Fix boundaries in applyClampedDragUpdate (#14444)

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

@@ -895,6 +895,10 @@ class _NestedScrollController extends ScrollController {
   }
 }
 
+// The _NestedScrollPosition is used by both the inner and outer viewports of a
+// NestedScrollView. It tracks the offset to use for those viewports, and knows
+// about the _NestedScrollCoordinator, so that when activities are triggered on
+// this class, they can defer, or be influenced by, the coordinator.
 class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDelegate {
   _NestedScrollPosition({
     @required ScrollPhysics physics,
@@ -945,10 +949,31 @@ class _NestedScrollPosition extends ScrollPosition implements ScrollActivityDele
   }
 
   // Returns the amount of delta that was not used.
+  //
+  // Positive delta means going down (exposing stuff above), negative delta
+  // going up (exposing stuff below).
   double applyClampedDragUpdate(double delta) {
     assert(delta != 0.0);
-    final double min = delta < 0.0 ? -double.INFINITY : minScrollExtent;
+    // If we are going towards the maxScrollExtent (negative scroll offset),
-    final double max = delta > 0.0 ? double.INFINITY : maxScrollExtent;
+    // then the furthest we can be in the minScrollExtent direction is negative
+    // infinity. For example, if we are already overscrolled, then scrolling to
+    // reduce the overscroll should not disallow the overscroll.
+    //
+    // If we are going towards the minScrollExtent (positive scroll offset),
+    // then the furthest we can be in the minScrollExtent direction is wherever
+    // we are now, if we are already overscrolled (in which case pixels is less
+    // than the minScrollExtent), or the minScrollExtent if we are not.
+    //
+    // In other words, we cannot, via applyClampedDragUpdate, _enter_ an
+    // overscroll situation.
+    //
+    // An overscroll situation might be nonetheless entered via several means.
+    // One is if the physics allow it, via applyFullDragUpdate (see below). An
+    // overscroll situation can also be forced, e.g. if the scroll position is
+    // artificially set using the scroll controller.
+    final double min = delta < 0.0 ? -double.INFINITY : math.min(minScrollExtent, pixels);
+    // The logic for max is equivalent but on the other side.
+    final double max = delta > 0.0 ? double.INFINITY : math.max(maxScrollExtent, pixels);
     final double oldPixels = pixels;
     final double newPixels = (pixels - delta).clamp(min, max);
     final double clampedDelta = newPixels - pixels;

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -566,4 +566,84 @@ void main() {
     expect(buildCount, expectedBuildCount);
     expect(find.byType(NestedScrollView), isNot(paints..shadow()));
   });
+
+  testWidgets('NestedScrollView and iOS bouncing', (WidgetTester tester) async {
+    // This verifies that overscroll bouncing works correctly on iOS. For
+    // example, this checks that if you pull to overscroll, friction is applied;
+    // it also makes sure that if you scroll back the other way, the scroll
+    // positions of the inner and outer list don't have a discontinuity.
+    debugDefaultTargetPlatformOverride = TargetPlatform.iOS;
+    const Key key1 = const ValueKey<int>(1);
+    const Key key2 = const ValueKey<int>(2);
+    await tester.pumpWidget(
+      new MaterialApp(
+        home: new Material(
+          child: new DefaultTabController(
+            length: 1,
+            child: new NestedScrollView(
+              headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
+                return <Widget>[
+                  const SliverPersistentHeader(
+                    delegate: const TestHeader(key: key1),
+                  ),
+                ];
+              },
+              body: new SingleChildScrollView(
+                child: new Container(
+                  height: 1000.0,
+                  child: const Placeholder(key: key2),
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+    expect(tester.getRect(find.byKey(key1)), new Rect.fromLTWH(0.0, 0.0, 800.0, 100.0));
+    expect(tester.getRect(find.byKey(key2)), new Rect.fromLTWH(0.0, 100.0, 800.0, 1000.0));
+    final TestGesture gesture = await tester.startGesture(const Offset(10.0, 10.0));
+    await gesture.moveBy(const Offset(0.0, -10.0)); // scroll up
+    await tester.pump();
+    expect(tester.getRect(find.byKey(key1)), new Rect.fromLTWH(0.0, -10.0, 800.0, 100.0));
+    expect(tester.getRect(find.byKey(key2)), new Rect.fromLTWH(0.0, 90.0, 800.0, 1000.0));
+    await gesture.moveBy(const Offset(0.0, 10.0)); // scroll back to origin
+    await tester.pump();
+    expect(tester.getRect(find.byKey(key1)), new Rect.fromLTWH(0.0, 0.0, 800.0, 100.0));
+    expect(tester.getRect(find.byKey(key2)), new Rect.fromLTWH(0.0, 100.0, 800.0, 1000.0));
+    await gesture.moveBy(const Offset(0.0, 10.0)); // overscroll
+    await gesture.moveBy(const Offset(0.0, 10.0)); // overscroll
+    await gesture.moveBy(const Offset(0.0, 10.0)); // overscroll
+    await tester.pump();
+    expect(tester.getRect(find.byKey(key1)), new Rect.fromLTWH(0.0, 0.0, 800.0, 100.0));
+    expect(tester.getRect(find.byKey(key2)).top, greaterThan(100.0));
+    expect(tester.getRect(find.byKey(key2)).top, lessThan(130.0));
+    await gesture.moveBy(const Offset(0.0, -1.0)); // scroll back a little
+    await tester.pump();
+    expect(tester.getRect(find.byKey(key1)), new Rect.fromLTWH(0.0, -1.0, 800.0, 100.0));
+    expect(tester.getRect(find.byKey(key2)).top, greaterThan(100.0));
+    expect(tester.getRect(find.byKey(key2)).top, lessThan(129.0));
+    await gesture.moveBy(const Offset(0.0, -10.0)); // scroll back a lot
+    await tester.pump();
+    expect(tester.getRect(find.byKey(key1)), new Rect.fromLTWH(0.0, -11.0, 800.0, 100.0));
+    await gesture.moveBy(const Offset(0.0, 20.0)); // overscroll again
+    await tester.pump();
+    expect(tester.getRect(find.byKey(key1)), new Rect.fromLTWH(0.0, 0.0, 800.0, 100.0));
+    await gesture.up();
+    debugDefaultTargetPlatformOverride = null;
+  });
+}
+
+class TestHeader extends SliverPersistentHeaderDelegate {
+  const TestHeader({ this.key });
+  final Key key;
+  @override
+  double get minExtent => 100.0;
+  @override
+  double get maxExtent => 100.0;
+  @override
+  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
+    return new Placeholder(key: key);
+  }
+  @override
+  bool shouldRebuild(TestHeader oldDelegate) => false;
 }