commit 1c02c2009de772ce140d8a3d80bd67feddcef213
author Taha Tesser <tessertaha@gmail.com> Wed Jan 29 14:40:21 2025 +0200
committer dart-internal-monorepo <dart-internal-monorepo@dart-ci-internal.iam.gserviceaccount.com> Wed Jan 29 05:22:25 2025 -0800
tree 09bae67926ca151d006fc488fc4bdec08a6e6bca
parent f255d03bb72673b13c181ec590faa161980f9cde

Fix `Tab` linear and elastic animation blink (#162315)

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.
- [x] I read the [Tree Hygiene] wiki page, which explains my
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
on [Discord].

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[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
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[Features we expect every widget to implement]:
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[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md
https://dart.googlesource.com/external/github.com/flutter/flutter/+/b007899d3a7a072e880e3984856c777cc9c5082b