Fix NestedScrollView inner ballistic activity for 0 velocity (#61386)

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

@@ -918,9 +918,7 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
   ScrollActivity createInnerBallisticScrollActivity(_NestedScrollPosition position, double velocity) {
     return position.createBallisticScrollActivity(
       position.physics.createBallisticSimulation(
-        velocity == 0
-          ? position as ScrollMetrics
-          : _getMetrics(position, velocity),
+        _getMetrics(position, velocity),
         velocity,
       ),
       mode: _NestedBallisticScrollActivityMode.inner,
@@ -929,7 +927,8 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
 
   _NestedScrollMetrics _getMetrics(_NestedScrollPosition innerPosition, double velocity) {
     assert(innerPosition != null);
-    double pixels, minRange, maxRange, correctionOffset, extra;
+    double pixels, minRange, maxRange, correctionOffset;
+    double extra = 0.0;
     if (innerPosition.pixels == innerPosition.minScrollExtent) {
       pixels = _outerPosition.pixels.clamp(
         _outerPosition.minScrollExtent,
@@ -939,7 +938,6 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
       maxRange = _outerPosition.maxScrollExtent;
       assert(minRange <= maxRange);
       correctionOffset = 0.0;
-      extra = 0.0;
     } else {
       assert(innerPosition.pixels != innerPosition.minScrollExtent);
       if (innerPosition.pixels < innerPosition.minScrollExtent) {
@@ -974,8 +972,7 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
         if (velocity > 0.0) {
           // shrinking
           extra = _outerPosition.minScrollExtent - _outerPosition.pixels;
-        } else {
-          assert(velocity < 0.0);
+        } else if (velocity < 0.0) {
           // growing
           extra = _outerPosition.pixels - (_outerPosition.maxScrollExtent - _outerPosition.minScrollExtent);
         }

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -1817,6 +1817,97 @@ void main() {
       verifyGeometry(key: appBarKey, paintExtent: 200.0, visible: true);
     });
   });
+
+  group('Correctly handles 0 velocity inner ballistic scroll activity:', () {
+    // Regression tests for https://github.com/flutter/flutter/issues/17096
+    Widget _buildBallisticTest(ScrollController controller) {
+      return MaterialApp(
+        home: Scaffold(
+          body: NestedScrollView(
+            controller: controller,
+            headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
+              return <Widget>[
+                const SliverAppBar(
+                  pinned: true,
+                  expandedHeight: 200.0,
+                ),
+              ];
+            },
+            body: ListView.builder(
+              itemCount: 50,
+              itemBuilder: (BuildContext context, int index) {
+                return Container(
+                  child: Padding(
+                    padding: const EdgeInsets.all(8.0),
+                    child: Text('Item $index'),
+                  )
+                );
+              }
+            ),
+          ),
+        ),
+      );
+    }
+
+    testWidgets('overscroll, hold for 0 velocity, and release', (WidgetTester tester) async {
+      // Dragging into an overscroll and holding so that when released, the
+      // ballistic scroll activity has a 0 velocity.
+      final ScrollController controller = ScrollController();
+      await tester.pumpWidget(_buildBallisticTest(controller));
+      // Last item of the inner scroll view.
+      expect(find.text('Item 49'), findsNothing);
+
+      // Scroll to bottom
+      await tester.fling(find.text('Item 3'), const Offset(0.0, -50.0), 10000.0);
+      await tester.pumpAndSettle();
+
+      // End of list
+      expect(find.text('Item 49'), findsOneWidget);
+      expect(tester.getCenter(find.text('Item 49')).dy, equals(585.0));
+
+      // Overscroll, dragging like this will release with 0 velocity.
+      await tester.drag(find.text('Item 49'), const Offset(0.0, -50.0));
+      await tester.pump();
+      // If handled correctly, the last item should still be visible and
+      // progressing  back down to the bottom edge, instead of jumping further
+      // up the list and out of view.
+      expect(find.text('Item 49'), findsOneWidget);
+      await tester.pumpAndSettle();
+      expect(tester.getCenter(find.text('Item 49')).dy, equals(585.0));
+    }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS }));
+
+    testWidgets('overscroll, release, and tap', (WidgetTester tester) async {
+      // Tapping while an inner ballistic scroll activity is in progress will
+      // trigger a secondary ballistic scroll activity with a 0 velocity.
+      final ScrollController controller = ScrollController();
+      await tester.pumpWidget(_buildBallisticTest(controller));
+      // Last item of the inner scroll view.
+      expect(find.text('Item 49'), findsNothing);
+
+      // Scroll to bottom
+      await tester.fling(find.text('Item 3'), const Offset(0.0, -50.0), 10000.0);
+      await tester.pumpAndSettle();
+
+      // End of list
+      expect(find.text('Item 49'), findsOneWidget);
+      expect(tester.getCenter(find.text('Item 49')).dy, equals(585.0));
+
+      // Fling again to trigger first ballistic activity.
+      await tester.fling(find.text('Item 48'), const Offset(0.0, -50.0), 10000.0);
+      await tester.pump();
+
+      // Tap after releasing the overscroll to trigger secondary inner ballistic
+      // scroll activity with 0 velocity.
+      await tester.tap(find.text('Item 49'));
+      await tester.pumpAndSettle();
+
+      // If handled correctly, the ballistic scroll activity should finish
+      // closing out the overscrolled area, with the last item visible at the
+      // bottom.
+      expect(find.text('Item 49'), findsOneWidget);
+      expect(tester.getCenter(find.text('Item 49')).dy, equals(585.0));
+    }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS }));
+  });
 }
 
 class TestHeader extends SliverPersistentHeaderDelegate {