title: Position widgets in stack without overlapping
body: <p>I have a login screen with a logo on top. I want the login form to be exactly in the screen center (not in the center of the space below the logo). So far I managed to achieve that using <code>Stack</code>, roughly like this:</p>
  
  <pre><code>Stack(
     children: [
        Positioned(
          child: Logo(),
          left: 0,
          top: 0,
        ),
        Column(
            children: [LoginForm()],
            mainAxisAlignment: MainAxisAlignment.center,
        ),
     ],
  )
  </code></pre>
  
  <p>In general, I am quite happy with how it looks, however, when the soft keyboard pops up, the visible screen size and the notion of the center change, the form moves up and overlaps with the logo. I know I can prevent widgets from resizing at all, but can I just allow the form move up only as long as it does not overlap with the logo?  </p>