title: Why do NSOutlineView cells overlap group cells when floatsGroupRows is Enabled
body: <p>I'm creating a new app on Yosemite with an NSOutlineView created via the "Source List" item in the Object Library. By default, <code>floatsGroupRows</code> is enabled. I have top-level items as group items, and others as normal cell items. Which is great, except that, when I scroll up, the regular items don't disappear under the group items, but overlap with them:</p>
  
  <p><img src="https://i.stack.imgur.com/NLFgu.png" alt="NSOutlineView Overlap"></p>
  
  <p>Here I would expect the "Foo" item to be half invisible, but it's not, it's just flowing over (under?) the "THINGS" group item. Is there any way to fix this, other than disabling <code>floatsGroupRows</code>?</p>