Fix NestedScrollView inner position logic (flutter#157756)

closes flutter#40740

<p align="center">
  <img 
    src="https://github.com/user-attachments/assets/8a48abd0-466b-4e06-90f3-dd05869bac27"
    alt="NestedScrollController fix"
    height="720px"
  />
</p>

<details> <summary><h4>Demo source code</h4> (click to expand)</summary>

(this is a slightly more concise version of the code sample from flutter#40740)

```dart
import 'package:flutter/material.dart';

void main() {
  runApp(
    const MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(body: NewsScreen()),
    ),
  );
}

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

  static const List<String> _tabs = <String>['Featured', 'Popular', 'Latest'];

  static final List<Widget> _tabViews = <Widget>[
    for (final String name in _tabs)
      SafeArea(
        top: false,
        bottom: false,
        child: Builder(builder: (BuildContext context) {
          final handle = NestedScrollView.sliverOverlapAbsorberHandleFor(context);

          return NotificationListener<ScrollNotification>(
            onNotification: (ScrollNotification notification) => true,
            child: CustomScrollView(
              key: PageStorageKey<String>(name),
              slivers: <Widget>[
                SliverOverlapInjector(handle: handle),
                SliverPadding(
                  padding: const EdgeInsets.all(8.0),
                  sliver: SliverList(
                    delegate: SliverChildBuilderDelegate(
                      childCount: 30,
                      (BuildContext context, int index) => Container(
                        margin: const EdgeInsets.only(bottom: 8),
                        width: double.infinity,
                        height: 150,
                        color: const Color(0xFFB0A4C8),
                        alignment: Alignment.center,
                        child: Text(
                          '$name $index',
                          style: const TextStyle(fontWeight: FontWeight.w600),
                        ),
                      ),
                    ),
                  ),
                ),
              ],
            ),
          );
        }),
      ),
  ];

  @OverRide
  Widget build(BuildContext context) {
    return DefaultTabController(
      length: _tabs.length,
      child: NestedScrollView(
        headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) => <Widget>[
          SliverOverlapAbsorber(
            handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
            sliver: SliverSafeArea(
              top: false,
              sliver: SliverAppBar(
                title: const Text('Tab Demo'),
                floating: true,
                pinned: true,
                snap: true,
                forceElevated: innerBoxIsScrolled,
                bottom: TabBar(
                  tabs: _tabs.map((String name) => Tab(text: name)).toList(),
                ),
              ),
            ),
          ),
        ],
        body: TabBarView(children: _tabViews),
      ),
    );
  }
}
```

<br>

</details>

<br>

This bug can be traced to a return statement inside `_NestedScrollPosition`:

```dart
double applyClampedDragUpdate(double delta) {
  // ...
  return delta + offset;
}
```

Thanks to some quirks of floating-point arithmetic, `applyClampedDragUpdate` would sometimes return a tiny non-zero value, which ends up ruining one of the scroll coordinator's equality checks.

https://github.com/flutter/flutter/blob/8990ed6538592fc8aee63b0e137b7b52804a030f/packages/flutter/lib/src/widgets/nested_scroll_view.dart#L658-L664

Author: nate-thegrate

packages/flutter/lib/src/widgets/nested_scroll_view.dart

@@ -1320,7 +1320,12 @@ class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDele
       forcePixels(actualNewPixels);
       didUpdateScrollPositionBy(offset);
     }
-    return delta + offset;
+
+    final double result = delta + offset;
+    if (result.abs() < precisionErrorTolerance) {
+      return 0.0;
+    }
+    return result;
   }
 
   // Returns the overscroll.

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -3628,6 +3628,117 @@ void main() {
     });
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/40740.
+  testWidgets('Maintains scroll position of inactive tab', (WidgetTester tester) async {
+    const List<String> tabs = <String>['Featured', 'Popular', 'Latest'];
+    final List<Widget> tabViews = <Widget>[
+      for (final String name in tabs)
+        SafeArea(
+          top: false,
+          bottom: false,
+          child: Builder(
+            builder: (BuildContext context) => CustomScrollView(
+              key: PageStorageKey<String>(name),
+              slivers: <Widget>[
+                SliverOverlapInjector(
+                  handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
+                ),
+                SliverPadding(
+                  padding: const EdgeInsets.all(8.0),
+                  sliver: SliverList(
+                    delegate: SliverChildBuilderDelegate(
+                      childCount: 30,
+                      (BuildContext context, int index) {
+                        return ListTile(title: Text('Item $index'));
+                      },
+                    ),
+                  ),
+                ),
+              ],
+            ),
+          ),
+        ),
+    ];
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: DefaultTabController(
+            length: tabs.length,
+            child: NestedScrollView(
+              headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) => <Widget>[
+                SliverOverlapAbsorber(
+                  handle: NestedScrollView.sliverOverlapAbsorberHandleFor(context),
+                  sliver: SliverSafeArea(
+                    top: false,
+                    sliver: SliverAppBar(
+                      title: const Text('Tab Demo'),
+                      floating: true,
+                      pinned: true,
+                      snap: true,
+                      forceElevated: innerBoxIsScrolled,
+                      bottom: TabBar(
+                        tabs: tabs.map((String name) => Tab(text: name)).toList(),
+                      ),
+                    ),
+                  ),
+                ),
+              ],
+              body: TabBarView(children: tabViews),
+            ),
+          ),
+        ),
+      ),
+    );
+
+    final Finder finder = find.text('Item 14', skipOffstage: false);
+    final Finder findAny = find.descendant(
+      of: find.byType(SliverList),
+      matching: find.byType(ListTile),
+    ).first;
+
+    Future<void> scroll(VerticalDirection direction) async {
+      switch (direction) {
+        case VerticalDirection.down:
+          while (!tester.any(finder)) {
+            await tester.fling(find.byType(Scaffold), const Offset(0, -50), 20);
+            await tester.pumpAndSettle();
+          }
+          await tester.ensureVisible(finder);
+        case VerticalDirection.up:
+          await tester.fling(find.byType(Scaffold), const Offset(0, 20), 20);
+          await tester.pumpAndSettle();
+      }
+    }
+
+    double getScrollPosition() {
+      return Scrollable.of(tester.element(findAny)).position.pixels;
+    }
+
+    expect(getScrollPosition(), 0.0);
+
+    // Scroll toward the bottom of Tab 1.
+    await scroll(VerticalDirection.down);
+    final double tab1Position = getScrollPosition();
+
+    // Switch to second tab.
+    await tester.tap(find.text('Popular'));
+    await tester.pumpAndSettle();
+
+    // Scroll toward the bottom of the second tab.
+    await scroll(VerticalDirection.down);
+    final double tab2Position = getScrollPosition();
+
+    // Scroll up a bit in the second tab.
+    await scroll(VerticalDirection.up);
+    expect(getScrollPosition(), lessThan(tab2Position));
+
+    // Switch back to the first tab.
+    await tester.tap(find.text('Featured'));
+    await tester.pumpAndSettle();
+    expect(getScrollPosition(), tab1Position);
+  });
+
   testWidgets('$SliverOverlapAbsorberHandle dispatches creation in constructor', (WidgetTester widgetTester) async {
     await expectLater(
       await memoryEvents(() => SliverOverlapAbsorberHandle().dispose(), SliverOverlapAbsorberHandle),