Fix: Update CupertinoSheetRoute transition rounded corner (#163700)

Fix: Update CupertinoSheetRoute transition rounded corner
fixes: #163334 

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

@@ -11,6 +11,19 @@ import 'interface_level.dart';
 import 'route.dart';
 import 'theme.dart';
 
+// Smoothing factor applied to the device's top padding (which approximates the corner radius)
+// to achieve a smoother end to the corner radius animation.  A value of 1.0 would use
+// the full top padding. Values less than 1.0 reduce the effective corner radius, improving
+// the animation's appearance.  Determined through empirical testing.
+const double _kDeviceCornerRadiusSmoothingFactor = 0.9;
+
+// Threshold in logical pixels. If the calculated device corner radius (after applying
+// the smoothing factor) is below this value, the corner radius transition animation will
+// start from zero. This prevents abrupt transitions for devices with small or negligible
+// corner radii.  This value, combined with the smoothing factor, corresponds roughly
+// to double the targeted radius of 12.  Determined through testing and visual inspection.
+const double _kRoundedDeviceCornersThreshold = 20.0;
+
 // The distance from the top of the open sheet to the top of the screen, as a ratio
 // of the total height of the screen. Found from eyeballing a simulator running
 // iOS 18.0.
@@ -221,10 +234,15 @@ class CupertinoSheetTransition extends StatefulWidget {
       reverseCurve: reverseCurve,
       parent: secondaryAnimation,
     );
-    final double deviceCornerRadius = MediaQuery.maybeViewPaddingOf(context)?.top ?? 0;
+
+    final double deviceCornerRadius =
+        (MediaQuery.maybeViewPaddingOf(context)?.top ?? 0) * _kDeviceCornerRadiusSmoothingFactor;
+    final bool roundedDeviceCorners = deviceCornerRadius > _kRoundedDeviceCornersThreshold;
 
     final Animatable<BorderRadiusGeometry> decorationTween = Tween<BorderRadiusGeometry>(
-      begin: BorderRadius.circular(deviceCornerRadius),
+      begin: BorderRadius.vertical(
+        top: Radius.circular(roundedDeviceCorners ? deviceCornerRadius : 0),
+      ),
       end: BorderRadius.circular(12),
     );
 
@@ -262,7 +280,13 @@ class CupertinoSheetTransition extends StatefulWidget {
           animation: radiusAnimation,
           child: child,
           builder: (BuildContext context, Widget? child) {
-            return ClipRRect(borderRadius: radiusAnimation.value, child: contrastedChild);
+            return ClipRRect(
+              borderRadius:
+                  !secondaryAnimation.isDismissed
+                      ? radiusAnimation.value
+                      : BorderRadius.circular(0),
+              child: contrastedChild,
+            );
           },
         ),
       ),

packages/flutter/test/cupertino/sheet_test.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter/cupertino.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 import '../widgets/navigator_utils.dart';
@@ -745,6 +746,74 @@ void main() {
     expect(sheetNavBarHeight, lessThan(homeNavBardHeight));
   });
 
+  testWidgets('Previous route corner radius goes to same when sheet route is popped', (
+    WidgetTester tester,
+  ) async {
+    final GlobalKey scaffoldKey = GlobalKey();
+
+    await tester.pumpWidget(
+      CupertinoApp(
+        home: CupertinoPageScaffold(
+          key: scaffoldKey,
+          child: Column(
+            children: <Widget>[
+              const Text('Page 1'),
+              CupertinoButton(
+                onPressed: () {
+                  Navigator.push<void>(
+                    scaffoldKey.currentContext!,
+                    CupertinoSheetRoute<void>(
+                      builder: (BuildContext context) {
+                        return CupertinoPageScaffold(
+                          child: GestureDetector(
+                            onTap: () => Navigator.pop(context),
+                            child: const Icon(Icons.arrow_back_ios),
+                          ),
+                        );
+                      },
+                    ),
+                  );
+                },
+                child: const Text('Push Page 2'),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+
+    expect(find.text('Page 1'), findsOneWidget);
+    expect(
+      tester
+          .getTopLeft(
+            find.ancestor(of: find.text('Page 1'), matching: find.byType(CupertinoPageScaffold)),
+          )
+          .dy,
+      equals(0.0),
+    );
+    expect(find.byType(Icon), findsNothing);
+
+    await tester.tap(find.text('Push Page 2'));
+    await tester.pumpAndSettle();
+
+    // Previous page is still visible behind the new sheet.
+    expect(find.text('Page 1'), findsOneWidget);
+    final Offset pageOneOffset = tester.getTopLeft(
+      find.ancestor(of: find.text('Page 1'), matching: find.byType(CupertinoPageScaffold)),
+    );
+    expect(pageOneOffset.dy, greaterThan(0.0));
+    expect(pageOneOffset.dx, greaterThan(0.0));
+    expect(find.byType(Icon), findsOneWidget);
+
+    // Pop Sheet Route
+    await tester.tap(find.byType(Icon));
+    await tester.pumpAndSettle();
+
+    final Finder clipRRectFinder = find.byType(ClipRRect);
+    final Rect clipRect = tester.getRect(clipRRectFinder);
+    expect(clipRect.center, equals(const Offset(400, 300)));
+  });
+
   group('drag dismiss gesture', () {
     Widget dragGestureApp(GlobalKey homeScaffoldKey, GlobalKey sheetScaffoldKey) {
       return CupertinoApp(