iOS 27: SwiftUI TextField text invisible while typing when the app runs in Arabic (FB25410049)

Since iOS 27, if an app runs in Arabic (or, presumably, any right-to-left language), typing digits or Latin letters into a SwiftUI TextField makes the field look empty. The binding updates and the caret shows, but the text isn't drawn until editing ends.

What I measured:

• The text's layout fragment is placed at about x = −39,655 pt in a 370 pt wide field, so it's drawn far outside the field.
• A plain TextField and one with .environment(\.layoutDirection, .leftToRight) (phone and card numbers) are both affected.
• A UIKit UITextField on the same screen isn't affected.
• The same build on iOS 18.0 works, and iOS 27 in English works.

Minimal repro: one SwiftUI file with four fields. Run it with App Language set to Arabic, tap the first field and type 523. Launch with -autoProbe to have it type into each field and print where the text was laid out.

PROBE iOS 27.0, language ar, layout RTL
PROBE field 1 text="523" fieldWidth=370 fragmentX=[-39655] visible=false   // SwiftUI TextField
PROBE field 2 text="523" fieldWidth=370 fragmentX=[-39655] visible=false   // SwiftUI, LTR pinned
PROBE field 3 text="523" fieldWidth=370 fragmentX=[314]    visible=true    // UIKit UITextField

Workaround that works for us, installed once at launch:

import UIKit

enum TextFieldDirection {
    static func install() {
        for name in [UITextField.textDidBeginEditingNotification, UITextField.textDidChangeNotification] {
            NotificationCenter.default.addObserver(forName: name, object: nil, queue: .main) { note in
                guard let field = note.object as? UITextField else { return }
                MainActor.assumeIsolated { settle(field) }
                // SwiftUI rebuilds the paragraph style right after; settle again.
                DispatchQueue.main.async { MainActor.assumeIsolated { settle(field) } }
            }
        }
    }

    @MainActor static func settle(_ field: UITextField) {
        let direction: NSWritingDirection =
            field.effectiveUserInterfaceLayoutDirection == .rightToLeft ? .rightToLeft : .leftToRight
        if let whole = field.textRange(from: field.beginningOfDocument, to: field.endOfDocument) {
            field.setBaseWritingDirection(direction, for: whole)
        }
        func directed(_ attributes: [NSAttributedString.Key: Any]) -> [NSAttributedString.Key: Any] {
            var attributes = attributes
            let style = (attributes[.paragraphStyle] as? NSParagraphStyle)?.mutableCopy() as? NSMutableParagraphStyle
                ?? NSMutableParagraphStyle()
            style.baseWritingDirection = direction
            attributes[.paragraphStyle] = style
            return attributes
        }
        if let typing = field.typingAttributes { field.typingAttributes = directed(typing) }
        field.defaultTextAttributes = directed(field.defaultTextAttributes)
        field.setNeedsLayout()
        field.layoutIfNeeded()
    }
}

Don't pin textAlignment instead. SwiftUI puts .natural back on every keystroke and the caret jumps from side to side.

If you see this too, please file a duplicate referencing FB25410049. Duplicates help Apple prioritise it.