title: Document view detect enclosing NSScrollView resize
body: <p>I have a custom view as the document view of an <code>NSScrollView</code>.</p>
  
  <p>The document view needs to know how wide the scroll view is, to determine its own height and adjust drawing (wrapped content based on the width).</p>
  
  <p>How can the document view watch for changes to the width of the enclosing scroll view?</p>