Flutter position stack widget in center

You can use the Positioned.fill with Align inside a Stack:

Stack(
  children: <Widget>[      
    Positioned.fill(
      child: Align(
        alignment: Alignment.centerRight,
        child: ....                
      ),
    ),
  ],
),

Probably the most elegant way.

You can simply use the alignment option present in Stack

child: Stack(
  alignment: Alignment.center
)

Remove everything, but this:

Align(
  alignment: Alignment.bottomCenter,
  child: new ButtonBar(
    alignment: MainAxisAlignment.center,
    children: <Widget>[
      new OutlineButton(
        onPressed: () {
          Navigator.push(
            context,
            new MaterialPageRoute(
              builder: (context) => new LoginPage()));
        },
        child: new Text(
          "Login",
          style: new TextStyle(color: Colors.white),
        ),
      ),
      new RaisedButton(
        color: Colors.white,
        onPressed: () {
          Navigator.push(
            context,
            new MaterialPageRoute(
              builder: (context) =>
                new RegistrationPage()));
        },
        child: new Text(
          "Register",
          style: new TextStyle(color: Colors.black),
        ),
      )
    ],
  ),
)

In my theory, the additional Container is destroying it. I would advise you to just surround this by adding Padding:

Padding(
  padding: EdgeInsets.only(bottom: 20.0),
  child: Align...
),

This seems a lot more reasonable to me than the Positioned and also I do not quite understand your Column with a single child only.

For anyone who is reaching here and is not able to solve their issue, I used to make my widget horizontally center by setting both right and left to 0 like below:

Stack(
  children: <Widget>[
    Positioned(
      top: 100,
      left: 0,
      right: 0,
      child: Text("Search",
        style: TextStyle(
          color: Color(0xff757575),
          fontWeight: FontWeight.w700,
          fontFamily: "Roboto",
          fontStyle: FontStyle.normal,
          fontSize: 56.0,
        ),
        textAlign: TextAlign.center,
      ),
    ),
  ]
)

A Stack allows you to stack elements on top of each other, with the last element in the array taking the highest priority. You can use Align, Positioned, or Container to position the children of a stack.

Align

Widgets are moved by setting the alignment with Alignment, which has static properties like topCenter, bottomRight, and so on. Or you can take full control and set Alignment(1.0, -1.0), which takes x,y values ranging from 1.0 to -1.0, with (0,0) being the center of the screen.

  Stack(
      children: [
        Align(
          alignment: Alignment.topCenter,
          child: Container(
              height: 80,
              width: 80, color: Colors.blueAccent
          ),
        ),
        Align(
          alignment: Alignment.center,
          child: Container(
              height: 80,
              width: 80, color: Colors.deepPurple
          ),
        ),
        Container(
          alignment: Alignment.bottomCenter,
          // alignment: Alignment(1.0, -1.0),
          child: Container(
              height: 80,
              width: 80, color: Colors.amber
          ),
        )
      ]
  )