title: constrain only top
body: <p>With no constraint I have the following issue:  The user is disabled from dragging if they drag the header above the top of the window.</p>
  
  <p>Like this:</p>
  
  <pre><code>  ________
    |header|
  __|______|_____
  | |      |    |
  | |______|    |
  |             |
  |             |
  |_____________|
  </code></pre>
  
  <p>constrain:true is too limiting for my uses as drag-ability off the sides and bottom is required.  This applies to constrained as well because the window will not be able to be dragged off the sides.</p>
  
  <p>Is there a way to prevent dragging off the top of the page only?</p>