Move drawing code for background below tracks

src/TrackArtist.h

@@ -70,6 +70,7 @@ public:
       PassBorders,
       PassControls,
       PassZooming,
+      PassBackground,
       
       NPasses
    };

src/TrackPanel.cpp

@@ -973,16 +973,6 @@ void TrackPanel::DrawEverythingElse(TrackPanelDrawingContext &context,
 
    auto target = Target();
 
-   // Paint over the part below the tracks
-   trackRect.y += trackRect.height;
-   if (trackRect.y < clip.GetBottom()) {
-      AColor::TrackPanelBackground(dc, false);
-      dc->DrawRectangle(trackRect.x,
-                        trackRect.y,
-                        trackRect.width,
-                        clip.height - trackRect.y);
-   }
-
    // Sometimes highlight is not drawn on backing bitmap. I thought
    // it was because FindFocus did not return "this" on Mac, but
    // when I removed that test, yielding this condition:

src/tracks/ui/BackgroundCell.cpp

@@ -11,14 +11,18 @@ Paul Licameli split from TrackPanel.cpp
 #include "../../Audacity.h"
 #include "BackgroundCell.h"
 
+#include "../../AColor.h"
 #include "../../HitTestResult.h"
 #include "../../Project.h"
 #include "../../RefreshCode.h"
 #include "../../SelectionState.h"
 #include "../../Track.h"
+#include "../../TrackArtist.h"
 #include "../../TrackPanel.h"
+#include "../../TrackPanelDrawingContext.h"
 #include "../../TrackPanelMouseEvent.h"
 #include "../../UIHandle.h"
+#include "../../ViewInfo.h"
 
 #include <wx/cursor.h>
 #include <wx/event.h>
@@ -117,3 +121,30 @@ std::shared_ptr<Track> BackgroundCell::DoFindTrack()
    return {};
 }
 
+void BackgroundCell::Draw(
+   TrackPanelDrawingContext &context,
+   const wxRect &rect, unsigned iPass )
+{
+   if ( iPass == TrackArtist::PassBackground ) {
+      auto &dc = context.dc;
+      // Paint over the part below the tracks
+      AColor::TrackPanelBackground( &dc, false );
+      dc.DrawRectangle( rect );
+   }
+}
+
+wxRect BackgroundCell::DrawingArea(
+   const wxRect &rect, const wxRect &, unsigned iPass )
+{
+   if ( iPass == TrackArtist::PassBackground )
+      // If there are any tracks, extend the drawing area up, to cover the
+      // bottom ends of any zooming guide lines.
+      return {
+         rect.x,
+         rect.y - kTopMargin,
+         rect.width,
+         rect.height + kTopMargin
+      };
+   else
+      return rect;
+}

src/tracks/ui/BackgroundCell.h

@@ -44,6 +44,14 @@ protected:
    std::shared_ptr<Track> DoFindTrack() override;
 
 private:
+   // TrackPanelDrawable implementation
+   void Draw(
+      TrackPanelDrawingContext &context,
+      const wxRect &rect, unsigned iPass ) override;
+
+   wxRect DrawingArea(
+      const wxRect &rect, const wxRect &panelRect, unsigned iPass ) override;
+   
    AudacityProject *mpProject;
 
    std::weak_ptr<BackgroundHandle> mHandle;