fix a CupertinoDatePicker bug (#112697)

Author: xu-baolin

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

@@ -681,10 +681,14 @@ class RenderListWheelViewport
   /// by [childManager].
   @override
   void performLayout() {
-    // Apply the dimensions first in case it changes the scroll offset which
-    // determines what should be shown.
     offset.applyViewportDimension(_viewportExtent);
-    offset.applyContentDimensions(_minEstimatedScrollExtent, _maxEstimatedScrollExtent);
+    // Apply the content dimensions first if it has exact dimensions in case it
+    // changes the scroll offset which determines what should be shown. Such as
+    // if the child count decrease, we should correct the pixels first, otherwise,
+    // it may be shown blank null children.
+    if (childManager.childCount != null) {
+      offset.applyContentDimensions(_minEstimatedScrollExtent, _maxEstimatedScrollExtent);
+    }
 
     // The height, in pixel, that children will be visible and might be laid out
     // and painted.

packages/flutter/test/widgets/list_wheel_scroll_view_test.dart

@@ -288,6 +288,44 @@ void main() {
   });
 
   group('layout', () {
+    testWidgets('Flings with high velocity should not break the children lower and upper limits', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/112526
+      final FixedExtentScrollController controller = FixedExtentScrollController();
+      Widget buildFrame() {
+        return Directionality(
+          textDirection: TextDirection.ltr,
+          child: ListWheelScrollView.useDelegate(
+            physics: const FixedExtentScrollPhysics(),
+            controller: controller,
+            itemExtent: 400.0,
+            onSelectedItemChanged: (_) { },
+            childDelegate: ListWheelChildBuilderDelegate(
+              builder: (BuildContext context, int index) {
+                if (index < 0 || index > 5) {
+                  return null;
+                }
+                return SizedBox(
+                  width: 400.0,
+                  height: 400.0,
+                  child: Text(index.toString()),
+                );
+              },
+            ),
+          ),
+        );
+      }
+
+      await tester.pumpWidget(buildFrame());
+      expect(tester.renderObject(find.text('0')).attached, true);
+      expect(tester.renderObject(find.text('1')).attached, true);
+      expect(find.text('2'), findsNothing);
+      expect(controller.selectedItem, 0);
+
+      // Flings with high velocity and stop at the child boundary.
+      await tester.fling(find.byType(ListWheelScrollView), const Offset(0.0, 40000.0), 8000.0);
+      expect(controller.selectedItem, 0);
+    }, variant: TargetPlatformVariant(TargetPlatform.values.toSet()));
+
     // Regression test for https://github.com/flutter/flutter/issues/90953
     testWidgets('ListWheelScrollView childDelegate update test 2', (WidgetTester tester) async {
       final FixedExtentScrollController controller = FixedExtentScrollController( initialItem: 2 );