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>

apps/learn/halkyon_learn_flutter/macos/Runner/MainFlutterWindow.swift

@@ -2,6 +2,13 @@ import Cocoa
 import FlutterMacOS
 
 class MainFlutterWindow: NSWindow {
+  /// The channel halkyon_app's SecretScreen asks to keep the window out of
+  /// screenshots and screen recordings while something secret shows (the
+  /// recovery phrase, recovery codes, an authenticator's key), and how many
+  /// show: counted, as several may at once.
+  private var screenGuard: FlutterMethodChannel?
+  private var secretsShown = 0
+
   override func awakeFromNib() {
     let flutterViewController = FlutterViewController()
     let windowFrame = self.frame
@@ -9,7 +16,34 @@ class MainFlutterWindow: NSWindow {
     self.setFrame(windowFrame, display: true)
 
     RegisterGeneratedPlugins(registry: flutterViewController)
+    guardScreen(messenger: flutterViewController.engine.binaryMessenger)
 
     super.awakeFromNib()
   }
+
+  private func guardScreen(messenger: FlutterBinaryMessenger) {
+    let channel = FlutterMethodChannel(
+      name: "app.halkyon/screen_guard", binaryMessenger: messenger)
+    channel.setMethodCallHandler { [weak self] call, result in
+      guard let self = self else {
+        result(nil)
+        return
+      }
+      switch call.method {
+      case "hold":
+        self.secretsShown += 1
+        self.sharingType = .none
+        result(nil)
+      case "release":
+        self.secretsShown = max(0, self.secretsShown - 1)
+        if self.secretsShown == 0 {
+          self.sharingType = .readOnly
+        }
+        result(nil)
+      default:
+        result(FlutterMethodNotImplemented)
+      }
+    }
+    screenGuard = channel
+  }
 }

apps/learn/halkyon_learn_flutter/windows/runner/flutter_window.cpp

@@ -1,9 +1,17 @@
 #include "flutter_window.h"
 
+#include <flutter/standard_method_codec.h>
+
 #include <optional>
 
 #include "flutter/generated_plugin_registrant.h"
 
+// Kept out of captures altogether (Windows 10 2004 and later); older SDKs
+// don't name it.
+#ifndef WDA_EXCLUDEFROMCAPTURE
+#define WDA_EXCLUDEFROMCAPTURE 0x00000011
+#endif
+
 FlutterWindow::FlutterWindow(const flutter::DartProject& project)
     : project_(project) {}
 
@@ -25,6 +33,7 @@ bool FlutterWindow::OnCreate() {
     return false;
   }
   RegisterPlugins(flutter_controller_->engine());
+  GuardScreen();
   SetChildContent(flutter_controller_->view()->GetNativeWindow());
 
   flutter_controller_->engine()->SetNextFrameCallback([&]() {
@@ -39,7 +48,38 @@ bool FlutterWindow::OnCreate() {
   return true;
 }
 
+void FlutterWindow::GuardScreen() {
+  screen_guard_ =
+      std::make_unique<flutter::MethodChannel<flutter::EncodableValue>>(
+          flutter_controller_->engine()->messenger(),
+          "app.halkyon/screen_guard",
+          &flutter::StandardMethodCodec::GetInstance());
+  screen_guard_->SetMethodCallHandler(
+      [this](const flutter::MethodCall<flutter::EncodableValue>& call,
+             std::unique_ptr<flutter::MethodResult<flutter::EncodableValue>>
+                 result) {
+        if (call.method_name() == "hold") {
+          secrets_shown_++;
+          // Before Windows 10 2004 the exclusion fails: the window shows
+          // black in captures instead.
+          if (!SetWindowDisplayAffinity(GetHandle(), WDA_EXCLUDEFROMCAPTURE)) {
+            SetWindowDisplayAffinity(GetHandle(), WDA_MONITOR);
+          }
+          result->Success();
+        } else if (call.method_name() == "release") {
+          secrets_shown_ = secrets_shown_ > 0 ? secrets_shown_ - 1 : 0;
+          if (secrets_shown_ == 0) {
+            SetWindowDisplayAffinity(GetHandle(), WDA_NONE);
+          }
+          result->Success();
+        } else {
+          result->NotImplemented();
+        }
+      });
+}
+
 void FlutterWindow::OnDestroy() {
+  screen_guard_ = nullptr;
   if (flutter_controller_) {
     flutter_controller_ = nullptr;
   }

apps/learn/halkyon_learn_flutter/windows/runner/flutter_window.h

@@ -2,7 +2,9 @@
 #define RUNNER_FLUTTER_WINDOW_H_
 
 #include <flutter/dart_project.h>
+#include <flutter/encodable_value.h>
 #include <flutter/flutter_view_controller.h>
+#include <flutter/method_channel.h>
 
 #include <memory>
 
@@ -28,6 +30,17 @@ class FlutterWindow : public Win32Window {
 
   // The Flutter instance hosted by this window.
   std::unique_ptr<flutter::FlutterViewController> flutter_controller_;
+
+  // The channel halkyon_app's SecretScreen asks to keep the window out of
+  // screen captures while something secret shows (the recovery phrase,
+  // recovery codes, an authenticator's key), and how many show: counted, as
+  // several may at once.
+  std::unique_ptr<flutter::MethodChannel<flutter::EncodableValue>>
+      screen_guard_;
+  int secrets_shown_ = 0;
+
+  // Answers |screen_guard_|.
+  void GuardScreen();
 };
 
 #endif  // RUNNER_FLUTTER_WINDOW_H_

apps/todo/halkyon_todo_flutter/macos/Runner/MainFlutterWindow.swift

@@ -2,6 +2,13 @@ import Cocoa
 import FlutterMacOS
 
 class MainFlutterWindow: NSWindow {
+  /// The channel halkyon_app's SecretScreen asks to keep the window out of
+  /// screenshots and screen recordings while something secret shows (the
+  /// recovery phrase, recovery codes, an authenticator's key), and how many
+  /// show: counted, as several may at once.
+  private var screenGuard: FlutterMethodChannel?
+  private var secretsShown = 0
+
   override func awakeFromNib() {
     let flutterViewController = FlutterViewController()
     let windowFrame = self.frame
@@ -9,7 +16,34 @@ class MainFlutterWindow: NSWindow {
     self.setFrame(windowFrame, display: true)
 
     RegisterGeneratedPlugins(registry: flutterViewController)
+    guardScreen(messenger: flutterViewController.engine.binaryMessenger)
 
     super.awakeFromNib()
   }
+
+  private func guardScreen(messenger: FlutterBinaryMessenger) {
+    let channel = FlutterMethodChannel(
+      name: "app.halkyon/screen_guard", binaryMessenger: messenger)
+    channel.setMethodCallHandler { [weak self] call, result in
+      guard let self = self else {
+        result(nil)
+        return
+      }
+      switch call.method {
+      case "hold":
+        self.secretsShown += 1
+        self.sharingType = .none
+        result(nil)
+      case "release":
+        self.secretsShown = max(0, self.secretsShown - 1)
+        if self.secretsShown == 0 {
+          self.sharingType = .readOnly
+        }
+        result(nil)
+      default:
+        result(FlutterMethodNotImplemented)
+      }
+    }
+    screenGuard = channel
+  }
 }

apps/todo/halkyon_todo_flutter/windows/runner/flutter_window.cpp

@@ -1,9 +1,17 @@
 #include "flutter_window.h"
 
+#include <flutter/standard_method_codec.h>
+
 #include <optional>
 
 #include "flutter/generated_plugin_registrant.h"
 
+// Kept out of captures altogether (Windows 10 2004 and later); older SDKs
+// don't name it.
+#ifndef WDA_EXCLUDEFROMCAPTURE
+#define WDA_EXCLUDEFROMCAPTURE 0x00000011
+#endif
+
 FlutterWindow::FlutterWindow(const flutter::DartProject& project)
     : project_(project) {}
 
@@ -25,6 +33,7 @@ bool FlutterWindow::OnCreate() {
     return false;
   }
   RegisterPlugins(flutter_controller_->engine());
+  GuardScreen();
   SetChildContent(flutter_controller_->view()->GetNativeWindow());
 
   flutter_controller_->engine()->SetNextFrameCallback([&]() {
@@ -39,7 +48,38 @@ bool FlutterWindow::OnCreate() {
   return true;
 }
 
+void FlutterWindow::GuardScreen() {
+  screen_guard_ =
+      std::make_unique<flutter::MethodChannel<flutter::EncodableValue>>(
+          flutter_controller_->engine()->messenger(),
+          "app.halkyon/screen_guard",
+          &flutter::StandardMethodCodec::GetInstance());
+  screen_guard_->SetMethodCallHandler(
+      [this](const flutter::MethodCall<flutter::EncodableValue>& call,
+             std::unique_ptr<flutter::MethodResult<flutter::EncodableValue>>
+                 result) {
+        if (call.method_name() == "hold") {
+          secrets_shown_++;
+          // Before Windows 10 2004 the exclusion fails: the window shows
+          // black in captures instead.
+          if (!SetWindowDisplayAffinity(GetHandle(), WDA_EXCLUDEFROMCAPTURE)) {
+            SetWindowDisplayAffinity(GetHandle(), WDA_MONITOR);
+          }
+          result->Success();
+        } else if (call.method_name() == "release") {
+          secrets_shown_ = secrets_shown_ > 0 ? secrets_shown_ - 1 : 0;
+          if (secrets_shown_ == 0) {
+            SetWindowDisplayAffinity(GetHandle(), WDA_NONE);
+          }
+          result->Success();
+        } else {
+          result->NotImplemented();
+        }
+      });
+}
+
 void FlutterWindow::OnDestroy() {
+  screen_guard_ = nullptr;
   if (flutter_controller_) {
     flutter_controller_ = nullptr;
   }

apps/todo/halkyon_todo_flutter/windows/runner/flutter_window.h

@@ -2,7 +2,9 @@
 #define RUNNER_FLUTTER_WINDOW_H_
 
 #include <flutter/dart_project.h>
+#include <flutter/encodable_value.h>
 #include <flutter/flutter_view_controller.h>
+#include <flutter/method_channel.h>
 
 #include <memory>
 
@@ -28,6 +30,17 @@ class FlutterWindow : public Win32Window {
 
   // The Flutter instance hosted by this window.
   std::unique_ptr<flutter::FlutterViewController> flutter_controller_;
+
+  // The channel halkyon_app's SecretScreen asks to keep the window out of
+  // screen captures while something secret shows (the recovery phrase,
+  // recovery codes, an authenticator's key), and how many show: counted, as
+  // several may at once.
+  std::unique_ptr<flutter::MethodChannel<flutter::EncodableValue>>
+      screen_guard_;
+  int secrets_shown_ = 0;
+
+  // Answers |screen_guard_|.
+  void GuardScreen();
 };
 
 #endif  // RUNNER_FLUTTER_WINDOW_H_

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);
   });
 }