SliverList perform layout start from the initial child when there is no valid...

SliverList perform layout start from the initial child when there is no valid layout offset (#66262)

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

@@ -101,14 +101,22 @@ class RenderSliverList extends RenderSliverMultiBoxAdaptor {
     // offset.
     if (childScrollOffset(firstChild!) == null) {
       int leadingChildrenWithoutLayoutOffset = 0;
-      while (childScrollOffset(earliestUsefulChild!) == null) {
-        earliestUsefulChild = childAfter(firstChild!);
+      while (earliestUsefulChild != null && childScrollOffset(earliestUsefulChild) == null) {
+        earliestUsefulChild = childAfter(earliestUsefulChild);
         leadingChildrenWithoutLayoutOffset += 1;
       }
       // We should be able to destroy children with null layout offset safely,
       // because they are likely outside of viewport
       collectGarbage(leadingChildrenWithoutLayoutOffset, 0);
-      assert(firstChild != null);
+      // If can not find a valid layout offset, start from the initial child.
+      if (firstChild == null) {
+        if (!addInitialChild()) {
+          // There are no children.
+          geometry = SliverGeometry.zero;
+          childManager.didFinishLayout();
+          return;
+        }
+      }
     }
 
     // Find the last child that is at or before the scrollOffset.

packages/flutter/test/widgets/sliver_list_test.dart

@@ -277,6 +277,65 @@ void main() {
     expect(find.text('Tile 18'), findsOneWidget);
     expect(find.text('Tile 3'), findsOneWidget);
   });
+
+  testWidgets('SliverList should start to perform layout from the initial child when there is no valid offset', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/66198.
+    bool isShow = true;
+    final ScrollController controller = ScrollController();
+    Widget buildSliverList(ScrollController controller) {
+      return Directionality(
+        textDirection: TextDirection.ltr,
+        child: Center(
+          child: Container(
+            height: 200,
+            child: ListView(
+              controller: controller,
+              children: <Widget>[
+                if (isShow)
+                  for (int i = 0; i < 20; i++)
+                    Container(
+                      height: 50,
+                      child: Text('Tile $i'),
+                    ),
+                const SizedBox(), // Use this widget to occupy the position where the offset is 0 when rebuild
+                const SizedBox(key: Key('key0'), height: 50.0),
+                const SizedBox(key: Key('key1'), height: 50.0),
+              ],
+            ),
+          ),
+        ),
+      );
+    }
+
+    await tester.pumpWidget(buildSliverList(controller));
+    await tester.pumpAndSettle();
+
+    // Scrolling to the bottom.
+    await tester.drag(find.text('Tile 2'), const Offset(0.0, -1000.0));
+    await tester.pumpAndSettle();
+
+    // Viewport should be scrolled to the end of list.
+    expect(controller.offset, 900.0);
+    expect(find.text('Tile 17'), findsNothing);
+    expect(find.text('Tile 18'), findsOneWidget);
+    expect(find.text('Tile 19'), findsOneWidget);
+    expect(find.byKey(const Key('key0')), findsOneWidget);
+    expect(find.byKey(const Key('key1')), findsOneWidget);
+
+    // Trigger rebuild.
+    isShow = false;
+    await tester.pumpWidget(buildSliverList(controller));
+
+    // After rebuild, [ContainerRenderObjectMixin] has two children, and
+    // neither of them has a valid layout offset.
+    // SliverList can layout normally without any assert or dead loop.
+    // Only the 'SizeBox' show in the viewport.
+    expect(controller.offset, 0.0);
+    expect(find.text('Tile 0'), findsNothing);
+    expect(find.text('Tile 19'), findsNothing);
+    expect(find.byKey(const Key('key0')), findsOneWidget);
+    expect(find.byKey(const Key('key1')), findsOneWidget);
+  });
 }
 
 Widget _buildSliverListRenderWidgetChild(List<String> items) {