Revert "Fixed TrackingScrollController cleaning offset on position's detach (#11798)" (#11897)

This reverts commit ec25f6bd5142d9c3b70d81a7ccf7c77f27dc63f7.

AUTHORS

@@ -15,4 +15,3 @@ Alexandre Ardhuin <alexandre.ardhuin@gmail.com>
 Luke Freeman <luke@goposse.com>
 Vincent Le Quéméner <eu.lequem@gmail.com>
 Mike Hoolehan <mike@hoolehan.com>
-German Saprykin <saprykin.h@gmail.com>

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

@@ -323,7 +323,6 @@ class TrackingScrollController extends ScrollController {
 
   final Map<ScrollPosition, VoidCallback> _positionToListener = <ScrollPosition, VoidCallback>{};
   ScrollPosition _lastUpdated;
-  double _lastUpdatedOffset;
 
   /// The last [ScrollPosition] to change. Returns null if there aren't any
   /// attached scroll positions, or there hasn't been any scrolling yet, or the
@@ -337,16 +336,13 @@ class TrackingScrollController extends ScrollController {
   ///
   ///  * [ScrollController.initialScrollOffset], which this overrides.
   @override
-  double get initialScrollOffset => _lastUpdatedOffset ?? super.initialScrollOffset;
+  double get initialScrollOffset => _lastUpdated?.pixels ?? super.initialScrollOffset;
 
   @override
   void attach(ScrollPosition position) {
     super.attach(position);
     assert(!_positionToListener.containsKey(position));
-    _positionToListener[position] = () {
+    _positionToListener[position] = () { _lastUpdated = position; };
-      _lastUpdated = position;
-      _lastUpdatedOffset = position.pixels;
-    };
     position.addListener(_positionToListener[position]);
   }
 
@@ -358,8 +354,6 @@ class TrackingScrollController extends ScrollController {
     _positionToListener.remove(position);
     if (_lastUpdated == position)
       _lastUpdated = null;
-    if (_positionToListener.isEmpty)
-      _lastUpdatedOffset = null;
   }
 
   @override

packages/flutter/test/widgets/tracking_scroll_controller_test.dart

@@ -1,57 +0,0 @@
-// Copyright 2017 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'package:flutter/widgets.dart';
-import 'package:flutter_test/flutter_test.dart';
-
-void main() {
-  testWidgets('TrackingScrollController saves offset',
-      (WidgetTester tester) async {
-    final TrackingScrollController controller = new TrackingScrollController();
-
-    await tester.pumpWidget(
-      new PageView.builder(
-        itemBuilder: (BuildContext context, int index) {
-          return new ListView(
-              controller: controller,
-              children: new List<Widget>.generate(
-                10,
-                (int i) => new Container(
-                    height: 100.0, child: new Text("Page$index-Item$i")),
-              ).toList());
-        },
-      ),
-    );
-
-    expect(find.text('Page0-Item1'), findsOneWidget);
-    expect(find.text('Page1-Item1'), findsNothing);
-    expect(find.text('Page2-Item0'), findsNothing);
-    expect(find.text('Page2-Item1'), findsNothing);
-
-    controller.jumpTo(110.0);
-    await (tester.pumpAndSettle());
-
-    await tester.fling(
-        find.text('Page0-Item1'), const Offset(-100.0, 0.0), 10000.0);
-    await (tester.pumpAndSettle());
-
-    expect(find.text('Page0-Item1'), findsNothing);
-    expect(find.text('Page1-Item1'), findsOneWidget);
-    expect(find.text('Page2-Item0'), findsNothing);
-    expect(find.text('Page2-Item1'), findsNothing);
-
-    await tester.fling(
-        find.text('Page1-Item1'), const Offset(-100.0, 0.0), 10000.0);
-    await (tester.pumpAndSettle());
-
-    expect(find.text('Page0-Item1'), findsNothing);
-    expect(find.text('Page1-Item1'), findsNothing);
-    expect(find.text('Page2-Item0'), findsNothing);
-    expect(find.text('Page2-Item1'), findsOneWidget);
-
-    await tester.pumpWidget(new Text("Another page"));
-
-    expect(controller.initialScrollOffset, 0.0);
-  });
-}