Flutter Layout: How can I put a Row inside a Flex (or another Row) in Flutter? Also using Stack widget

Wrap BOTH your Container and TextFormField inside a Flexible widget.

Widget boxText = new Flexible(child: new Container(
      decoration: new BoxDecoration(
        border: new Border.all(
          color: Colors.cyan[100],
          width: 3.0,
          style: BorderStyle.solid,
        ),
      ),
      margin: new EdgeInsets.all(5.0),
      padding: new EdgeInsets.all(8.0),
      child: new Row(
        crossAxisAlignment: CrossAxisAlignment.start,
        children: <Widget>[

          new Text(
            'Text',
            style: null,
          ),
          new Text(
            'Text',
            style: null,
          ),
          new Flexible (child: new TextFormField(
            decoration: const InputDecoration(
              hintText: '',
              labelText: 'label',
            ),
            obscureText: true,
          ),),
        ],
      ),
    ));

The problem is that your Container is relying on its parent to decide its width. Since it has no parent, it is infinitely wide.

To fix the problem, give it a parent who tells its child how to handle width. A Flexible or Expanded does that. Expanded tells its child to be as big as possible (to expand) and a Flexible tells its child to be as small as possible (to shrink).

It sounds to me like an Expanded would be your best bet:

Change your code:

// Composed Widget
Widget boxText = new Container(
  decoration: new BoxDecoration(
  // all your other code here
);

to

// Composed Widget
Widget boxText = Expanded(   // <-- Add this Expanded widget
  child: new Container(
    decoration: new BoxDecoration(
    // all your other code here
  ),
);