Flutter v1.22 optimization bug: a widget rebuild does not happen

main.dart

import 'dart:async';

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  Timer _timer;
  Color _backgroundColor = Colors.blue;

  @override
  void initState() {
    super.initState();
    print('set timer');
    _timer = Timer(
        const Duration(seconds: 1),
        () => setState(() {
              print('set new color');
              _backgroundColor = Colors.red;
            }));
  }

  @override
  void dispose() {
    _timer?.cancel();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    print('backgroundColor: $_backgroundColor');
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Column(
        children: [
          Expanded(
            child: CustomScrollView(
              slivers: [
                SliverPersistentHeader(
                  pinned: true,
                  delegate: HeaderDelegate(
                    // widget #1: this color should be changed but not.
                    child: Container(color: _backgroundColor),
                  ),
                ),
                const SliverPadding(padding: EdgeInsets.only(top: 2)),
                SliverToBoxAdapter(
                  // widget #2: this color should be changed and it will be.
                  child: Container(height: 100, color: _backgroundColor),
                ),
                SliverToBoxAdapter(
                  // widget #2: this color should be changed and it will be.
                  child: Container(
                    padding: const EdgeInsets.symmetric(
                      horizontal: 40,
                      vertical: 40,
                    ),
                    child: const Text('both of two widgets above are programmed'
                        ' to be turned to red after a second.'),
                  ),
                ),
              ],
            ),
          )
        ],
      ),
    );
  }
}

class HeaderDelegate extends SliverPersistentHeaderDelegate {
  HeaderDelegate({this.child});

  Widget child;

  @override
  double get minExtent => 100;

  @override
  double get maxExtent => 100;

  @override
  bool shouldRebuild(covariant SliverPersistentHeaderDelegate oldDelegate) =>
      false;

  @override
  Widget build(
          BuildContext context, double shrinkOffset, bool overlapsContent) =>
      SizedBox(height: 100, child: child);
}