Add error message and documentation when a `SnackBar` is off screen (#102073)

examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.1.dart

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

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

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

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

@@ -264,6 +264,23 @@ class ScaffoldMessengerState extends State<ScaffoldMessenger> with TickerProvide
   ///
   /// ** See code in examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.0.dart **
   /// {@end-tool}
+  ///
+  /// ## Relative positioning of floating SnackBars
+  ///
+  /// A [SnackBar] with [SnackBar.behavior] set to [SnackBarBehavior.floating] is
+  /// positioned above the widgets provided to [Scaffold.floatingActionButton],
+  /// [Scaffold.persistentFooterButtons], and [Scaffold.bottomNavigationBar].
+  /// If some or all of these widgets take up enough space such that the SnackBar
+  /// would not be visible when positioned above them, an error will be thrown.
+  /// In this case, consider constraining the size of these widgets to allow room for
+  /// the SnackBar to be visible.
+  ///
+  /// {@tool dartpad}
+  /// Here is an example showing that a floating [SnackBar] appears above [Scaffold.floatingActionButton].
+  ///
+  /// ** See code in examples/api/lib/material/scaffold/scaffold_messenger_state.show_snack_bar.1.dart **
+  /// {@end-tool}
+  ///
   ScaffoldFeatureController<SnackBar, SnackBarClosedReason> showSnackBar(SnackBar snackBar) {
     assert(
       _scaffolds.isNotEmpty,
@@ -1118,6 +1135,32 @@ class _ScaffoldLayout extends MultiChildLayoutDelegate {
 
       final double xOffset = hasCustomWidth ? (size.width - snackBarWidth!) / 2 : 0.0;
       positionChild(_ScaffoldSlot.snackBar, Offset(xOffset, snackBarYOffsetBase - snackBarSize.height));
+
+      assert((){
+        // Whether a floating SnackBar has been offsetted too high.
+        //
+        // To improve the developper experience, this assert is done after the call to positionChild.
+        // if we assert sooner the SnackBar is visible because its defaults position is (0,0) and
+        // it can cause confusion to the user as the error message states that the SnackBar is off screen.
+        if (isSnackBarFloating) {
+          final bool snackBarVisible = (snackBarYOffsetBase - snackBarSize.height) > 0;
+          if (!snackBarVisible) {
+            throw FlutterError.fromParts(<DiagnosticsNode>[
+              ErrorSummary('Floating SnackBar presented off screen.'),
+              ErrorDescription(
+                'A SnackBar with behavior property set to SnackBarBehavior.floating is fully '
+                'or partially off screen because some or all the widgets provided to '
+                'Scaffold.floatingActionButton, Scaffold.persistentFooterButtons and '
+                'Scaffold.bottomNavigationBar take up too much vertical space.\n'
+              ),
+              ErrorHint(
+                'Consider constraining the size of these widgets to allow room for the SnackBar to be visible.',
+              ),
+            ]);
+          }
+        }
+        return true;
+      }());
     }
 
     if (hasChild(_ScaffoldSlot.statusBar)) {

packages/flutter/test/material/snack_bar_test.dart

@@ -1652,6 +1652,72 @@ void main() {
       },
     );
 
+    void openFloatingSnackBar(WidgetTester tester) {
+      final ScaffoldMessengerState scaffoldMessengerState = tester.state(find.byType(ScaffoldMessenger));
+      scaffoldMessengerState.showSnackBar(
+        const SnackBar(
+          content: Text('SnackBar text'),
+          behavior: SnackBarBehavior.floating,
+        ),
+      );
+    }
+
+    void expectSnackBarNotVisibleError(WidgetTester tester) {
+      final AssertionError exception = tester.takeException() as AssertionError;
+      const String message = 'Floating SnackBar presented off screen.\n'
+        'A SnackBar with behavior property set to SnackBarBehavior.floating is fully '
+        'or partially off screen because some or all the widgets provided to '
+        'Scaffold.floatingActionButton, Scaffold.persistentFooterButtons and '
+        'Scaffold.bottomNavigationBar take up too much vertical space.\n'
+        'Consider constraining the size of these widgets to allow room for the SnackBar to be visible.';
+      expect(exception.message, message);
+    }
+
+    testWidgets('Snackbar with SnackBarBehavior.floating will assert when offsetted too high by a large Scaffold.floatingActionButton', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/84263
+      await tester.pumpWidget(
+        MaterialApp(
+          home: Scaffold(
+            floatingActionButton: Container(),
+          ),
+        ),
+      );
+
+      openFloatingSnackBar(tester);
+      await tester.pumpAndSettle(); // Have the SnackBar fully animate out.
+      expectSnackBarNotVisibleError(tester);
+    });
+
+    testWidgets('Snackbar with SnackBarBehavior.floating will assert when offsetted too high by a large Scaffold.persistentFooterButtons', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/84263
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Scaffold(
+            persistentFooterButtons: <Widget>[SizedBox(height: 1000)],
+          ),
+        ),
+      );
+
+      openFloatingSnackBar(tester);
+      await tester.pumpAndSettle(); // Have the SnackBar fully animate out.
+      expectSnackBarNotVisibleError(tester);
+    });
+
+    testWidgets('Snackbar with SnackBarBehavior.floating will assert when offsetted too high by a large Scaffold.bottomNavigationBar', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/84263
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Scaffold(
+            bottomNavigationBar: SizedBox(height: 1000),
+          ),
+        ),
+      );
+
+      openFloatingSnackBar(tester);
+      await tester.pumpAndSettle(); // Have the SnackBar fully animate out.
+      expectSnackBarNotVisibleError(tester);
+    });
+
     testWidgets(
       'SnackBar has correct end padding when it contains an action with fixed behavior',
       (WidgetTester tester) async {