Flutter position stack widget in center

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,
      ),
    ),
  ]
)

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.