Fix the layout calculation in sliver list where the scroll offset cor… (#59888)

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

@@ -120,7 +120,6 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
         earliestScrollOffset = childScrollOffset(earliestUsefulChild)) {
       // We have to add children before the earliestUsefulChild.
       earliestUsefulChild = insertAndLayoutLeadingChild(childConstraints, parentUsesSize: true);
-
       if (earliestUsefulChild == null) {
         final SliverMultiBoxAdaptorParentData childParentData = firstChild.parentData as SliverMultiBoxAdaptorParentData;
         childParentData.layoutOffset = 0.0;
@@ -148,29 +147,14 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
       final double firstChildScrollOffset = earliestScrollOffset - paintExtentOf(firstChild);
       // firstChildScrollOffset may contain double precision error
       if (firstChildScrollOffset < -precisionErrorTolerance) {
-        // The first child doesn't fit within the viewport (underflow) and
-        // there may be additional children above it. Find the real first child
-        // and then correct the scroll position so that there's room for all and
-        // so that the trailing edge of the original firstChild appears where it
-        // was before the scroll offset correction.
-        // TODO(hansmuller): do this work incrementally, instead of all at once,
-        // i.e. find a way to avoid visiting ALL of the children whose offset
-        // is < 0 before returning for the scroll correction.
-        double correction = 0.0;
-        while (earliestUsefulChild != null) {
-          assert(firstChild == earliestUsefulChild);
-          correction += paintExtentOf(firstChild);
-          earliestUsefulChild = insertAndLayoutLeadingChild(childConstraints, parentUsesSize: true);
-        }
-        earliestUsefulChild = firstChild;
-        if ((correction - earliestScrollOffset).abs() > precisionErrorTolerance) {
-          geometry = SliverGeometry(
-            scrollOffsetCorrection: correction - earliestScrollOffset,
-          );
-          final SliverMultiBoxAdaptorParentData childParentData = firstChild.parentData as SliverMultiBoxAdaptorParentData;
-          childParentData.layoutOffset = 0.0;
-          return;
-        }
+        // Let's assume there is no child before the first child. We will
+        // correct it on the next layout if it is not.
+        geometry = SliverGeometry(
+          scrollOffsetCorrection: -firstChildScrollOffset,
+        );
+        final SliverMultiBoxAdaptorParentData childParentData = firstChild.parentData as SliverMultiBoxAdaptorParentData;
+        childParentData.layoutOffset = 0.0;
+        return;
       }
 
       final SliverMultiBoxAdaptorParentData childParentData = earliestUsefulChild.parentData as SliverMultiBoxAdaptorParentData;
@@ -180,6 +164,28 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
       trailingChildWithLayout ??= earliestUsefulChild;
     }
 
+    assert(childScrollOffset(firstChild) > -precisionErrorTolerance);
+
+    // If the scroll offset is at zero, we should make sure we are
+    // actually at the beginning of the list.
+    if (scrollOffset < precisionErrorTolerance) {
+      if (indexOf(firstChild) > 0) {
+        final double earliestScrollOffset = childScrollOffset(firstChild);
+        // We correct one child at a time. If there are more children before
+        // the earliestUsefulChild, we will correct it once the scroll offset
+        // reach zero again.
+        earliestUsefulChild = insertAndLayoutLeadingChild(childConstraints, parentUsesSize: true);
+        assert(earliestUsefulChild != null);
+        final double firstChildScrollOffset = earliestScrollOffset - paintExtentOf(firstChild);
+        geometry = SliverGeometry(
+          scrollOffsetCorrection: -firstChildScrollOffset,
+        );
+        final SliverMultiBoxAdaptorParentData childParentData = firstChild.parentData as SliverMultiBoxAdaptorParentData;
+        childParentData.layoutOffset = 0.0;
+        return;
+      }
+    }
+
     // At this point, earliestUsefulChild is the first child, and is a child
     // whose scrollOffset is at or before the scrollOffset, and
     // leadingChildWithLayout and trailingChildWithLayout are either null or

packages/flutter/test/widgets/slivers_test.dart

@@ -308,6 +308,109 @@ void main() {
     },
   );
 
+  testWidgets(
+    'SliverList can handle inaccurate scroll offset due to changes in children list',
+      (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/pull/59888.
+      bool skip = true;
+      Widget _buildItem(BuildContext context, int index) {
+        return !skip || index.isEven
+          ? Card(
+          child: ListTile(
+            title: Text(
+              'item$index',
+              style: const TextStyle(fontSize: 80),
+            ),
+          ),
+        )
+          : Container();
+      }
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: CustomScrollView(
+              slivers: <Widget> [
+                SliverList(
+                  delegate: SliverChildBuilderDelegate(
+                    _buildItem,
+                    childCount: 30,
+                  ),
+                ),
+              ],
+            )
+          ),
+        ),
+      );
+      // Only even items 0~12 are on the screen.
+      expect(find.text('item0'), findsOneWidget);
+      expect(find.text('item12'), findsOneWidget);
+      expect(find.text('item14'), findsNothing);
+
+      await tester.drag(find.byType(CustomScrollView), const Offset(0.0, -750.0));
+      await tester.pump();
+      // Only even items 16~28 are on the screen.
+      expect(find.text('item15'), findsNothing);
+      expect(find.text('item16'), findsOneWidget);
+      expect(find.text('item28'), findsOneWidget);
+
+      skip = false;
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            body: CustomScrollView(
+              slivers: <Widget> [
+                SliverList(
+                  delegate: SliverChildBuilderDelegate(
+                    _buildItem,
+                    childCount: 30,
+                  ),
+                ),
+              ],
+            )
+          ),
+        ),
+      );
+
+      // Only items 12~19 are on the screen.
+      expect(find.text('item11'), findsNothing);
+      expect(find.text('item12'), findsOneWidget);
+      expect(find.text('item19'), findsOneWidget);
+      expect(find.text('item20'), findsNothing);
+
+      await tester.drag(find.byType(CustomScrollView), const Offset(0.0, 250.0));
+      await tester.pump();
+
+      // Only items 10~16 are on the screen.
+      expect(find.text('item9'), findsNothing);
+      expect(find.text('item10'), findsOneWidget);
+      expect(find.text('item16'), findsOneWidget);
+      expect(find.text('item17'), findsNothing);
+
+      // The inaccurate scroll offset should reach zero at this point
+      await tester.drag(find.byType(CustomScrollView), const Offset(0.0, 250.0));
+      await tester.pump();
+
+      // Only items 7~13 are on the screen.
+      expect(find.text('item6'), findsNothing);
+      expect(find.text('item7'), findsOneWidget);
+      expect(find.text('item13'), findsOneWidget);
+      expect(find.text('item14'), findsNothing);
+
+      // It will be corrected as we scroll, so we have to drag multiple times.
+      await tester.drag(find.byType(CustomScrollView), const Offset(0.0, 250.0));
+      await tester.pump();
+      await tester.drag(find.byType(CustomScrollView), const Offset(0.0, 250.0));
+      await tester.pump();
+      await tester.drag(find.byType(CustomScrollView), const Offset(0.0, 250.0));
+      await tester.pump();
+
+      // Only items 0~6 are on the screen.
+      expect(find.text('item0'), findsOneWidget);
+      expect(find.text('item6'), findsOneWidget);
+      expect(find.text('item7'), findsNothing);
+    },
+  );
+
   testWidgets(
     'SliverFixedExtentList Correctly layout children after rearranging',
     (WidgetTester tester) async {