Fix Tab linear and elastic animation blink (flutter#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].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[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

packages/flutter/lib/src/material/tabs.dart

@@ -586,27 +586,10 @@ class _IndicatorPainter extends CustomPainter {
     _painter ??= indicator.createBoxPainter(markNeedsPaint);
 
     final double value = controller.animation!.value;
-    final int to =
-        controller.indexIsChanging
-            ? controller.index
-            : switch (textDirection) {
-              TextDirection.ltr => value.ceil(),
-              TextDirection.rtl => value.floor(),
-            }.clamp(0, maxTabIndex);
-    final int from =
-        controller.indexIsChanging
-            ? controller.previousIndex
-            : switch (textDirection) {
-              TextDirection.ltr => (to - 1),
-              TextDirection.rtl => (to + 1),
-            }.clamp(0, maxTabIndex);
-    final Rect toRect = indicatorRect(size, to);
-    final Rect fromRect = indicatorRect(size, from);
-    _currentRect = Rect.lerp(fromRect, toRect, (value - from).abs());
 
     _currentRect = switch (indicatorAnimation) {
-      TabIndicatorAnimation.linear => _currentRect,
-      TabIndicatorAnimation.elastic => _applyElasticEffect(fromRect, toRect, _currentRect!),
+      TabIndicatorAnimation.linear => _applyLinearEffect(size: size, value: value),
+      TabIndicatorAnimation.elastic => _applyElasticEffect(size: size, value: value),
     };
 
     assert(_currentRect != null);
@@ -628,6 +611,17 @@ class _IndicatorPainter extends CustomPainter {
     _painter!.paint(canvas, _currentRect!.topLeft, configuration);
   }
 
+  /// Applies the linear effect to the indicator.
+  Rect? _applyLinearEffect({required Size size, required double value}) {
+    final double index = controller.index.toDouble();
+    final bool ltr = index > value;
+    final int from = (ltr ? value.floor() : value.ceil()).clamp(0, maxTabIndex);
+    final int to = (ltr ? from + 1 : from - 1).clamp(0, maxTabIndex);
+    final Rect fromRect = indicatorRect(size, from);
+    final Rect toRect = indicatorRect(size, to);
+    return Rect.lerp(fromRect, toRect, (value - from).abs());
+  }
+
   // Ease out sine (decelerating).
   double decelerateInterpolation(double fraction) {
     return math.sin((fraction * math.pi) / 2.0);
@@ -639,20 +633,38 @@ class _IndicatorPainter extends CustomPainter {
   }
 
   /// Applies the elastic effect to the indicator.
-  Rect _applyElasticEffect(Rect fromRect, Rect toRect, Rect currentRect) {
+  Rect? _applyElasticEffect({required Size size, required double value}) {
+    final double index = controller.index.toDouble();
+    double progressLeft = (index - value).abs();
+
+    final int to =
+        progressLeft == 0.0 || !controller.indexIsChanging
+            ? switch (textDirection) {
+              TextDirection.ltr => value.ceil(),
+              TextDirection.rtl => value.floor(),
+            }.clamp(0, maxTabIndex)
+            : controller.index;
+    final int from =
+        progressLeft == 0.0 || !controller.indexIsChanging
+            ? switch (textDirection) {
+              TextDirection.ltr => (to - 1),
+              TextDirection.rtl => (to + 1),
+            }.clamp(0, maxTabIndex)
+            : controller.previousIndex;
+    final Rect toRect = indicatorRect(size, to);
+    final Rect fromRect = indicatorRect(size, from);
+    final Rect rect = Rect.lerp(fromRect, toRect, (value - from).abs())!;
+
     // If the tab animation is completed, there is no need to stretch the indicator
     // This only works for the tab change animation via tab index, not when
     // dragging a [TabBarView], but it's still ok, to avoid unnecessary calculations.
     if (controller.animation!.isCompleted) {
-      return currentRect;
+      return rect;
     }
 
-    final double index = controller.index.toDouble();
-    final double value = controller.animation!.value;
     final double tabChangeProgress;
 
     if (controller.indexIsChanging) {
-      double progressLeft = (index - value).abs();
       final int tabsDelta = (controller.index - controller.previousIndex).abs();
       if (tabsDelta != 0) {
         progressLeft /= tabsDelta;
@@ -664,7 +676,7 @@ class _IndicatorPainter extends CustomPainter {
 
     // If the animation has finished, there is no need to apply the stretch effect.
     if (tabChangeProgress == 1.0) {
-      return currentRect;
+      return rect;
     }
 
     final double leftFraction;
@@ -701,7 +713,7 @@ class _IndicatorPainter extends CustomPainter {
       };
     }
 
-    return Rect.fromLTRB(lerpRectLeft, currentRect.top, lerpRectRight, currentRect.bottom);
+    return Rect.fromLTRB(lerpRectLeft, rect.top, lerpRectRight, rect.bottom);
   }
 
   @override