Problem

Since Flutter 3.35, MediaQuery.of(context) inside CupertinoNavigationBar components (trailing, leading, middle, etc.) returns Size(0.0, 0.0) instead of the correct screen dimensions. This regression breaks responsive logic and layout decisions made within navigation bar components.

Root Cause

The issue occurs because CupertinoNavigationBar components wrap their child widgets with MediaQuery widgets that only provide textScaler data using MediaQueryData(textScaler: ...). When creating a MediaQueryData with only specific properties, all other properties default to their initial values, causing size to default to Size.zero.

Affected components:

• Leading widget
• Trailing widget
• Middle widget
• Back chevron
• Back label
• Large title
• Bottom widget

Solution

Replace MediaQueryData() constructors with MediaQuery.of(context).copyWith() to preserve all existing MediaQuery properties (size, padding, devicePixelRatio, etc.) while only modifying the text scaling behavior.

Changes

Core Fix

• Modified 6 MediaQueryData constructor calls in packages/flutter/lib/src/cupertino/nav_bar.dart
• Preserves all inherited MediaQuery properties while maintaining text scaling limits
• Ensures size, padding, devicePixelRatio and other properties are not reset to defaults

Testing

• Added comprehensive test suite with 3 focused test cases in packages/flutter/test/cupertino/nav_bar_test.dart
• Tests verify MediaQuery size preservation in trailing/leading widgets
• Regression test specifically prevents Size(0,0) from returning
• Tests are minimal and focused on the core issue

Impact

Fixed Issues

• ✅ MediaQuery.of(context).size returns correct screen dimensions
• ✅ MediaQuery.of(context).padding returns correct safe area padding
• ✅ MediaQuery.of(context).devicePixelRatio returns correct pixel ratio
• ✅ Responsive navigation bar components work correctly
• ✅ No impact on existing text scaling behavior

Compatibility

• ✅ Backward compatible - no breaking changes
• ✅ No impact on Material design widgets
• ✅ All existing CupertinoNavigationBar functionality preserved
• ✅ Performance neutral - same number of MediaQuery widgets

Testing Instructions

To verify the fix:

CupertinoNavigationBar(
  trailing: Builder(
    builder: (context) {
      final size = MediaQuery.of(context).size;
      print('Size: $size'); // Should print actual screen size, not Size(0.0, 0.0)
      return Icon(CupertinoIcons.info);
    },
  ),
)

Fixes #174771