Implement AlertDialog title/content overflow scroll (#43226)

* Wrap alert dialog title and content in single child scroll view

* Scrollable alert dialog title and content tests

* Remove unnecessary comment

* Fix mainAxisSize and crossAxisAlignment issue

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

@@ -133,15 +133,6 @@ class Dialog extends StatelessWidget {
 /// of actions. The title is displayed above the content and the actions are
 /// displayed below the content.
 ///
-/// If the content is too large to fit on the screen vertically, the dialog will
-/// display the title and the actions and let the content overflow, which is
-/// rarely desired. Consider using a scrolling widget for [content], such as
-/// [SingleChildScrollView], to avoid overflow. (However, be aware that since
-/// [AlertDialog] tries to size itself using the intrinsic dimensions of its
-/// children, widgets such as [ListView], [GridView], and [CustomScrollView],
-/// which use lazy viewports, will not work. If this is a problem, consider
-/// using [Dialog] directly.)
-///
 /// For dialogs that offer the user a choice between several options, consider
 /// using a [SimpleDialog].
 ///
@@ -161,13 +152,11 @@ class Dialog extends StatelessWidget {
 ///     builder: (BuildContext context) {
 ///       return AlertDialog(
 ///         title: Text('Rewind and remember'),
-///         content: SingleChildScrollView(
-///           child: ListBody(
-///             children: <Widget>[
-///               Text('You will never be satisfied.'),
-///               Text('You\’re like me. I’m never satisfied.'),
-///             ],
-///           ),
+///         content: Column(
+///           children: <Widget>[
+///             Text('You will never be satisfied.'),
+///             Text('You\’re like me. I’m never satisfied.'),
+///           ],
 ///         ),
 ///         actions: <Widget>[
 ///           FlatButton(
@@ -321,25 +310,34 @@ class AlertDialog extends StatelessWidget {
         mainAxisSize: MainAxisSize.min,
         crossAxisAlignment: CrossAxisAlignment.stretch,
         children: <Widget>[
-          if (title != null)
-            Padding(
-              padding: titlePadding ?? EdgeInsets.fromLTRB(24.0, 24.0, 24.0, content == null ? 20.0 : 0.0),
-              child: DefaultTextStyle(
-                style: titleTextStyle ?? dialogTheme.titleTextStyle ?? theme.textTheme.title,
-                child: Semantics(
-                  child: title,
-                  namesRoute: true,
-                  container: true,
-                ),
-              ),
-            ),
-          if (content != null)
+          if (title != null || content != null)
             Flexible(
-              child: Padding(
-                padding: contentPadding,
-                child: DefaultTextStyle(
-                  style: contentTextStyle ?? dialogTheme.contentTextStyle ?? theme.textTheme.subhead,
-                  child: content,
+              child: SingleChildScrollView(
+                child: Column(
+                  mainAxisSize: MainAxisSize.min,
+                  crossAxisAlignment: CrossAxisAlignment.stretch,
+                  children: <Widget>[
+                    if (title != null)
+                      Padding(
+                        padding: titlePadding ?? EdgeInsets.fromLTRB(24.0, 24.0, 24.0, content == null ? 20.0 : 0.0),
+                        child: DefaultTextStyle(
+                          style: titleTextStyle ?? dialogTheme.titleTextStyle ?? theme.textTheme.title,
+                          child: Semantics(
+                            child: title,
+                            namesRoute: true,
+                            container: true,
+                          ),
+                        ),
+                      ),
+                    if (content != null)
+                      Padding(
+                        padding: contentPadding,
+                        child: DefaultTextStyle(
+                          style: contentTextStyle ?? dialogTheme.contentTextStyle ?? theme.textTheme.subhead,
+                          child: content,
+                        ),
+                      ),
+                  ],
                 ),
               ),
             ),

packages/flutter/test/material/dialog_test.dart

@@ -645,4 +645,81 @@ void main() {
     await tester.pumpWidget(buildFrame(UniqueKey()));
     await tester.pump();
   });
+
+  group('Scrollable title and content', () {
+    testWidgets('Title is scrollable', (WidgetTester tester) async {
+      final Key titleKey = UniqueKey();
+      final AlertDialog dialog = AlertDialog(
+        title: Container(
+          key: titleKey,
+          color: Colors.green,
+          height: 1000,
+        ),
+      );
+      await tester.pumpWidget(_appWithAlertDialog(tester, dialog));
+      await tester.tap(find.text('X'));
+      await tester.pumpAndSettle();
+
+      final RenderBox box = tester.renderObject(find.byKey(titleKey));
+      final Offset originalOffset = box.localToGlobal(Offset.zero);
+      await tester.drag(find.byKey(titleKey), const Offset(0.0, -200.0));
+      expect(box.localToGlobal(Offset.zero), equals(originalOffset.translate(0.0, -200.0)));
+    });
+
+    testWidgets('Content is scrollable', (WidgetTester tester) async {
+      final Key contentKey = UniqueKey();
+      final AlertDialog dialog = AlertDialog(
+        content: Container(
+          key: contentKey,
+          color: Colors.orange,
+          height: 1000,
+        ),
+      );
+      await tester.pumpWidget(_appWithAlertDialog(tester, dialog));
+      await tester.tap(find.text('X'));
+      await tester.pumpAndSettle();
+
+      final RenderBox box = tester.renderObject(find.byKey(contentKey));
+      final Offset originalOffset = box.localToGlobal(Offset.zero);
+      await tester.drag(find.byKey(contentKey), const Offset(0.0, -200.0));
+      expect(box.localToGlobal(Offset.zero), equals(originalOffset.translate(0.0, -200.0)));
+    });
+
+    testWidgets('Title and content are scrollable', (WidgetTester tester) async {
+      final Key titleKey = UniqueKey();
+      final Key contentKey = UniqueKey();
+      final AlertDialog dialog = AlertDialog(
+        title: Container(
+          key: titleKey,
+          color: Colors.green,
+          height: 400,
+        ),
+        content: Container(
+          key: contentKey,
+          color: Colors.orange,
+          height: 400,
+        ),
+      );
+      await tester.pumpWidget(_appWithAlertDialog(tester, dialog));
+      await tester.tap(find.text('X'));
+      await tester.pumpAndSettle();
+
+      final RenderBox title = tester.renderObject(find.byKey(titleKey));
+      final RenderBox content = tester.renderObject(find.byKey(contentKey));
+      final Offset titleOriginalOffset = title.localToGlobal(Offset.zero);
+      final Offset contentOriginalOffset = content.localToGlobal(Offset.zero);
+
+      // Dragging the title widget should scroll both the title
+      // and the content widgets.
+      await tester.drag(find.byKey(titleKey), const Offset(0.0, -200.0));
+      expect(title.localToGlobal(Offset.zero), equals(titleOriginalOffset.translate(0.0, -200.0)));
+      expect(content.localToGlobal(Offset.zero), equals(contentOriginalOffset.translate(0.0, -200.0)));
+
+      // Dragging the content widget should scroll both the title
+      // and the content widgets.
+      await tester.drag(find.byKey(contentKey), const Offset(0.0, 200.0));
+      expect(title.localToGlobal(Offset.zero), equals(titleOriginalOffset));
+      expect(content.localToGlobal(Offset.zero), equals(contentOriginalOffset));
+    });
+  });
 }

packages/flutter/test/material/dialog_theme_test.dart

@@ -10,23 +10,23 @@ MaterialApp _appWithAlertDialog(WidgetTester tester, AlertDialog dialog, { Theme
   return MaterialApp(
     theme: theme,
     home: Material(
-        child: Builder(
-            builder: (BuildContext context) {
-              return Center(
-                  child: RaisedButton(
-                      child: const Text('X'),
-                      onPressed: () {
-                        showDialog<void>(
-                          context: context,
-                          builder: (BuildContext context) {
-                            return RepaintBoundary(key: _painterKey, child: dialog);
-                          },
-                        );
-                      },
-                  ),
-              );
-            }
-        ),
+      child: Builder(
+        builder: (BuildContext context) {
+          return Center(
+            child: RaisedButton(
+              child: const Text('X'),
+              onPressed: () {
+                showDialog<void>(
+                  context: context,
+                  builder: (BuildContext context) {
+                    return RepaintBoundary(key: _painterKey, child: dialog);
+                  },
+                );
+              },
+            ),
+          );
+        },
+      ),
     ),
   );
 }