gistfile1.txt

// See https://github.com/flutter/flutter/issues/10531

/*
In release mode the app is often asked to layout before the engine
has initialized to the point where the size of the screen is known.
In that case the app is built and laid out with 0x0 constraints and
then later with the screen's actual size.

The phantom tabbar scrolling seen here upon app startup is due to the
scrollable tabbar's scroll offset being "corrected" once the tab bar's
size is constrained to a non-zero value. The correction happens
in ScrollPosition.applyNewDimensions() which calls the IdleScrollActivity
with the same name, which ballistic-scrolls the scroll offset back to 0.

The phantom scroll can be worked around but not building anything
until the app's size is non-zero.
*/

import 'package:flutter/material.dart';

class AppBarBug extends StatefulWidget {
  @override
  _AppBarBugState createState() => new _AppBarBugState();
}

class _AppBarBugState extends State<AppBarBug> with SingleTickerProviderStateMixin {
  final List<Widget> tabs = <Widget>[
    new Tab(text: 'NEW MEXICO'),
    new Tab(text: 'GABBA'),
    new Tab(text: 'HEY'),
  ];

  TabController _tabController;

  @override
  void initState() {
    super.initState();
    _tabController = new TabController(vsync: this, length: tabs.length);
  }

  @override
  void dispose() {
    _tabController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return new LayoutBuilder(
      builder: (_, BoxConstraints constraints) {
        // In release mode, layout may begin before the engine has initialized
        // to the point where the size of the screen is known. We'll be built
        // again, when the incoming constraints are non-zero.
        if (constraints.biggest == Size.zero)
          return new Container();
        return new Scaffold(
          appBar: new AppBar(
            title: new Text('AppBarBug'),
            bottom: new PreferredSize(
              preferredSize: const Size.fromHeight(30.0),
              child: new Padding(
                padding: const EdgeInsets.symmetric(horizontal: 15.0),
                child: new Align(
                  alignment: FractionalOffset.center,
                  child: new LayoutBuilder(
                    builder: (BuildContext _, BoxConstraints constraints) {
                      return new TabBar(
                        controller: _tabController,
                        isScrollable: true,
                        tabs: tabs,
                      );
                    },
                  ),
                ),
              ),
            ),
          ),
          body: new Center(
            child: new Text('Hello World'),
          ),
        );
      },
    );
  }
}

void main() {
  runApp(new MaterialApp(home: new AppBarBug()));
}