Flutter – Zero widgets with type "SliverAppBar" which is an ancestor of text "SliverAppBar"

I’m trying to write test for my SliverAppBar example app. But failing to find title text ("SliverAppBar") of flexibleSpace after dragging down first item of SliverList by test.

Here is the code of my app:

import 'package:flutter/material.dart';

void main() {
  runApp(const StretchableSliverAppBar());
}

class StretchableSliverAppBar extends StatefulWidget {
  const StretchableSliverAppBar({super.key});

  @override
  State<StretchableSliverAppBar> createState() =>
      _StretchableSliverAppBarState();
}

class _StretchableSliverAppBarState extends State<StretchableSliverAppBar> {
  bool _stretch = true;
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
      body: CustomScrollView(
        physics: const BouncingScrollPhysics(),
        slivers: <Widget>[
          SliverAppBar(
            stretch: _stretch,
            onStretchTrigger: () async {},
            expandedHeight: 200.0,
            flexibleSpace: const FlexibleSpaceBar(
              title: Text('SliverAppBar'),
              background: FlutterLogo(),
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate(
              (BuildContext context, int index) {
                return Container(
                  color: index.isOdd ? Colors.white : Colors.black12,
                  height: 100.0,
                  child: Center(
                    child: Text('$index', textScaleFactor: 5),
                  ),
                );
              },
              childCount: 20,
            ),
          ),
        ],
      ),
    ));
  }
}

The test code:

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:test_project/main.dart';

const Offset _kOffset = Offset(0.0, -200.0);

void main() {
  testWidgets('SliverAppbar can be stretched', (WidgetTester tester) async {
    await tester.pumpWidget(
      const StretchableSliverAppBar(),
    );

    expect(find.text('SliverAppBar'), findsOneWidget);
    expect(tester.getBottomLeft(find.text('SliverAppBar')).dy, 184.0);

    await tester.drag(find.text('0'), _kOffset,
        touchSlopY: 0, warnIfMissed: false);
    await tester.pump();
    await tester.pump(const Duration(milliseconds: 500));
    // Failing to find this text "SliverAppBar"
    expect(find.text('SliverAppBar'), findsOneWidget);
 });
}

And this the error found in the test result

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following TestFailure was thrown running a test:
Expected: exactly one matching node in the widget tree
  Actual: _TextFinder:<zero widgets with text "SliverAppBar" (ignoring offstage widgets)>
   Which: means none were found but one was expected

When the exception was thrown, this was the stack:
#4      main.<anonymous closure> (file:///D:/Flutter_Projects/test_project/test/widget_test.dart:56:5)
<asynchronous suspension>
<asynchronous suspension>
(elided one frame from package:stack_trace)

This was caught by the test expectation on the following line:
  file:///D:/Flutter_Projects/test_project/test/widget_test.dart line 56
The test description was:
  SliverAppbar can be stretched
════════════════════════════════════════════════════════════════════════════════════════════════════
Test failed. See exception logs above.
The test description was: SliverAppbar can be stretched

✖ SliverAppbar can be stretched

Now, shouldn’t the test be passed?