An improved IndexStack, suppress flickering on switching to another widget.

optimized_indexed_stack.dart

import 'package:flutter/material.dart';
import 'package:flutter_hooks/flutter_hooks.dart';

class OptimizedIndexedStack extends HookWidget {
  const OptimizedIndexedStack({
    Key key,
    @required this.pages,
    this.index = 0,
  }) : super(key: key);

  final List<Widget> pages;
  final int index;

  @override
  Widget build(BuildContext context) {
    // Suppress flickering.
    // When switching to another:
    // 1. Show both of widgets.
    // 2. At the next frame, hide the old one.
    final prevIndex = useState(index);
    if (prevIndex.value != index) {
      Future<void>.delayed(Duration.zero).then((_) => prevIndex.value = index);
    }

    return Stack(
      children: List.generate(pages.length, (i) {
        final page = pages[i];
        return Visibility(
          visible: i == index || i == prevIndex.value,
          maintainState: true,
          child: page,
        );
      }).toList(),
    );
  }
}