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].
 ///
@@ -161,14 +152,12 @@ class Dialog extends StatelessWidget {
 ///     builder: (BuildContext context) {
 ///       return AlertDialog(
 ///         title: Text('Rewind and remember'),
-///         content: SingleChildScrollView(
-///           child: ListBody(
+///         content: Column(
 ///           children: <Widget>[
 ///             Text('You will never be satisfied.'),
 ///             Text('You\’re like me. I’m never satisfied.'),
 ///           ],
 ///         ),
-///         ),
 ///         actions: <Widget>[
 ///           FlatButton(
 ///             child: Text('Regret'),
@@ -317,6 +306,13 @@ class AlertDialog extends StatelessWidget {
     }
 
     Widget dialogChild = IntrinsicWidth(
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        children: <Widget>[
+          if (title != null || content != null)
+            Flexible(
+              child: SingleChildScrollView(
                 child: Column(
                   mainAxisSize: MainAxisSize.min,
                   crossAxisAlignment: CrossAxisAlignment.stretch,
@@ -334,14 +330,16 @@ class AlertDialog extends StatelessWidget {
                         ),
                       ),
                     if (content != null)
-            Flexible(
-              child: Padding(
+                      Padding(
                         padding: contentPadding,
                         child: DefaultTextStyle(
                           style: contentTextStyle ?? dialogTheme.contentTextStyle ?? theme.textTheme.subhead,
                           child: content,
                         ),
                       ),
+                  ],
+                ),
+              ),
             ),
           if (actions != null)
             ButtonBar(

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

@@ -25,7 +25,7 @@ MaterialApp _appWithAlertDialog(WidgetTester tester, AlertDialog dialog, { Theme
               },
             ),
           );
-            }
+        },
       ),
     ),
   );