CupertinoAlertDialog should not create ScrollController on every build, if null values are passed in constructor. (#134075)

Relanding of https://github.com/flutter/flutter/pull/134071

Verified failed tests succeeded now:

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

@@ -188,7 +188,7 @@ bool _isInAccessibilityMode(BuildContext context) {
 ///  * [CupertinoDialogAction], which is an iOS-style dialog button.
 ///  * [AlertDialog], a Material Design alert dialog.
 ///  * <https://developer.apple.com/ios/human-interface-guidelines/views/alerts/>
-class CupertinoAlertDialog extends StatelessWidget {
+class CupertinoAlertDialog extends StatefulWidget {
   /// Creates an iOS-style alert dialog.
   ///
   /// The [actions] must not be null.
@@ -233,9 +233,6 @@ class CupertinoAlertDialog extends StatelessWidget {
   ///    section when there are many actions.
   final ScrollController? scrollController;
 
-  ScrollController get _effectiveScrollController =>
-    scrollController ?? ScrollController();
-
   /// A scroll controller that can be used to control the scrolling of the
   /// actions in the dialog.
   ///
@@ -247,37 +244,49 @@ class CupertinoAlertDialog extends StatelessWidget {
   ///    section when it is long.
   final ScrollController? actionScrollController;
 
-  ScrollController get _effectiveActionScrollController =>
-    actionScrollController ?? ScrollController();
-
   /// {@macro flutter.material.dialog.insetAnimationDuration}
   final Duration insetAnimationDuration;
 
   /// {@macro flutter.material.dialog.insetAnimationCurve}
   final Curve insetAnimationCurve;
 
+  @override
+  State<CupertinoAlertDialog> createState() => _CupertinoAlertDialogState();
+}
+
+class _CupertinoAlertDialogState extends State<CupertinoAlertDialog> {
+  ScrollController? _backupScrollController;
+
+  ScrollController? _backupActionScrollController;
+
+  ScrollController get _effectiveScrollController =>
+    widget.scrollController ?? (_backupScrollController ??= ScrollController());
+
+  ScrollController get _effectiveActionScrollController =>
+    widget.actionScrollController ?? (_backupActionScrollController ??= ScrollController());
+
   Widget _buildContent(BuildContext context) {
     final double textScaleFactor = MediaQuery.textScalerOf(context).textScaleFactor;
 
     final List<Widget> children = <Widget>[
-      if (title != null || content != null)
+      if (widget.title != null || widget.content != null)
         Flexible(
           flex: 3,
           child: _CupertinoAlertContentSection(
-            title: title,
+            title: widget.title,
-            message: content,
+            message: widget.content,
             scrollController: _effectiveScrollController,
             titlePadding: EdgeInsets.only(
               left: _kDialogEdgePadding,
               right: _kDialogEdgePadding,
-              bottom: content == null ? _kDialogEdgePadding : 1.0,
+              bottom: widget.content == null ? _kDialogEdgePadding : 1.0,
               top: _kDialogEdgePadding * textScaleFactor,
             ),
             messagePadding: EdgeInsets.only(
               left: _kDialogEdgePadding,
               right: _kDialogEdgePadding,
               bottom: _kDialogEdgePadding * textScaleFactor,
-              top: title == null ? _kDialogEdgePadding : 1.0,
+              top: widget.title == null ? _kDialogEdgePadding : 1.0,
             ),
             titleTextStyle: _kCupertinoDialogTitleStyle.copyWith(
               color: CupertinoDynamicColor.resolve(CupertinoColors.label, context),
@@ -303,10 +312,10 @@ class CupertinoAlertDialog extends StatelessWidget {
     Widget actionSection = Container(
       height: 0.0,
     );
-    if (actions.isNotEmpty) {
+    if (widget.actions.isNotEmpty) {
       actionSection = _CupertinoAlertActionSection(
         scrollController: _effectiveActionScrollController,
-        children: actions,
+        children: widget.actions,
       );
     }
 
@@ -330,8 +339,8 @@ class CupertinoAlertDialog extends StatelessWidget {
               return AnimatedPadding(
                 padding: MediaQuery.viewInsetsOf(context) +
                     const EdgeInsets.symmetric(horizontal: 40.0, vertical: 24.0),
-                duration: insetAnimationDuration,
+                duration: widget.insetAnimationDuration,
-                curve: insetAnimationCurve,
+                curve: widget.insetAnimationCurve,
                 child: MediaQuery.removeViewInsets(
                   removeLeft: true,
                   removeTop: true,
@@ -368,6 +377,13 @@ class CupertinoAlertDialog extends StatelessWidget {
       ),
     );
   }
+
+  @override
+  void dispose() {
+    _backupScrollController?.dispose();
+    _backupActionScrollController?.dispose();
+    super.dispose();
+  }
 }
 
 /// Rounded rectangle surface that looks like an iOS popup surface, e.g., alert dialog

packages/flutter/test/material/dialog_test.dart

@@ -2664,7 +2664,7 @@ void main() {
     okNode.dispose();
   });
 
-  testWidgets('Adaptive AlertDialog shows correct widget on each platform', (WidgetTester tester) async {
+  testWidgetsWithLeakTracking('Adaptive AlertDialog shows correct widget on each platform', (WidgetTester tester) async {
     final AlertDialog dialog = AlertDialog.adaptive(
       content: Container(
         height: 5000.0,