ignore sliver underflow if the last children is no longer at the prev… (#71864)

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

@@ -1131,19 +1131,21 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
   void performRebuild() {
     super.performRebuild();
     _currentBeforeChild = null;
+    bool childrenUpdated = false;
     assert(_currentlyUpdatingChildIndex == null);
     try {
       final SplayTreeMap<int, Element?> newChildren = SplayTreeMap<int, Element?>();
       final Map<int, double> indexToLayoutOffset = HashMap<int, double>();
-
       void processElement(int index) {
         _currentlyUpdatingChildIndex = index;
         if (_childElements[index] != null && _childElements[index] != newChildren[index]) {
           // This index has an old child that isn't used anywhere and should be deactivated.
           _childElements[index] = updateChild(_childElements[index], null, index);
+          childrenUpdated = true;
         }
         final Element? newChild = updateChild(newChildren[index], _build(index), index);
         if (newChild != null) {
+          childrenUpdated = childrenUpdated || _childElements[index] != newChild;
           _childElements[index] = newChild;
           final SliverMultiBoxAdaptorParentData parentData = newChild.renderObject!.parentData! as SliverMultiBoxAdaptorParentData;
           if (index == 0) {
@@ -1154,6 +1156,7 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
           if (!parentData.keptAlive)
             _currentBeforeChild = newChild.renderObject as RenderBox?;
         } else {
+          childrenUpdated = true;
           _childElements.remove(index);
         }
       }
@@ -1185,7 +1188,16 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
 
       renderObject.debugChildIntegrityEnabled = false; // Moving children will temporary violate the integrity.
       newChildren.keys.forEach(processElement);
-      if (_didUnderflow) {
+      // An element rebuild only updates existing children. The underflow check
+      // is here to make sure we look ahead one more child if we were at the end
+      // of the child list before the update. By doing so, we can update the max
+      // scroll offset during the layout phase. Otherwise, the layout phase may
+      // be skipped, and the scroll view may be stuck at the previous max
+      // scroll offset.
+      //
+      // This logic is not needed if any existing children has been updated,
+      // because we will not skip the layout phase if that happens.
+      if (!childrenUpdated && _didUnderflow) {
         final int lastKey = _childElements.lastKey() ?? -1;
         final int rightBoundary = lastKey + 1;
         newChildren[rightBoundary] = _childElements[rightBoundary];

packages/flutter/test/widgets/slivers_test.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/widgets.dart';
@@ -271,6 +272,71 @@ void main() {
     expect(find.text('4'), findsOneWidget);
   });
 
+  testWidgets('SliverFixedExtentList handles underflow when its children changes', (WidgetTester tester) async {
+    final List<String> items = <String>['1', '2', '3', '4', '5', '6'];
+    final List<String> initializedChild = <String>[];
+    List<Widget> children = <Widget>[];
+    for (final String item in items) {
+      children.add(
+          StateInitSpy(
+            item, () => initializedChild.add(item), key: ValueKey<String>(item),
+          )
+      );
+    }
+    final ScrollController controller = ScrollController(initialScrollOffset: 5400);
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: CustomScrollView(
+          controller: controller,
+          slivers: <Widget>[
+            SliverFixedExtentList(
+              itemExtent: 900,
+              delegate: SliverChildListDelegate(children),
+            ),
+          ],
+        ),
+      ),
+    );
+    await tester.pumpAndSettle();
+    expect(find.text('1'), findsNothing);
+    expect(find.text('2'), findsNothing);
+    expect(find.text('3'), findsNothing);
+    expect(find.text('4'), findsNothing);
+    expect(find.text('5'), findsNothing);
+    expect(find.text('6'), findsOneWidget);
+    expect(listEquals<String>(initializedChild, <String>['6']), isTrue);
+
+    // move to item 1 and swap the children at the same time
+    controller.jumpTo(0);
+    final Widget temp = children[5];
+    children[5] = children[0];
+    children[0] = temp;
+    children = List<Widget>.from(children);
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: CustomScrollView(
+          controller: controller,
+          slivers: <Widget>[
+            SliverFixedExtentList(
+              itemExtent: 900,
+              delegate: SliverChildListDelegate(children),
+            ),
+          ],
+        ),
+      ),
+    );
+    expect(find.text('1'), findsNothing);
+    expect(find.text('2'), findsNothing);
+    expect(find.text('3'), findsNothing);
+    expect(find.text('4'), findsNothing);
+    expect(find.text('5'), findsNothing);
+    expect(find.text('6'), findsOneWidget);
+    // None of the children should be built.
+    expect(listEquals<String>(initializedChild, <String>['6']), isTrue);
+  });
+
   testWidgets(
     'SliverGrid Correctly layout children after rearranging',
     (WidgetTester tester) async {
@@ -1021,6 +1087,29 @@ class TestSliverFixedExtentList extends StatelessWidget {
   }
 }
 
+class StateInitSpy extends StatefulWidget {
+  const StateInitSpy(this.data, this.onStateInit, { Key? key }) : super(key: key);
+
+  final String data;
+  final VoidCallback onStateInit;
+
+  @override
+  StateInitSpyState createState() => StateInitSpyState();
+}
+
+class StateInitSpyState extends State<StateInitSpy> {
+  @override
+  void initState() {
+    super.initState();
+    widget.onStateInit();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Text(widget.data);
+  }
+}
+
 class KeepAlive extends StatefulWidget {
   const KeepAlive(this.data, { Key? key }) : super(key: key);