Use worksheet bounding box when the board is empty

Fixes: lp:1742140
* https://bugs.launchpad.net/kicad/+bug/1742140

common/tool/common_tools.cpp

@@ -114,21 +114,15 @@ int COMMON_TOOLS::ZoomFitScreen( const TOOL_EVENT& aEvent )
 
     if( bBox.GetWidth() == 0 || bBox.GetHeight() == 0 )
     {
-        // Empty view
-        view->SetScale( 17.0 );     // works fine for the standard worksheet frame
-
-        view->SetCenter( screenSize / 2.0 );
-    }
-    else
-    {
-        VECTOR2D vsize = bBox.GetSize();
-        double scale = view->GetScale() / std::max( fabs( vsize.x / screenSize.x ),
-                                                    fabs( vsize.y / screenSize.y ) );
-
-        view->SetScale( scale );
-        view->SetCenter( bBox.Centre() );
+        bBox = galCanvas->GetDefaultViewBBox();
     }
 
+    VECTOR2D vsize = bBox.GetSize();
+    double scale = view->GetScale() / std::max( fabs( vsize.x / screenSize.x ),
+                                                fabs( vsize.y / screenSize.y ) );
+
+    view->SetScale( scale );
+    view->SetCenter( bBox.Centre() );
 
     // Take scrollbars into account
     VECTOR2D worldScrollbarSize = view->ToWorld( scrollbarSize, false );

include/class_draw_panel_gal.h

@@ -33,6 +33,7 @@
 
 #include <wx/window.h>
 #include <wx/timer.h>
+#include <math/box2.h>
 #include <math/vector2d.h>
 #include <msgpanel.h>
 #include <memory>
@@ -224,6 +225,17 @@ public:
      */
     int GetCurrentCursor() const { return m_currentCursor; }
 
+    /**
+     * Returns the bounding box of the view that should be used if model is not valid
+     * For example, the worksheet bounding box for an empty PCB
+     *
+     * @return the default bounding box for the panel
+     */
+    virtual BOX2I GetDefaultViewBBox() const
+    {
+        return BOX2I();
+    }
+
 
 protected:
     void onPaint( wxPaintEvent& WXUNUSED( aEvent ) );

pcbnew/pcb_draw_panel_gal.cpp

@@ -395,6 +395,15 @@ void PCB_DRAW_PANEL_GAL::RedrawRatsnest()
 }
 
 
+BOX2I PCB_DRAW_PANEL_GAL::GetDefaultViewBBox() const
+{
+    if( m_worksheet )
+        return m_worksheet->ViewBBox();
+
+    return BOX2I();
+}
+
+
 void PCB_DRAW_PANEL_GAL::setDefaultLayerDeps()
 {
     // caching makes no sense for Cairo and other software renderers

pcbnew/pcb_draw_panel_gal.h

@@ -103,6 +103,9 @@ public:
     ///> Forces refresh of the ratsnest visual representation
     void RedrawRatsnest();
 
+    ///> @copydoc EDA_DRAW_PANEL_GAL::GetDefaultViewBBox()
+    BOX2I GetDefaultViewBBox() const override;
+
 protected:
 
     KIGFX::PCB_VIEW* view() const;