title: VirtualTreeview enabling and disabling columns
body: <p>I have a VirtualTreeView which has some columns initially hidden (<code>coVisible</code> is not present).</p>
  
  <p>After enabling them (adding <code>coVisible</code> to the column Options) a strange thing happens - column appears but overlaps with previous column. I can fix the problem by changing width to 1 pixel smaller, then back to original width of the column which forces some repaint which then displays column correctly.</p>
  
  <p>Is there something I need to do additionally except adding <code>coVisible</code> to make the columns repaint properly?</p>