WKNavigationAction.sourceFrame is nil on iOS 15.4 & 16.6 despite non-optional Swift declaration (fixed in iOS 18.6)

Reproducibility 100% on iOS 15.4 and iOS 16.6
Zero crash on iOS 18.6

Xcode 26.1

Steps to Reproduce

1. Xcode 26.1 → New iOS App
2. Replace ViewController.swift with the 20-line code below
3. Run on real device
   • iPhone XR iOS 15.4
   • iPhone 13 iOS 16.6
4. Tap the link → breakpoint in decidePolicyFor
5. lldb → po navigationAction.sourceFrame

Actual Result (lldb) po navigationAction.sourceFrame nil

Swift declaration lies: public var sourceFrame: WKFrameInfo { get } // non-optional

→ Instant EXC_BREAKPOINT
libswiftFoundation.dylib`URLRequest._unconditionallyBridgeFromObjectiveC

Objective-C tells the truth: po [(WKNavigationAction *)navigationAction fixedSourceFrame] nil

iOS 18.6 → same code prints a valid WKFrameInfo, no crash.

Expected sourceFrame must be declared WKFrameInfo? in Swift
or at least documented “can be nil on iOS 15–16”.

Impact Every WKWebView app that touches sourceFrame on iOS 15.4 & 16.6 ships with a latent crash.

Production Workaround @implementation WKNavigationAction (Safe)

• (WKFrameInfo *)fixedSourceFrame { return self.sourceFrame ? self.sourceFrame : nil;

} @end

Minimal Test (copy-paste) import UIKit import WebKit

class ViewController: UIViewController, WKNavigationDelegate { lazy var web = WKWebView(frame: view.bounds)

override func viewDidLoad() {
    super.viewDidLoad()
    web.navigationDelegate = self
    view.addSubview(web)
    web.load(URLRequest(url: URL(string: "https://www.apple.com")!))
}

func webView(_ webView: WKWebView,
             decidePolicyFor navigationAction: WKNavigationAction,
             preferences: WKWebpagePreferences,
             decisionHandler: @escaping (WKNavigationActionPolicy, WKWebpagePreferences)->Void) {
    print(navigationAction.sourceFrame) // ← crashes on 15.4 & 16.6
    decisionHandler(.allow, preferences)
}

}