Fix border drawing glitch in wxGrid column header

See #18980

src/generic/grid.cpp

@@ -6901,8 +6901,11 @@ void wxGrid::DrawColLabel(wxDC& dc, int col)
     {
         // It is reported that we need to erase the background to avoid display
         // artefacts, see #12055.
+        {
             wxDCBrushChanger setBrush(dc, m_colLabelWin->GetBackgroundColour());
+            wxDCPenChanger setPen(dc, *wxTRANSPARENT_PEN);
             dc.DrawRectangle(rect);
+        }
 
         rend.DrawBorder(*this, dc, rect);
     }