title: Expand widgets inside the Stack widget
body: <p>I have a stack widgets with two widgets inside.
  One of them draws the background, the other one draws on top of that.
  I want both widgets to have the same size.</p>
  <p>I want the top widget, which is a column, to expand and fill the stack vertically to the size that is set by the background widget.
  I tried setting the MainAxis <code>mainAxisSize: MainAxisSize.max</code> but it didn't work.</p>
  <p>How can I make it work?</p>