What's the big green right arrow (-->) mean in DevTools with Flutter?

What's the big green right arrow (-->) mean in DevTools with Flutter?

I guess it's related to the interactable area, but I'm not sure.

The code is the same as in Flutter, Page two can't click when it's on Page 1. Here is the simplified code:

PageController _controller = PageController(initialPage: 0, viewportFraction: 0.5);

@override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.yellow,
      child: PageView(
        controller: _controller,
        children: <Widget>[
          Center(
            child: FlatButton(
              onPressed: () {},
              color: Colors.red,
              child: Text('First Tab'),
            ),
          ),
          Center(
            child: FlatButton(
              onPressed: () {},
              color: Colors.blue,
              child: Text('Second Tab'),
            ),
          ),
          Center(
            child: FlatButton(
              onPressed: () {},
              color: Colors.green,
              child: Text('Third Tab'),
            ),
          ),
        ],
      ),
    );
}