Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

Second pass at addressing #2177. In order to avoid potential conflicts and confusion, this breaking change removes all extended UINavigationItem properties and places them onto a single configuration object, the FluentNavigationItemConfiguration class. This removes the conflict with subtitle.

However, now that we know this new property is coming, this change also builds fallbacks to handle the existence of UINavigationItem.subtitle, and to prefer it to the existing Fluent value. This is done with a few changes:

• Marking subtitle deprecated on iOS/visionOS/macCatalyst 26+
• Adding observers for the new property when using a compiler version >=6.2 (which ships with the new iOS SDK)
• Configuring the navigation item using the appropriate property based on compiler version, preferring the system API over our own if possible.

Verification

Confirmed expected behavior across iOS 18 and 26, using both Xcode 16.4 and 26.0b1.

Visual Verification

iOS 17	iOS 18	iOS 26	iOS 18 with iOS 26 SDK

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow