Flutter: line spacing affecting first line, problem with Positioned widget as a solution - flutter

Question

I need to modify the line spacing. Unfortunately, the height parameter of TextStyle affects the first line as well what you can see here:
class Screen extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.blue,
appBar: AppBar(backgroundColor: Colors.blue.withOpacity(0.5)),
body: Container(
color: Colors.yellow,
child: Container(
color: Colors.red,
child: Text(
'first line\nsecond line',
style: TextStyle(height: 2.5, fontSize: 20),
),
),
),
);
}
}
I immediately thought of a solution: Stack with Positioned widget, with negative top value. Let's do it in two steps, first embed it in Stack.
class Screen extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.blue,
appBar: AppBar(backgroundColor: Colors.blue.withOpacity(0.5)),
body: Container(
color: Colors.yellow,
child: Stack(
children: <Widget>[
Container(
color: Colors.red,
child: Text(
'first line\nsecond line',
style: TextStyle(height: 2.5, fontSize: 20),
),
),
],
),
),
);
}
}
Output looks exactly like before:
Second step: wrap it with Positioned widget.
class Screen extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: Colors.blue,
appBar: AppBar(backgroundColor: Colors.blue.withOpacity(0.5)),
body: Container(
color: Colors.yellow,
child: Stack(
// fit: StackFit.loose,
children: <Widget>[
Positioned( // .fill
top: -20,
// bottom: 0,
// left: 50,
// right: 0,
child: Container(
color: Colors.red,
child: Text(
'first line\nsecond line',
style: TextStyle(height: 2.5, fontSize: 20),
),
),
),
],
),
),
);
}
}
This is almost what I wanted. However, if you pay attention to the colors, what was before blue, is now yellow. Adding Positioned widget makes Stack to be expanded rather than fit the content - that prevents me from using it in more complex layouts - e.g. ListView. Flutter outputs warning RenderStack object was given an infinite size during layout.
I noticed that Positioned widget does it as soon as you add any of top/left/bottom/right - before it fits the content. I tried to experiment with Stack's fit property, of Positioned with .fill initializer, but I didn't succeed.