title: Why does a newly added widget have no allocation?
body: <p>I want to add a widget to the bottom of a scrolled window, then scroll to the bottom of that window. However, the window thinks it is already at the bottom, because the widget has still not been allocated. I.e., this returns -1:</p>
  
  <pre><code>widget.get_allocation().y
  </code></pre>
  
  <p>Why is this? Is there any way to force the widget to be allocated immediately, so I can adjust the window accordingly? Or is there a better solution?</p>