title: Flutter Center align widget to specific coordinate
body: <p>Ive create a popup menu widget which takes two arguments :a button widget, and a menu widget . When the button is pressed the menu widget is passed to an OverlayEntry as the child which is then added to the Overlay.of(context)</p>
  
  <p>I'd like the resulting display to be as such:</p>
  
  <pre><code>     O   (Button)
   ____|____
   | MENU  |
   |_______|
  </code></pre>
  
  <p>I can place the menu widget inside a Positioned Widget to move it freely around the Overlay, but I dont know how to get the correct coordinates. I can get the center of the Button by finding it's Renderbox, but I cant know the size, and therefore the correct position, of the Menu until after it's draw (as it can be any widget).</p>
  
  <p>Is there some soft of layout option to tell a Widget of any size to vertically or horzontally align itself with a given coordinate?</p>