Fix #5744: Hide recovery phrase when app is backgrounded (#5755)

Add new `CustomPrivacySensitiveModifier` SwiftUI view modifier that hides the content when in background or recording using `redacted(reason: .placeholder)`. Not using `.privacySensitive()` / `.redacted(reason: .privacy)` as they not working as expected due to `redactionReasons` not propagating through the environment in modals.

BraveWallet/Crypto/Onboarding/BackupRecoveryPhraseView.swift

@@ -59,7 +59,7 @@ struct BackupRecoveryPhraseView: View {
           .padding(.vertical)
         RecoveryPhraseGrid(data: recoveryWords, id: \.self) { word in
           Text(verbatim: "\(word.index + 1). \(word.value)")
-            .noCapture()
+            .customPrivacySensitive()
             .font(.footnote.bold())
             .padding(8)
             .frame(maxWidth: .infinity)

BraveWallet/Crypto/Onboarding/VerifyRecoveryPhraseView.swift

@@ -56,7 +56,7 @@ struct VerifyRecoveryPhraseView: View {
             tappedWord(word)
           }) {
             Text(verbatim: word.value)
-              .noCapture()
+              .customPrivacySensitive()
               .font(.footnote.bold())
               .foregroundColor(.primary)
               .fixedSize(horizontal: false, vertical: true)
@@ -176,7 +176,7 @@ private struct SelectedWordsBox: View {
       case .word(let word, let index, let isCorrect):
         Button(action: { tappedWord(atIndex: index) }) {
           Text(verbatim: "\(index + 1). \(word)")
-            .noCapture()
+            .customPrivacySensitive()
             .padding(8)
             .frame(maxWidth: .infinity)
             .fixedSize(horizontal: false, vertical: true)

Sources/BraveUI/SwiftUI/CustomPrivacySensitiveModifier.swift

@@ -0,0 +1,52 @@
+// Copyright 2021 The Brave Authors. All rights reserved.
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import SwiftUI
+
+/// Custom modifier to behave similar to `.privacySensitive()`
+/// The `.privacySensitive()` doesn't work in sheets because the
+/// `redactedReasons` doesn't get passed through the environment in sheets.
+private struct CustomPrivacySensitiveModifier: ViewModifier {
+
+  @State private var isBackgrounded: Bool = false
+  @State private var isCaptured: Bool = false
+
+  func body(content: Content) -> some View {
+    Group {
+      if isBackgrounded || isCaptured {
+        content
+          .redacted(reason: .placeholder)
+      } else {
+        content
+      }
+    }
+    .onAppear {
+      isCaptured = UIScreen.main.isCaptured
+    }
+    .onReceive(
+      NotificationCenter.default.publisher(for: UIScreen.capturedDidChangeNotification, object: nil)
+    ) { notification in
+      isCaptured = (notification.object as? UIScreen)?.isCaptured ?? UIScreen.main.isCaptured
+    }
+    .onReceive(
+      NotificationCenter.default.publisher(for: UIApplication.willResignActiveNotification)
+    ) { _ in
+      isBackgrounded = true
+    }
+    .onReceive(
+      NotificationCenter.default.publisher(for: UIApplication.didBecomeActiveNotification)
+    ) { _ in
+      isBackgrounded = false
+    }
+  }
+}
+
+extension View {
+  /// Redacts the given view if the app is backgrounded, or the system is actively
+  /// recording, mirroring, or using AirPlay to stream the contents of the screen.
+  public func customPrivacySensitive() -> some View {
+    modifier(CustomPrivacySensitiveModifier())
+  }
+}