feat(app): secrets kept out of screenshots on macOS and Windows too

The reading of the full reading's fixes noted that macOS and Windows can
keep a window out of screen captures, which SecretScreen didn't ask.

PlatformScreenGuard now asks the app's window on Android, macOS and
Windows (asksOn), and both apps' runners answer app.halkyon/screen_guard,
counting what shows: macOS's MainFlutterWindow sets sharingType = .none,
Windows' FlutterWindow WDA_EXCLUDEFROMCAPTURE (Windows 10 2004 on;
before, WDA_MONITOR, black in captures). Neither runner was built here
(Linux): to be tried on each system.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/README.md

@@ -87,10 +87,13 @@ Written as the apps are: Dart 3.13's primary constructors, Material from
   through Flutter, so clipboard histories and cloud clipboards would keep
   it): the recovery phrase and codes are saved or written down, an
   authenticator's key scanned, opened or typed; all of them show in a
-  `SecretScreen`, kept out of screenshots and recents on Android
-  (`FLAG_SECURE`: the app's `MainActivity` answers `app.halkyon/screen_guard`,
-  as Learn's and Todo's `ScreenGuard.kt` do) and covered on iOS while the app
-  isn't in front; keyboards learn nothing typed into a password, a code or
+  `SecretScreen` (the phrase typed back too), kept out of screenshots,
+  screen recordings and recents where the app's window answers
+  `app.halkyon/screen_guard`: Android's `FLAG_SECURE` (as Learn's and Todo's
+  `ScreenGuard.kt` in their `MainActivity`), macOS's `sharingType = .none`
+  (`MainFlutterWindow.swift`) and Windows' `WDA_EXCLUDEFROMCAPTURE`
+  (`flutter_window.cpp`); covered on iOS while the app isn't in front;
+  nothing on Linux; keyboards learn nothing typed into a password, a code or
   the phrase; no state's text (which a debug build's bloc observer logs) holds
   a secret, a code or the address; only an `otpauth:` URI is shown as a QR
   code or opened; and anything a step didn't expect (bytes of the wrong

packages/halkyon_app/lib/src/password/widget/secret_screen.dart

@@ -1,5 +1,6 @@
 import 'dart:io';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/services.dart';
 import 'package:material_ui/material_ui.dart';
 
@@ -15,16 +16,29 @@ abstract interface class ScreenGuard() {
   Future<void> release();
 }
 
-/// [ScreenGuard] through Android's `FLAG_SECURE`, which each app's
-/// `MainActivity` sets while any is held (`ScreenGuard.kt`, counting them,
-/// as several may show at once); nothing to do elsewhere: iOS has no such
-/// flag (its app switcher's picture is covered by [SecretScreen] instead),
-/// and desktops none either.
-class const PlatformScreenGuard({final bool? android}) implements ScreenGuard {
-  /// The channel each app's activity answers.
+/// [ScreenGuard] through the app's window, which each app's runner keeps
+/// out while any is held, counting them, as several may show at once:
+/// Android's `FLAG_SECURE` (`ScreenGuard.kt` in its `MainActivity`), macOS's
+/// `sharingType = .none` (`MainFlutterWindow.swift`) and Windows'
+/// `WDA_EXCLUDEFROMCAPTURE` (`flutter_window.cpp`; from Windows 10 2004 on).
+/// Nothing to ask elsewhere: iOS has no such flag (its app switcher's
+/// picture is covered by [SecretScreen] instead), and Linux's desktops
+/// none either.
+class const PlatformScreenGuard({final bool? asks}) implements ScreenGuard {
+  /// The channel each app's window answers.
   static const channel = MethodChannel('app.halkyon/screen_guard');
 
-  bool get _android => android ?? Platform.isAndroid;
+  /// Whether the app's window on [platform] answers [channel].
+  static bool asksOn(TargetPlatform platform) => switch (platform) {
+    TargetPlatform.android ||
+    TargetPlatform.macOS ||
+    TargetPlatform.windows => true,
+    TargetPlatform.iOS ||
+    TargetPlatform.linux ||
+    TargetPlatform.fuchsia => false,
+  };
+
+  bool get _asks => asks ?? asksOn(defaultTargetPlatform);
 
   @override
   Future<void> hold() => _ask('hold');
@@ -33,7 +47,7 @@ class const PlatformScreenGuard({final bool? android}) implements ScreenGuard {
   Future<void> release() => _ask('release');
 
   Future<void> _ask(String method) async {
-    if (!_android) {
+    if (!_asks) {
       return;
     }
     try {

packages/halkyon_app/test/password/widget/secret_screen_test.dart

@@ -91,9 +91,9 @@ void main() {
     }
   });
 
-  testWidgets("the platform's guard asks Android's activity, counted", (
-    tester,
-  ) async {
+  testWidgets("the platform's guard asks the app's window, counted: "
+      "Android's activity, and macOS's and Windows' windows (the reading of "
+      'the fixes)', (tester) async {
     final calls = <String>[];
     tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(
       PlatformScreenGuard.channel,
@@ -108,14 +108,22 @@ void main() {
         null,
       ),
     );
-    const guard = PlatformScreenGuard(android: true);
+    const guard = PlatformScreenGuard(asks: true);
     await guard.hold();
     await guard.release();
     expect(calls, ['hold', 'release']);
 
-    // Elsewhere, nothing to ask.
+    // Elsewhere (iOS, Linux), nothing to ask.
     calls.clear();
-    await const PlatformScreenGuard(android: false).hold();
+    await const PlatformScreenGuard(asks: false).hold();
     expect(calls, isEmpty);
+    expect(
+      PlatformScreenGuard.asksOn(TargetPlatform.macOS),
+      isTrue,
+    );
+    expect(PlatformScreenGuard.asksOn(TargetPlatform.windows), isTrue);
+    expect(PlatformScreenGuard.asksOn(TargetPlatform.android), isTrue);
+    expect(PlatformScreenGuard.asksOn(TargetPlatform.iOS), isFalse);
+    expect(PlatformScreenGuard.asksOn(TargetPlatform.linux), isFalse);
   });
 }