Restore change of Waveform dB scale with change of magnification...

... This is done with Shift+Ctrl+wheel and pointer in the vertical ruler, and
the pointer y coordinate near the bottom of the dB scale.

If y coordinate is far from there, then Shift+Ctrl+wheel changes dB level
without change of magnification (as before this commit).

src/TrackArtist.cpp

@@ -1028,6 +1028,7 @@ void TrackArtist::DrawNegativeOffsetTrackArrows(wxDC &dc, const wxRect &rect)
 void TrackArtist::DrawWaveformBackground(wxDC &dc, int leftOffset, const wxRect &rect,
                                          const double env[],
                                          float zoomMin, float zoomMax,
+                                         int zeroLevelYCoordinate,
                                          bool dB, float dBRange,
                                          double t0, double t1,
                                          const ZoomInfo &zoomInfo,
@@ -1134,11 +1135,11 @@ void TrackArtist::DrawWaveformBackground(wxDC &dc, int leftOffset, const wxRect
 
    //OK, the display bounds are between min and max, which
    //is spread across rect.height.  Draw the line at the proper place.
-
-   if (zoomMin < 0 && zoomMax > 0) {
-      int half = (int)((zoomMax / (zoomMax - zoomMin)) * h);
+   if (zeroLevelYCoordinate >= rect.GetTop() &&
+       zeroLevelYCoordinate <= rect.GetBottom()) {
       dc.SetPen(*wxBLACK_PEN);
-      AColor::Line(dc, rect.x, rect.y + half, rect.x + rect.width, rect.y + half);
+      AColor::Line(dc, rect.x, zeroLevelYCoordinate,
+                   rect.x + rect.width, zeroLevelYCoordinate);
    }
 }
 
@@ -1788,6 +1789,7 @@ void TrackArtist::DrawClipWaveform(const WaveTrack *track,
       DrawWaveformBackground(dc, leftOffset, mid,
          env,
          zoomMin, zoomMax,
+         track->ZeroLevelYCoordinate(mid),
          dB, dBRange,
          t0, t1, zoomInfo, drawEnvelope,
          !track->GetSelected());

src/TrackArtist.h

@@ -142,6 +142,7 @@ class AUDACITY_DLL_API TrackArtist {
    void DrawWaveformBackground(wxDC & dc, int leftOffset, const wxRect &rect,
                                const double env[],
                                float zoomMin, float zoomMax,
+                               int zeroLevelYCoordinate,
                                bool dB, float dBRange,
                                double t0, double t1, const ZoomInfo &zoomInfo,
                                bool drawEnvelope, bool bIsSyncLockSelected);

src/TrackPanel.cpp

@@ -5861,14 +5861,26 @@ void TrackPanel::HandleWheelRotationInVRuler
                else
                   settings.NextHigherDBRange();
             }
-            const float extreme = (LINEAR_TO_DB(2) + newdBRange) / newdBRange;
-            max = std::min(extreme, max * olddBRange / newdBRange);
-            min = std::max(-extreme, min * olddBRange / newdBRange);
-            wt->SetLastdBRange();
-            wt->SetDisplayBounds(min, max);
-            if (partner) {
-               partner->SetLastdBRange();
-               partner->SetDisplayBounds(min, max);
+
+            // Is y coordinate within the rectangle half-height centered about
+            // the zero level?
+            const auto zeroLevel = wt->ZeroLevelYCoordinate(rect);
+            const bool fixedMagnification =
+               (4 * std::abs(event.GetY() - zeroLevel) < rect.GetHeight());
+
+            if (fixedMagnification) {
+               // Vary the db limit without changing
+               // magnification; that is, peaks and troughs move up and down
+               // rigidly, as parts of the wave near zero are exposed or hidden.
+               const float extreme = (LINEAR_TO_DB(2) + newdBRange) / newdBRange;
+               max = std::min(extreme, max * olddBRange / newdBRange);
+               min = std::max(-extreme, min * olddBRange / newdBRange);
+               wt->SetLastdBRange();
+               wt->SetDisplayBounds(min, max);
+               if (partner) {
+                  partner->SetLastdBRange();
+                  partner->SetDisplayBounds(min, max);
+               }
             }
          }
       }

src/WaveTrack.cpp

@@ -350,6 +350,12 @@ void WaveTrack::SetSpectrumBounds(float min, float max) const
    mSpectrumMax = max;
 }
 
+int WaveTrack::ZeroLevelYCoordinate(wxRect rect) const
+{
+   return rect.GetTop() +
+      (int)((mDisplayMax / (mDisplayMax - mDisplayMin)) * rect.height);
+}
+
 Track::Holder WaveTrack::Duplicate() const
 {
    return Track::Holder{ safenew WaveTrack{ *this } };

src/WaveTrack.h

@@ -550,6 +550,10 @@ class AUDACITY_DLL_API WaveTrack final : public Track {
    void GetSpectrumBounds(float *min, float *max) const;
    void SetSpectrumBounds(float min, float max) const;
 
+   // For display purposes, calculate the y coordinate where the midline of
+   // the wave should be drawn, if display minimum and maximum map to the
+   // bottom and top.  Maybe that is out of bounds.
+   int ZeroLevelYCoordinate(wxRect rect) const;
 
  protected:
    //