Workaround rounding erros in cupertino nav bar transition (#88935)

packages/flutter/lib/src/rendering/stack.dart

@@ -496,7 +496,7 @@ class RenderStack extends RenderBox
 
     child.layout(childConstraints, parentUsesSize: true);
 
-    late final double x;
+    final double x;
     if (childParentData.left != null) {
       x = childParentData.left!;
     } else if (childParentData.right != null) {
@@ -508,7 +508,7 @@ class RenderStack extends RenderBox
     if (x < 0.0 || x + child.size.width > size.width)
       hasVisualOverflow = true;
 
-    late final double y;
+    final double y;
     if (childParentData.top != null) {
       y = childParentData.top!;
     } else if (childParentData.bottom != null) {

packages/flutter/test/cupertino/nav_bar_transition_test.dart

@@ -171,6 +171,27 @@ void main() {
     );
   });
 
+  testWidgets('Bottom middle never changes size during the animation', (WidgetTester tester) async {
+    await tester.binding.setSurfaceSize(const Size(1080.0 / 2.75, 600));
+    addTearDown(() async {
+      await tester.binding.setSurfaceSize(const Size(800.0, 600.0));
+    });
+
+    await startTransitionBetween(
+      tester,
+      fromTitle: 'Page 1',
+    );
+
+    final Size size = tester.getSize(find.text('Page 1'));
+
+    for (int i = 0; i < 150; i++) {
+      await tester.pump(const Duration(milliseconds: 1));
+      expect(flying(tester, find.text('Page 1')), findsNWidgets(2));
+      expect(tester.getSize(flying(tester, find.text('Page 1')).first), size);
+      expect(tester.getSize(flying(tester, find.text('Page 1')).last), size);
+    }
+  });
+
   testWidgets('Bottom middle and top back label transitions their font', (WidgetTester tester) async {
     await startTransitionBetween(tester, fromTitle: 'Page 1');
 
@@ -978,6 +999,29 @@ void main() {
     );
   });
 
+  testWidgets('Top middle never changes size during the animation', (WidgetTester tester) async {
+    await tester.binding.setSurfaceSize(const Size(1080.0 / 2.75, 600));
+    addTearDown(() async {
+      await tester.binding.setSurfaceSize(const Size(800.0, 600.0));
+    });
+
+    await startTransitionBetween(
+      tester,
+      toTitle: 'Page 2',
+    );
+
+    Size? previousSize;
+
+    for (int i = 0; i < 150; i++) {
+      await tester.pump(const Duration(milliseconds: 1));
+      expect(flying(tester, find.text('Page 2')), findsOneWidget);
+      final Size size = tester.getSize(flying(tester, find.text('Page 2')));
+      if (previousSize != null)
+        expect(size, previousSize);
+      previousSize = size;
+    }
+  });
+
   testWidgets('Top middle fades in and slides in from the left in RTL', (WidgetTester tester) async {
     await startTransitionBetween(
       tester,