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Add error message and documentation when a SnackBar is off screen (#102073)
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‎examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.1.dart‎

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// Copyright 2014 The Flutter Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// Flutter code sample for SnackBar
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import 'package:flutter/material.dart';
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void main() => runApp(const SnackBarApp());
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class SnackBarApp extends StatelessWidget {
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const SnackBarApp({super.key});
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@override
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Widget build(BuildContext context) {
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return const MaterialApp(
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home: SnackBarExample(),
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);
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}
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}
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class SnackBarExample extends StatefulWidget {
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const SnackBarExample({super.key});
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@override
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State<SnackBarExample> createState() => _SnackBarExampleState();
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}
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class _SnackBarExampleState extends State<SnackBarExample> {
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bool _largeLogo = false;
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@override
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Widget build(BuildContext context) {
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return Scaffold(
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appBar: AppBar(title: const Text('SnackBar Sample')),
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body: Padding(
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padding: const EdgeInsets.all(8.0),
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child: Column(
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children: <Widget>[
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ElevatedButton(
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onPressed: () {
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const SnackBar snackBar = SnackBar(
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content: Text('A SnackBar has been shown.'),
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behavior: SnackBarBehavior.floating,
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);
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ScaffoldMessenger.of(context).showSnackBar(snackBar);
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},
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child: const Text('Show SnackBar'),
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),
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const SizedBox(height: 8.0),
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ElevatedButton(
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onPressed: () {
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setState(() => _largeLogo = !_largeLogo);
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},
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child: Text(_largeLogo ? 'Shrink Logo' : 'Grow Logo'),
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),
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],
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),
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),
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// A floating [SnackBar] is positioned above [Scaffold.floatingActionButton].
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// If the Widget provided to the floatingActionButton slot takes up too much space
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// for the SnackBar to be visible, an error will be thrown.
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floatingActionButton: Container(
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constraints: BoxConstraints.tightFor(
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width: 150,
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height: _largeLogo ? double.infinity : 150,
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),
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decoration: const BoxDecoration(
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color: Colors.blueGrey,
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borderRadius: BorderRadius.all(Radius.circular(20)),
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),
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child: const FlutterLogo(),
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),
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);
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}
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}

‎examples/api/test/material/scaffold/scaffold_messenger_state.show_snack_bar.1_test.dart‎

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// Copyright 2014 The Flutter Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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import 'package:flutter/material.dart';
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import 'package:flutter_api_samples/material/scaffold/scaffold_messenger_state.show_snack_bar.1.dart' as example;
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import 'package:flutter_test/flutter_test.dart';
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void main() {
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testWidgets('Floating SnackBar is visible', (WidgetTester tester) async {
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await tester.pumpWidget(
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const example.SnackBarApp(),
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);
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final Finder buttonFinder = find.byType(ElevatedButton);
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await tester.tap(buttonFinder.first);
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// Have the SnackBar fully animate out.
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await tester.pumpAndSettle();
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final Finder snackBarFinder = find.byType(SnackBar);
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expect(snackBarFinder, findsOneWidget);
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// Grow logo to send SnackBar off screen.
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await tester.tap(buttonFinder.last);
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await tester.pumpAndSettle();
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final AssertionError exception = tester.takeException() as AssertionError;
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const String message = 'Floating SnackBar presented off screen.\n'
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'A SnackBar with behavior property set to SnackBarBehavior.floating is fully '
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'or partially off screen because some or all the widgets provided to '
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'Scaffold.floatingActionButton, Scaffold.persistentFooterButtons and '
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'Scaffold.bottomNavigationBar take up too much vertical space.\n'
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'Consider constraining the size of these widgets to allow room for the SnackBar to be visible.';
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expect(exception.message, message);
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});
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}

‎packages/flutter/lib/src/material/scaffold.dart‎

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///
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/// ** See code in examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.0.dart **
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/// {@end-tool}
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///
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/// ## Relative positioning of floating SnackBars
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///
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/// A [SnackBar] with [SnackBar.behavior] set to [SnackBarBehavior.floating] is
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/// positioned above the widgets provided to [Scaffold.floatingActionButton],
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/// [Scaffold.persistentFooterButtons], and [Scaffold.bottomNavigationBar].
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/// If some or all of these widgets take up enough space such that the SnackBar
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/// would not be visible when positioned above them, an error will be thrown.
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/// In this case, consider constraining the size of these widgets to allow room for
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/// the SnackBar to be visible.
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///
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/// {@tool dartpad}
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/// Here is an example showing that a floating [SnackBar] appears above [Scaffold.floatingActionButton].
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///
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/// ** See code in examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.1.dart **
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/// {@end-tool}
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///
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ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(SnackBar snackBar) {
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assert(
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_scaffolds.isNotEmpty,
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final double xOffset = hasCustomWidth ? (size.width - snackBarWidth!) / 2 : 0.0;
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positionChild(_ScaffoldSlot.snackBar, Offset(xOffset, snackBarYOffsetBase - snackBarSize.height));
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assert((){
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// Whether a floating SnackBar has been offsetted too high.
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//
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// To improve the developper experience, this assert is done after the call to positionChild.
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// if we assert sooner the SnackBar is visible because its defaults position is (0,0) and
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// it can cause confusion to the user as the error message states that the SnackBar is off screen.
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if (isSnackBarFloating) {
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final bool snackBarVisible = (snackBarYOffsetBase - snackBarSize.height) > 0;
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if (!snackBarVisible) {
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throw FlutterError.fromParts(<DiagnosticsNode>[
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ErrorSummary('Floating SnackBar presented off screen.'),
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ErrorDescription(
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'A SnackBar with behavior property set to SnackBarBehavior.floating is fully '
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'or partially off screen because some or all the widgets provided to '
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'Scaffold.floatingActionButton, Scaffold.persistentFooterButtons and '
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'Scaffold.bottomNavigationBar take up too much vertical space.\n'
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),
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ErrorHint(
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'Consider constraining the size of these widgets to allow room for the SnackBar to be visible.',
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),
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]);
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}
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}
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return true;
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}());
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}
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if (hasChild(_ScaffoldSlot.statusBar)) {

‎packages/flutter/test/material/snack_bar_test.dart‎

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},
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);
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void openFloatingSnackBar(WidgetTester tester) {
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final ScaffoldMessengerState scaffoldMessengerState = tester.state(find.byType(ScaffoldMessenger));
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scaffoldMessengerState.showSnackBar(
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const SnackBar(
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content: Text('SnackBar text'),
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behavior: SnackBarBehavior.floating,
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),
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);
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}
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void expectSnackBarNotVisibleError(WidgetTester tester) {
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final AssertionError exception = tester.takeException() as AssertionError;
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const String message = 'Floating SnackBar presented off screen.\n'
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'A SnackBar with behavior property set to SnackBarBehavior.floating is fully '
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'or partially off screen because some or all the widgets provided to '
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'Scaffold.floatingActionButton, Scaffold.persistentFooterButtons and '
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'Scaffold.bottomNavigationBar take up too much vertical space.\n'
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'Consider constraining the size of these widgets to allow room for the SnackBar to be visible.';
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expect(exception.message, message);
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}
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testWidgets('Snackbar with SnackBarBehavior.floating will assert when offsetted too high by a large Scaffold.floatingActionButton', (WidgetTester tester) async {
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// Regression test for https://github.com/flutter/flutter/issues/84263
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await tester.pumpWidget(
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MaterialApp(
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home: Scaffold(
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floatingActionButton: Container(),
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),
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),
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);
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openFloatingSnackBar(tester);
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await tester.pumpAndSettle(); // Have the SnackBar fully animate out.
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expectSnackBarNotVisibleError(tester);
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});
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testWidgets('Snackbar with SnackBarBehavior.floating will assert when offsetted too high by a large Scaffold.persistentFooterButtons', (WidgetTester tester) async {
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// Regression test for https://github.com/flutter/flutter/issues/84263
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await tester.pumpWidget(
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const MaterialApp(
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home: Scaffold(
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persistentFooterButtons: <Widget>[SizedBox(height: 1000)],
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),
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),
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);
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openFloatingSnackBar(tester);
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await tester.pumpAndSettle(); // Have the SnackBar fully animate out.
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expectSnackBarNotVisibleError(tester);
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});
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testWidgets('Snackbar with SnackBarBehavior.floating will assert when offsetted too high by a large Scaffold.bottomNavigationBar', (WidgetTester tester) async {
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// Regression test for https://github.com/flutter/flutter/issues/84263
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await tester.pumpWidget(
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const MaterialApp(
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home: Scaffold(
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bottomNavigationBar: SizedBox(height: 1000),
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),
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),
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);
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openFloatingSnackBar(tester);
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await tester.pumpAndSettle(); // Have the SnackBar fully animate out.
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expectSnackBarNotVisibleError(tester);
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});
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testWidgets(
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'SnackBar has correct end padding when it contains an action with fixed behavior',
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(WidgetTester tester) async {

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