title: Allow click and dragging a view to drag the window itself?
body: <p>I'm using a textured window that has a tab bar along the top of it, just below the title bar.</p>
  
  <p>I've used <a href="https://developer.apple.com/documentation/appkit/nswindow/1419541-setcontentborderthickness?language=objc#" rel="noreferrer"><code>-setContentBorderThickness:forEdge:</code></a> on the window to make the gradient look right, and to make sure sheets slide out from the right position.</p>
  
  <p>What's not working however, is dragging the window around.  It works if I click and drag the area that's actually the title bar, but since the title bar gradient spills into a (potentially/often empty) tab bar, it's <em>really</em> easy to click too low and it feels really frustrating when you try to drag and realise the window is not moving.</p>
  
  <p>I notice <code>NSToolbar</code>, while occupying roughly the same amount of space below the title bar, allows the window to be dragged around when the cursor is over it.  How does one implement this?</p>
  
  <p>Thanks.</p>
  
  <p><img src="https://i.stack.imgur.com/GawvH.png" alt="Tab Bar"></p>