title: Finder-Style floating group rows in view-based NSOutlineView
body: <p>I have implemented a view-based NSOutlineView in my project. I am using floating group rows. Now, I would like to have this NSOutlineView look basically like the Finder list-view (CMD-2) when it is in the "arranged-by" layout (e.g. "by kind": CTRL-CMD-2). That means, the top-most group row should display the column titles and as soon as the next lower group row is starting to nudge the previous one out of the view, the column titles fade in on the second group row (I hope this makes sense).</p>
  
  <p>Is there any out-of-the-box way to achieve this? So far I have successfully subclassed NSTableCellView to show the columns' titles, however, I cannot get the fade-in to work as I cannot seem to find out the position of the group row in relation to the floating one above it.</p>
  
  <p>Regards,
  Michael</p>