title: Hover effect issue in NSTableView
body: <p>I have a view based NSTableView in my application and I have added the hovering effect for the rows using the implementation in HoverTableDemo. I can get the hovering effect when the mouse is over a row without a problem , however when the tableview is scrolled down sometimes it leaves highlighting effect on the top cells when the table view is scrolled up again. I think this is due to NSTableView reuses the cell.  I highlight the row by overriding the </p>
  
  <pre><code> - (void)drawBackgroundInRect:(NSRect)dirtyRect
  </code></pre>
  
  <p>method in NSTableRowView. Could someone point me to the right direction to solve this issue ? </p>
  
  <p>Thanks in advance .</p>