[Reland] Fix Tab linear and elastic animation blink (#162315) (#162450)
Relands https://github.com/flutter/flutter/pull/162315
Removed animated sheet golden tests as they're not consistent for long
animation tests. Rewritten the tests to be more precise using mock
canvas checks.
---
Fixes
[https://github.com/flutter/flutter/issues/162098](https://github.com/flutter/flutter/issues/162098)
### Description
This PR fixes `Tab` linear and elastic animation blinks/flickers when
skipping multiple tabs. Previous attempt to fix elastic animation didn't
cover linear animation tests and didn't have enough number of tab items
which this PR fixes.
- Fixed Linear and elastic animation blink issue.
- Added tests for linear and elastic animation with various tab sizes
(LTR and RTL)
- Added tests for linear and elastic animation when skipping tabs (LTR
and RTL)
### Code Sample
<details>
<summary>expand to view the code sample</summary>
```dart
import 'package:flutter/material.dart';
// import 'package:flutter/scheduler.dart';
void main() {
// timeDilation = 10;
runApp(const TabBarDemo());
}
class TabBarDemo extends StatelessWidget {
const TabBarDemo({super.key});
@override
Widget build(BuildContext context) {
final List<Widget> tabs = <Widget>[
const Tab(text: 'Short'),
const Tab(text: 'A Bit Longer Text'),
const Tab(text: 'An Extremely Long Tab Label That Overflows'),
const Tab(text: 'Tiny'),
const Tab(text: 'Moderate Length'),
const Tab(text: 'Just Right'),
const Tab(text: 'Supercalifragilisticexpialidocious'),
const Tab(text: 'Longer Than Usual'),
];
return MaterialApp(
home: DefaultTabController(
length: tabs.length,
child: Scaffold(
appBar: AppBar(
bottom: TabBar(
tabAlignment: TabAlignment.start,
isScrollable: true,
indicatorAnimation: TabIndicatorAnimation.elastic,
tabs: tabs,
),
title: const Text('Tabs Demo'),
),
body: TabBarView(
children: <Widget>[
for (int i = 0; i < tabs.length; i++) const Icon(Icons.directions_car),
],
),
),
),
);
}
}
```
</details>
### Before
https://github.com/user-attachments/assets/5c271948-5a01-4520-90a3-921c20c79470
### After
https://github.com/user-attachments/assets/6af32d43-3588-488f-ba50-be59323ed692
### Linear animation before (left) and After (right) comparison.
<img width="1048" alt="Screenshot 2025-01-28 at 17 27 50"
src="https://github.com/user-attachments/assets/4ba587a5-24d0-40ce-817c-366d004abc05"
/>
## Pre-launch Checklist
- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
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responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.
If you need help, consider asking for advice on the #hackers-new channel
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