How Flutter adds a top-level view (similar to iOS adding to a window)

Add this to your MaterialApp to remove it

MaterialApp(
  debugShowCheckedModeBanner: false,
)

You can use Banner widget that actually displays debug banner.

docs link : https://api.flutter.dev/flutter/widgets/Banner-class.html

 Banner(
          location: BannerLocation.topEnd,
          message: "",
          child: Icon(Icons.help), //you can pass anything here as widget
          color: Colors.transparent,
        ),    

You can use Stack Widget to overlay Widgets over one another. For example:

Stack(
  children: <Widget>[
    Container(
      width: 100,
      height: 100,
      color: Colors.red,
    ),
    Container(
      width: 90,
      height: 90,
      color: Colors.green,
    ),
    Container(
      width: 80,
      height: 80,
      color: Colors.blue,
    ),
  ],
)

The Banner widget realises this by creating a CustomPainter, take a look at the Implementation. But I think a Stack would be the better choice here if you are using Widgets.

Have a look at this. Flutter provides a default Banner widget.It is somewhat similar to the debug banner what we are used to of seeing on the top-right corner on a flutter app in debug mode.

https://api.flutter.dev/flutter/widgets/Banner-class.html