Fix the depth of scroll notifications (#8156)

Fixes https://github.com/flutter/flutter/issues/8017

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

@@ -726,7 +726,7 @@ class _TabBarViewState extends State<TabBarView> {
     if (_warpUnderwayCount > 0)
       return false;
 
-    if (notification.depth != 1)
+    if (notification.depth != 0)
       return false;
 
     if (notification is ScrollStartNotification) {

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

@@ -188,7 +188,7 @@ class PageView extends BoxScrollView {
     final Widget scrollable = super.build(context);
     return new NotificationListener<ScrollNotification2>(
       onNotification: (ScrollNotification2 notification) {
-        if (notification.depth == 1 && onPageChanged != null && notification is ScrollEndNotification) {
+        if (notification.depth == 0 && onPageChanged != null && notification is ScrollEndNotification) {
           final ScrollMetrics metrics = notification.metrics;
           onPageChanged(metrics.extentBefore ~/ metrics.viewportDimension);
         }

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

@@ -75,15 +75,19 @@ abstract class ScrollNotification2 extends LayoutChangedNotification {
   /// size of the viewport, for instance.
   final BuildContext context;
 
-  /// The number of [Scrollable2] widgets that this notification has bubbled
-  /// through. Typically listeners only respond to notifications with a [depth]
-  /// of zero.
+  /// The number of viewports that this notification has bubbled through.
+  ///
+  /// Typically listeners only respond to notifications with a [depth] of zero.
+  ///
+  /// Specifically, this is the number of [Widget]s representing
+  /// [RenderAbstractViewport] render objects through which this notification
+  /// has bubbled.
   int get depth => _depth;
   int _depth = 0;
 
   @override
   bool visitAncestor(Element element) {
-    if (element.widget is Scrollable2)
+    if (element is RenderObjectElement && element.renderObject is RenderAbstractViewport)
       _depth += 1;
     return super.visitAncestor(element);
   }
@@ -93,7 +97,7 @@ abstract class ScrollNotification2 extends LayoutChangedNotification {
     super.debugFillDescription(description);
     description.add('$axisDirection');
     description.add('metrics: $metrics');
-    description.add('depth: $depth');
+    description.add('depth: $depth (${ depth == 0 ? "local" : "remote"})');
   }
 }
 

packages/flutter/test/widgets/scroll_notification_test.dart

@@ -23,7 +23,7 @@ void main() {
     TestGesture gesture = await tester.startGesture(const Point(100.0, 100.0));
     await tester.pump(const Duration(seconds: 1));
     expect(notification, const isInstanceOf<ScrollStartNotification>());
-    expect(notification.depth, equals(1));
+    expect(notification.depth, equals(0));
     ScrollStartNotification start = notification;
     expect(start.dragDetails, isNotNull);
     expect(start.dragDetails.globalPosition, equals(const Point(100.0, 100.0)));
@@ -31,7 +31,7 @@ void main() {
     await gesture.moveBy(const Offset(-10.0, -10.0));
     await tester.pump(const Duration(seconds: 1));
     expect(notification, const isInstanceOf<ScrollUpdateNotification>());
-    expect(notification.depth, equals(1));
+    expect(notification.depth, equals(0));
     ScrollUpdateNotification update = notification;
     expect(update.dragDetails, isNotNull);
     expect(update.dragDetails.globalPosition, equals(const Point(90.0, 90.0)));
@@ -40,7 +40,7 @@ void main() {
     await gesture.up();
     await tester.pump(const Duration(seconds: 1));
     expect(notification, const isInstanceOf<ScrollEndNotification>());
-    expect(notification.depth, equals(1));
+    expect(notification.depth, equals(0));
     ScrollEndNotification end = notification;
     expect(end.dragDetails, isNotNull);
     expect(end.dragDetails.velocity, equals(Velocity.zero));
@@ -93,9 +93,7 @@ void main() {
     expect(depth0Types, equals(types));
     expect(depth1Types, equals(types));
 
-    // These values might not be what we want in the end.
-    // See <https://github.com/flutter/flutter/issues/8017>.
-    expect(depth0Values, equals(<int>[1, 1, 1, 1, 1]));
-    expect(depth1Values, equals(<int>[2, 2, 2, 2, 2]));
+    expect(depth0Values, equals(<int>[0, 0, 0, 0, 0]));
+    expect(depth1Values, equals(<int>[1, 1, 1, 1, 1]));
   });
 }