Fix: Update sheet route bottom padding with top padding (flutter#164473)

Fix: Update sheet route bottom padding with top padding
fixes: flutter#163572 
fixes: flutter#164087 

## 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.

Author: rkishan516

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

@@ -35,7 +35,7 @@ const double _kTopGapRatio = 0.08;
 // the top of the screen. Values found from eyeballing a simulator running iOS 18.0.
 final Animatable<Offset> _kBottomUpTween = Tween<Offset>(
   begin: const Offset(0.0, 1.0),
-  end: const Offset(0.0, _kTopGapRatio),
+  end: Offset.zero,
 );
 
 // Offset change for when a new sheet covers another sheet. '0.0' represents the
@@ -427,10 +427,7 @@ class _CupertinoSheetTransitionState extends State<CupertinoSheetTransition> {
         scale: _secondaryScaleAnimation,
         filterQuality: FilterQuality.medium,
         alignment: Alignment.topCenter,
-        child: ClipRRect(
-          borderRadius: const BorderRadius.vertical(top: Radius.circular(12)),
-          child: child,
-        ),
+        child: child,
       ),
     );
   }
@@ -444,10 +441,7 @@ class _CupertinoSheetTransitionState extends State<CupertinoSheetTransition> {
           context,
           widget.primaryRouteAnimation,
           widget.linearTransition,
-          ClipRRect(
-            borderRadius: const BorderRadius.vertical(top: Radius.circular(12)),
-            child: widget.child,
-          ),
+          widget.child,
         ),
       ),
     );
@@ -498,16 +492,19 @@ class CupertinoSheetRoute<T> extends PageRoute<T> with _CupertinoSheetRouteTrans
 
   @override
   Widget buildContent(BuildContext context) {
-    final double bottomPadding = MediaQuery.sizeOf(context).height * _kTopGapRatio;
+    final double topPadding = MediaQuery.sizeOf(context).height * _kTopGapRatio;
     return MediaQuery.removePadding(
       context: context,
       removeTop: true,
       removeBottom: true,
       child: Padding(
-        padding: EdgeInsets.only(bottom: bottomPadding),
-        child: CupertinoUserInterfaceLevel(
-          data: CupertinoUserInterfaceLevelData.elevated,
-          child: _CupertinoSheetScope(child: builder(context)),
+        padding: EdgeInsets.only(top: topPadding),
+        child: ClipRRect(
+          borderRadius: const BorderRadius.vertical(top: Radius.circular(12)),
+          child: CupertinoUserInterfaceLevel(
+            data: CupertinoUserInterfaceLevelData.elevated,
+            child: _CupertinoSheetScope(child: builder(context)),
+          ),
         ),
       ),
     );

packages/flutter/test/cupertino/sheet_test.dart

@@ -1212,6 +1212,98 @@ void main() {
       expect(finalPosition, equals(middlePosition));
       expect(finalPosition, equals(initialPosition));
     });
+
+    // Regression test for https://github.com/flutter/flutter/issues/163572.
+    testWidgets('showCupertinoSheet shows snackbar at bottom of screen', (
+      WidgetTester tester,
+    ) async {
+      final GlobalKey<ScaffoldMessengerState> scaffoldKey = GlobalKey<ScaffoldMessengerState>();
+
+      void showSheet(BuildContext context) {
+        showCupertinoSheet<void>(
+          context: context,
+          pageBuilder: (BuildContext context) {
+            return Scaffold(
+              body: Column(
+                children: <Widget>[
+                  const Text('Cupertino Sheet'),
+                  CupertinoButton(
+                    onPressed: () {
+                      scaffoldKey.currentState?.showSnackBar(
+                        const SnackBar(content: Text('SnackBar'), backgroundColor: Colors.red),
+                      );
+                    },
+                    child: const Text('Show SnackBar'),
+                  ),
+                ],
+              ),
+            );
+          },
+        );
+      }
+
+      await tester.pumpWidget(
+        MaterialApp(
+          scaffoldMessengerKey: scaffoldKey,
+          home: Scaffold(
+            body: Center(
+              child: Column(
+                children: <Widget>[
+                  const Text('Page 1'),
+                  Builder(
+                    builder: (BuildContext context) {
+                      return CupertinoButton(
+                        onPressed: () {
+                          showSheet(context);
+                        },
+                        child: const Text('Show Cupertino Sheet'),
+                      );
+                    },
+                  ),
+                ],
+              ),
+            ),
+          ),
+        ),
+      );
+
+      expect(find.text('Page 1'), findsOneWidget);
+
+      await tester.tap(find.text('Show Cupertino Sheet'));
+      await tester.pumpAndSettle();
+
+      expect(
+        tester
+            .getTopLeft(
+              find.ancestor(of: find.text('Cupertino Sheet'), matching: find.byType(Scaffold)),
+            )
+            .dy,
+        greaterThan(0.0),
+      );
+
+      await tester.tap(find.text('Show SnackBar'));
+      await tester.pumpAndSettle();
+
+      expect(find.byType(SnackBar), findsAtLeast(1));
+      expect(
+        tester.getBottomLeft(find.byType(Scaffold).first).dy,
+        equals(tester.getBottomLeft(find.byType(SnackBar).first).dy),
+      );
+
+      final TestGesture gesture = await tester.startGesture(const Offset(200, 400));
+      await tester.pump();
+      expect(
+        tester.getBottomLeft(find.byType(Scaffold).first).dy,
+        equals(tester.getBottomLeft(find.byType(SnackBar).first).dy),
+      );
+
+      await gesture.up();
+      await tester.pumpAndSettle();
+      expect(
+        tester.getBottomLeft(find.byType(Scaffold).first).dy,
+        equals(tester.getBottomLeft(find.byType(SnackBar).first).dy),
+      );
+    });
   });
 
   testWidgets('CupertinoSheetTransition handles SystemUiOverlayStyle changes', (