Prioritize scrolling away nested overscroll (#55069)

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

@@ -909,21 +909,31 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
     if (_innerPositions.isEmpty) {
       _outerPosition.applyFullDragUpdate(delta);
     } else if (delta < 0.0) {
-      // dragging "up"
-      // TODO(ianh): prioritize first getting rid of overscroll, and then the
-      // outer view, so that the app bar will scroll out of the way asap.
-      // Right now we ignore overscroll. This works fine on Android but looks
-      // weird on iOS if you fling down then up. The problem is it's not at all
-      // clear what this should do when you have multiple inner positions at
-      // different levels of overscroll.
-      final double innerDelta = _outerPosition.applyClampedDragUpdate(delta);
-      if (innerDelta != 0.0) {
-        for (final _NestedScrollPosition position in _innerPositions)
-          position.applyFullDragUpdate(innerDelta);
+      // Dragging "up"
+      // Prioritize getting rid of any inner overscroll, and then the outer
+      // view, so that the app bar will scroll out of the way asap.
+      double outerDelta = delta;
+      for (final _NestedScrollPosition position in _innerPositions) {
+        if (position.pixels < 0.0) { // This inner position is in overscroll.
+          final double potentialOuterDelta = position.applyClampedDragUpdate(delta);
+          // In case there are multiple positions in varying states of
+          // overscroll, the first to 'reach' the outer view above takes
+          // precedence.
+          outerDelta = math.max(outerDelta, potentialOuterDelta);
+        }
+      }
+      if (outerDelta != 0.0) {
+        final double innerDelta = _outerPosition.applyClampedDragUpdate(
+          outerDelta
+        );
+        if (innerDelta != 0.0) {
+          for (final _NestedScrollPosition position in _innerPositions)
+            position.applyFullDragUpdate(innerDelta);
+        }
       }
     } else {
-      // dragging "down" - delta is positive
-      // prioritize the inner views, so that the inner content will move before
+      // Dragging "down" - delta is positive
+      // Prioritize the inner views, so that the inner content will move before
       // the app bar grows
       double outerDelta = 0.0; // it will go positive if it changes
       final List<double> overscrolls = <double>[];

packages/flutter/test/widgets/nested_scroll_view_test.dart

@@ -133,9 +133,7 @@ void main() {
     await tester.pump(const Duration(milliseconds: 20));
     final Offset point2 = tester.getCenter(find.text('aaa1'));
     expect(point2.dy, greaterThan(point1.dy));
-    // TODO(ianh): Once we improve how we handle scrolling down from overscroll,
-    // the following expectation should switch to 200.0.
-    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 120.0);
+    expect(tester.renderObject<RenderBox>(find.byType(AppBar)).size.height, 200.0);
   }, variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
 
   testWidgets('NestedScrollView overscroll and release and hold', (WidgetTester tester) async {
@@ -173,7 +171,6 @@ void main() {
     await tester.pumpAndSettle();
     expect(find.text('aaa2'), findsOneWidget);
   },
-  skip: true,  // https://github.com/flutter/flutter/issues/9040
   variant: const TargetPlatformVariant(<TargetPlatform>{ TargetPlatform.iOS,  TargetPlatform.macOS }));
 
   testWidgets('NestedScrollView', (WidgetTester tester) async {
@@ -780,7 +777,7 @@ void main() {
     await tester.pump();
     expect(
       tester.getRect(find.byKey(key1)),
-      const Rect.fromLTWH(0.0, -1.0, 800.0, 100.0),
+      const 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(129.0));
@@ -788,7 +785,7 @@ void main() {
     await tester.pump();
     expect(
       tester.getRect(find.byKey(key1)),
-      const Rect.fromLTWH(0.0, -11.0, 800.0, 100.0),
+      const Rect.fromLTWH(0.0, 0.0, 800.0, 100.0),
     );
     await gesture.moveBy(const Offset(0.0, 20.0)); // overscroll again
     await tester.pump();