Fix scrollable TabBar jittering (#150041)

fixes [TabBar with isScrollable set to true is broken](flutter/flutter#150000)

This regressed due to a tiny mistake in flutter/flutter#146293

### Code sample

<details>
<summary>expand to view the code sample</summary> 

```dart
import 'dart:ui';

import 'package:flutter/material.dart';

/// Flutter code sample for [TabBar].

void main() => runApp(const TabBarApp());

class TabBarApp extends StatelessWidget {
  const TabBarApp({super.key});

  @OverRide
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: TabBarExample(),
    );
  }
}

class TabBarExample extends StatelessWidget {
  const TabBarExample({super.key});

  @OverRide
  Widget build(BuildContext context) {
    final List<Tab> tabs =
        List<Tab>.generate(20, (int index) => Tab(text: 'Tab $index'));

    return ScrollConfiguration(
      behavior: ScrollConfiguration.of(context)
          .copyWith(dragDevices: <PointerDeviceKind>{
        PointerDeviceKind.touch,
        PointerDeviceKind.mouse,
      }),
      child: DefaultTabController(
        length: tabs.length,
        child: Scaffold(
          appBar: AppBar(
            title: const Text('TabBar Sample'),
            bottom: TabBar(
              isScrollable: true,
              tabs: tabs,
              tabAlignment: TabAlignment.start,
            ),
          ),
          body: TabBarView(
            children: <Widget>[
              for (int i = 0; i < tabs.length; i++)
                Center(
                  child: Text('Page $i'),
                ),
            ],
          ),
        ),
      ),
    );
  }
}

```

</details>

### Before

https://github.com/flutter/flutter/assets/48603081/b7aa98a2-a6a5-431e-8327-859a11efa129

### After

https://github.com/flutter/flutter/assets/48603081/0435719f-03d4-4d76-8b5a-532894fcf4a3

Author: TahaTesser

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

@@ -1549,12 +1549,11 @@ class _TabBarState extends State<TabBar> {
     final double index = _controller!.index.toDouble();
     final double value = _controller!.animation!.value;
     final double offset = switch (value - index) {
-      -1.0 || 1.0 => leadingPosition ?? middlePosition,
-      0 => middlePosition,
-      < 0 when leadingPosition == null => middlePosition,
-      > 0 when trailingPosition == null => middlePosition,
-      < 0 => lerpDouble(middlePosition, leadingPosition, index - value)!,
-      _   => lerpDouble(middlePosition, trailingPosition, value - index)!,
+      -1.0 => leadingPosition ?? middlePosition,
+      1.0  => trailingPosition ?? middlePosition,
+      0    => middlePosition,
+      < 0  => leadingPosition == null ? middlePosition : lerpDouble(middlePosition, leadingPosition, index - value)!,
+      _    => trailingPosition == null ? middlePosition : lerpDouble(middlePosition, trailingPosition, value - index)!,
     };
 
     _scrollController!.jumpTo(offset);

packages/flutter/test/material/tabs_test.dart

@@ -7158,4 +7158,96 @@ void main() {
     labelSize = tester.getSize(find.text('Tab 1'));
     expect(labelSize, equals(const Size(140.5, 40.0)));
   }, skip: isBrowser && !isSkiaWeb); // https://github.com/flutter/flutter/issues/87543
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/150000.
+  testWidgets('Scrollable TabBar does not jitter in the middle position', (WidgetTester tester) async {
+    final List<String> tabs = List<String>.generate(20, (int index) => 'Tab $index');
+
+    await tester.pumpWidget(MaterialApp(
+      home: DefaultTabController(
+        length: tabs.length,
+        initialIndex: 10,
+        child: Scaffold(
+          appBar: AppBar(
+            bottom: TabBar(
+              isScrollable: true,
+              tabs: tabs.map((String tab) => Tab(text: tab)).toList(),
+            ),
+          ),
+          body: TabBarView(
+            children: <Widget>[
+              for (int i = 0; i < tabs.length; i++)
+                Center(
+                  child: Text('Page $i'),
+                ),
+            ],
+          ),
+        ),
+      ),
+    ));
+
+    final SingleChildScrollView scrollable = tester.widget(find.byType(SingleChildScrollView));
+    expect(find.text('Page 10'), findsOneWidget);
+    expect(find.text('Page 11'), findsNothing);
+    expect(scrollable.controller!.position.pixels, closeTo(683.2, 0.1));
+
+    // Drag the TabBarView to the left.
+    await tester.drag(find.byType(TabBarView), const Offset(-800, 0));
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 200));
+
+    expect(find.text('Page 10'), findsNothing);
+    expect(find.text('Page 11'), findsOneWidget);
+    expect(scrollable.controller!.position.pixels, closeTo(799.8, 0.1));
+  });
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/150000.
+  testWidgets('Scrollable TabBar does not jitter when the tab bar reaches the start', (WidgetTester tester) async {
+    final List<String> tabs = List<String>.generate(20, (int index) => 'Tab $index');
+
+    await tester.pumpWidget(MaterialApp(
+      home: DefaultTabController(
+        length: tabs.length,
+        initialIndex: 4,
+        child: Scaffold(
+          appBar: AppBar(
+            bottom: TabBar(
+              isScrollable: true,
+              tabs: tabs.map((String tab) => Tab(text: tab)).toList(),
+            ),
+          ),
+          body: TabBarView(
+            children: <Widget>[
+              for (int i = 0; i < tabs.length; i++)
+                Center(
+                  child: Text('Page $i'),
+                ),
+            ],
+          ),
+        ),
+      ),
+    ));
+
+    final SingleChildScrollView scrollable = tester.widget(find.byType(SingleChildScrollView));
+
+    expect(find.text('Page 4'), findsOneWidget);
+    expect(find.text('Page 3'), findsNothing);
+    expect(scrollable.controller!.position.pixels, closeTo(61.25, 0.1));
+
+    // Drag the TabBarView to the right.
+    final TestGesture gesture = await tester.startGesture(tester.getCenter(find.text('Page 4')));
+    await gesture.moveBy(const Offset(600.0, 0.0));
+    await gesture.up();
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    expect(find.text('Page 4'), findsOneWidget);
+    expect(find.text('Page 3'), findsOneWidget);
+    expect(scrollable.controller!.position.pixels, closeTo(0.2, 0.1));
+
+    await tester.pumpAndSettle();
+    expect(find.text('Page 4'), findsNothing);
+    expect(find.text('Page 3'), findsOneWidget);
+    expect(scrollable.controller!.position.pixels, equals(0.0));
+  });
 }