title: NSOutlineView - How to remove the expand/collapse toggle
body: <p>I have a NSOutlive view that I would like to remove the toggle from some rows. The Zones and Assets rows are group items (header rows) and should not be collapsable. The problem is that is I answer <code>NO</code> to <code>- (BOOL)outlineView:(NSOutlineView *)outlineView isItemExpandable:(id)item</code> for these items, the toggles do go away, but then I can't programmatically expand then either.</p>
  
  <p>Is there any way to remove the toggle from specific rows?</p>
  
  <p><img src="https://i.stack.imgur.com/Ub0sN.png" alt="Example"></p>