title: Flutter Stack doesn't make children overlap
body: <p>I'm trying to put a horizontal line(currently <code>Divider</code> but could be changed as a <code>CustomPaint</code>) as a background of a <code>Row</code>. However, regardless of the order of children, the line always goes front of the <code>Row</code>. I can't figure out what's wrong. Here's the code.</p>
  
  <pre><code>Stack(
    children: &lt;Widget&gt;[
      Positioned.fill(
        child: const Divider(color: Colors.black),
      ),
      Row(
        mainAxisAlignment: MainAxisAlignment.spaceBetween,
        children: &lt;Widget&gt;[
          Ink(
            decoration: ShapeDecoration(
              color: Colors.grey[350],
              shape: CircleBorder(),
            ),
            child: Padding(
              padding: const EdgeInsets.all(4),
              child: Icon(Icons.arrow_forward, size: 32)
            ),
          ),
          Ink(
            decoration: ShapeDecoration(
              color: Colors.grey[350],
              shape: CircleBorder(),
            ),
            child: Padding(
              padding: const EdgeInsets.all(4),
              child: Icon(Icons.location_on, size: 32)
            ),
          ),
          Ink(
            decoration: ShapeDecoration(
              color: Colors.grey[350],
              shape: CircleBorder(),
            ),
            child: Padding(
              padding: const EdgeInsets.all(4),
              child: Icon(Icons.flag, size: 32)
            ),
          ),
        ],
      ),
    ],
  ),
  </code></pre>
  
  <p>and here's the result image.</p>
  
  <p><a href="https://i.stack.imgur.com/4gKJq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4gKJq.png" alt=""></a></p>