms in/out, and shift moves up and down.

src/tracks/playabletrack/notetrack/ui/NoteTrackVRulerControls.cpp

@@ -203,11 +203,42 @@ HitTestResult NoteTrackVRulerControls::HitTest
 (const TrackPanelMouseEvent &evt,
  const AudacityProject *)
 {
-#ifdef USE_MIDI
    return NoteTrackVZoomHandle::HitTest(evt.event);
-#else
-   return {};
-#endif
 }
 
-#endif
+unsigned NoteTrackVRulerControls::HandleWheelRotation
+(const TrackPanelMouseEvent &evt, AudacityProject *pProject)
+{
+   using namespace RefreshCode;
+   const wxMouseEvent &event = evt.event;
+
+   if (!(event.ShiftDown() || event.CmdDown()))
+      return RefreshNone;
+
+   // Always stop propagation even if the ruler didn't change.  The ruler
+   // is a narrow enough target.
+   evt.event.Skip(false);
+
+   const auto pTrack = FindTrack();
+   if (!pTrack)
+      return RefreshNone;
+   wxASSERT(pTrack->GetKind() == Track::Note);
+   auto steps = evt.steps;
+
+   const auto nt = static_cast<NoteTrack*>(pTrack.get());
+   if (event.CmdDown() && !event.ShiftDown()) {
+      nt->Zoom(evt.rect, evt.event.m_y, (int) (steps));
+   } else if (!event.CmdDown() && event.ShiftDown()) {
+      // Scroll some fixed number of notes, independent of zoom level or track height:
+      static const int movement = 6; // 6 semitones is half an octave
+      nt->SetBottomNote(nt->GetBottomNote() + (int) (steps * movement));
+   } else {
+      return RefreshNone;
+   }
+
+   pProject->ModifyState(true);
+
+   return RefreshCell | UpdateVRuler;
+}
+
+#endif

src/tracks/playabletrack/notetrack/ui/NoteTrackVRulerControls.h

@@ -27,6 +27,10 @@ public:
    HitTestResult HitTest
       (const TrackPanelMouseEvent &event,
        const AudacityProject *pProject) override;
+
+   unsigned HandleWheelRotation
+      (const TrackPanelMouseEvent &event,
+       AudacityProject *pProject) override;
 };
 
 #endif