Choose half-grid for mirror

Using the half-grid allows for odd-grid counts to remain on-grid,
closer to the actual center.

Fixes https://gitlab.com/kicad/code/kicad/issues/7493

common/eda_draw_frame.cpp

@@ -744,6 +744,20 @@ wxPoint EDA_DRAW_FRAME::GetNearestGridPosition( const wxPoint& aPosition ) const
 }
 
 
+wxPoint EDA_DRAW_FRAME::GetNearestHalfGridPosition( const wxPoint& aPosition ) const
+{
+    const wxPoint& gridOrigin = GetGridOrigin();
+    VECTOR2D       gridSize = GetCanvas()->GetGAL()->GetGridSize() / 2.0;
+
+    double xOffset = fmod( gridOrigin.x, gridSize.x );
+    int    x = KiROUND( (aPosition.x - xOffset) / gridSize.x );
+    double yOffset = fmod( gridOrigin.y, gridSize.y );
+    int    y = KiROUND( (aPosition.y - yOffset) / gridSize.y );
+
+    return wxPoint( KiROUND( x * gridSize.x + xOffset ), KiROUND( y * gridSize.y + yOffset ) );
+}
+
+
 const BOX2I EDA_DRAW_FRAME::GetDocumentExtents( bool aIncludeAllVisible ) const
 {
     return BOX2I();

eeschema/tools/sch_edit_tool.cpp

@@ -699,7 +699,7 @@ int SCH_EDIT_TOOL::Mirror( const TOOL_EVENT& aEvent )
 
         case SCH_SHEET_T:
             // Mirror the sheet on itself. Sheets do not have a anchor point.
-            mirrorPoint = m_frame->GetNearestGridPosition( item->GetBoundingBox().Centre() );
+            mirrorPoint = m_frame->GetNearestHalfGridPosition( item->GetBoundingBox().Centre() );
 
             if( xAxis )
                 item->MirrorX( mirrorPoint.y );
@@ -717,7 +717,7 @@ int SCH_EDIT_TOOL::Mirror( const TOOL_EVENT& aEvent )
     }
     else if( selection.GetSize() > 1 )
     {
-        mirrorPoint = m_frame->GetNearestGridPosition( (wxPoint)selection.GetCenter() );
+        mirrorPoint = m_frame->GetNearestHalfGridPosition( (wxPoint)selection.GetCenter() );
 
         for( unsigned ii = 0; ii < selection.GetSize(); ii++ )
         {

eeschema/tools/symbol_editor_edit_tool.cpp

@@ -206,7 +206,7 @@ int SYMBOL_EDITOR_EDIT_TOOL::Mirror( const TOOL_EVENT& aEvent )
     if( selection.GetSize() == 1 )
         mirrorPoint = item->GetPosition();
     else
-        mirrorPoint = m_frame->GetNearestGridPosition( mapCoords( selection.GetCenter() ) );
+        mirrorPoint = m_frame->GetNearestHalfGridPosition( mapCoords( selection.GetCenter() ) );
 
     for( unsigned ii = 0; ii < selection.GetSize(); ii++ )
     {

include/eda_draw_frame.h

@@ -135,6 +135,17 @@ public:
      */
     wxPoint GetNearestGridPosition( const wxPoint& aPosition ) const;
 
+    /**
+     * Return the nearest \a aGridSize / 2 location to \a aPosition.
+     *
+     * This is useful when attempting for keep outer points on grid but
+     * not the middle point.
+     *
+     * @param aPosition The position to check.
+     * @return The nearest half-grid position.
+     */
+    wxPoint GetNearestHalfGridPosition( const wxPoint& aPosition ) const;
+
     /**
      * Return a reference to the default ORIGIN_TRANSFORMS object
      */