Issue: Changing one tab triggers the build method of other tabs.

I notice that changing 1 tab will trigger the build method of other tabs which is very bad for performance I think. For example, In `ColorsListPage` class, I added the log line in the build method, and when I change the tab I can see the build method of other tabs are called:
```dart
@override
Widget build(BuildContext context) {
log('Rebuild: $title.');
return Scaffold(
appBar: AppBar(
title: Text(
title,
),
backgroundColor: color,
),
body: Container(
color: Colors.white,
child: _buildList(),
));
}
```

Start with the ColorsListPage build method shown in the issue and reproduce the behavior by changing tabs while observing its log output. Trace the tab-change and nested-navigation entry points to identify why other tab pages rebuild; done means the unnecessary rebuilds no longer occur and the observed behavior is covered by an appropriate verification.

Written by the indexing model from the issue text.