Fixes RenderSliverFixedExtentBoxAdaptor correctly calculates leadingGarbage and trailingGarbage. (#36302)

packages/flutter/lib/src/rendering/sliver_fixed_extent_list.dart

@@ -142,6 +142,26 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
     return childManager.childCount * itemExtent;
   }
 
+  int _calculateLeadingGarbage(int firstIndex) {
+    RenderBox walker = firstChild;
+    int leadingGarbage = 0;
+    while(walker != null && indexOf(walker) < firstIndex){
+      leadingGarbage += 1;
+      walker = childAfter(walker);
+    }
+    return leadingGarbage;
+  }
+
+  int _calculateTrailingGarbage(int targetLastIndex) {
+    RenderBox walker = lastChild;
+    int trailingGarbage = 0;
+    while(walker != null && indexOf(walker) > targetLastIndex){
+      trailingGarbage += 1;
+      walker = childBefore(walker);
+    }
+    return trailingGarbage;
+  }
+
   @override
   void performLayout() {
     childManager.didStartLayout();
@@ -165,10 +185,8 @@ abstract class RenderSliverFixedExtentBoxAdaptor extends RenderSliverMultiBoxAda
         getMaxChildIndexForScrollOffset(targetEndScrollOffset, itemExtent) : null;
 
     if (firstChild != null) {
-      final int oldFirstIndex = indexOf(firstChild);
-      final int oldLastIndex = indexOf(lastChild);
-      final int leadingGarbage = (firstIndex - oldFirstIndex).clamp(0, childCount);
-      final int trailingGarbage = targetLastIndex == null ? 0 : (oldLastIndex - targetLastIndex).clamp(0, childCount);
+      final int leadingGarbage = _calculateLeadingGarbage(firstIndex);
+      final int trailingGarbage = _calculateTrailingGarbage(targetLastIndex);
       collectGarbage(leadingGarbage, trailingGarbage);
     } else {
       collectGarbage(0, 0);

packages/flutter/test/widgets/slivers_test.dart

@@ -25,6 +25,37 @@ Future<void> test(WidgetTester tester, double offset, { double anchor = 0.0 }) {
   );
 }
 
+Future<void> testSliverFixedExtentList(WidgetTester tester, List<String> items) {
+  return tester.pumpWidget(
+    Directionality(
+      textDirection: TextDirection.ltr,
+      child: CustomScrollView(
+        slivers: <Widget>[
+          SliverFixedExtentList(
+            itemExtent: 900,
+            delegate: SliverChildBuilderDelegate(
+                (BuildContext context, int index) {
+                return Center(
+                  key: ValueKey<String>(items[index]),
+                  child: KeepAlive(
+                    items[index],
+                  )
+                );
+              },
+              childCount : items.length,
+              findChildIndexCallback: (Key key) {
+                final ValueKey<String> valueKey = key;
+                final String data = valueKey.value;
+                return items.indexOf(data);
+              }
+            ),
+          ),
+        ],
+      ),
+    ),
+  );
+}
+
 void verify(WidgetTester tester, List<Offset> idealPositions, List<bool> idealVisibles) {
   final List<Offset> actualPositions = tester.renderObjectList<RenderBox>(find.byType(SizedBox, skipOffstage: false)).map<Offset>(
     (RenderBox target) => target.localToGlobal(const Offset(0.0, 0.0))
@@ -196,6 +227,47 @@ void main() {
     expect(find.text('BOTTOM'), findsOneWidget);
   });
 
+  testWidgets('SliverFixedExtentList correctly clears garbage', (WidgetTester tester) async {
+    final List<String> items = <String>['1', '2', '3', '4', '5', '6'];
+    await testSliverFixedExtentList(tester, items);
+    // Keep alive widgets require 1 frame to notify their parents. Pumps in between
+    // drags to ensure widgets are kept alive.
+    await tester.drag(find.byType(CustomScrollView),const Offset(0.0, -1200.0));
+    await tester.pump();
+    await tester.drag(find.byType(CustomScrollView),const Offset(0.0, -1200.0));
+    await tester.pump();
+    await tester.drag(find.byType(CustomScrollView),const Offset(0.0, -800.0));
+    await tester.pump();
+    expect(find.text('1'), findsNothing);
+    expect(find.text('2'), findsNothing);
+    expect(find.text('3'), findsNothing);
+    expect(find.text('4'), findsOneWidget);
+    expect(find.text('5'), findsOneWidget);
+    // Indexes [0, 1, 2] are kept alive and [3, 4] are in viewport, thus the sliver
+    // will need to keep updating the elements at these indexes whenever a rebuild is
+    // triggered. The current child list in RenderSliverFixedExtentList is
+    // '4' -> '5' -> null.
+    //
+    // With the insertion below, all items will get shifted back 1 position. The sliver
+    // will have to update indexes [0, 1, 2, 3, 4, 5]. Since this is the first time
+    // item '0' gets initialized, mounting the element will cause it to attach to
+    // child list in RenderSliverFixedExtentList. This will create a gap.
+    // '0' -> '4' -> '5' -> null.
+    items.insert(0, '0');
+    await testSliverFixedExtentList(tester, items);
+    // Sliver should collect leading and trailing garbage correctly.
+    //
+    // The child list update should occur in following order.
+    // '0' -> '4' -> '5' -> null Started with Original list.
+    // '4' -> null               Removed 1 leading garbage and 1 trailing garbage.
+    // '3' -> '4' -> null        Prepended '3' because viewport is still at [3, 4].
+    expect(find.text('0'), findsNothing);
+    expect(find.text('1'), findsNothing);
+    expect(find.text('2'), findsNothing);
+    expect(find.text('3'), findsOneWidget);
+    expect(find.text('4'), findsOneWidget);
+  });
+
   testWidgets('SliverGrid Correctly layout children after rearranging', (WidgetTester tester) async {
       await tester.pumpWidget(const TestSliverGrid(
         <Widget>[
@@ -332,3 +404,23 @@ class TestSliverFixedExtentList extends StatelessWidget {
     );
   }
 }
+
+class KeepAlive extends StatefulWidget {
+  const KeepAlive(this.data);
+
+  final String data;
+
+  @override
+  KeepAliveState createState() => KeepAliveState();
+}
+
+class KeepAliveState extends State<KeepAlive> with AutomaticKeepAliveClientMixin {
+  @override
+  bool get wantKeepAlive => true;
+
+  @override
+  Widget build(BuildContext context) {
+    super.build(context);
+    return Text(widget.data);
+  }
+}