Build BiometricPromptOverlay full-screen widget structure
epic-biometric-session-authentication-ui-and-lifecycle-task-004 — Create the BiometricPromptOverlay as a full-screen Flutter widget using design tokens exclusively (zero inline styles). Include the app logo, a localised prompt message string, and a fallback link to credential login. The overlay must use design token colors, spacing, and typography and meet WCAG 2.2 AA contrast ratios throughout.
Acceptance Criteria
Technical Requirements
Implementation Notes
Use `PopScope(canPop: false)` (Flutter 3.16+) rather than the deprecated WillPopScope for Android back interception. Structure the layout with a `Stack` containing a full-screen `Container` with the background color token, and a centered `Column` for logo + text + button. Use `SafeArea` inside the Stack child to position content within safe insets. Expose three callbacks as constructor parameters: `onFallbackTapped`, `onAuthRequested` (to be wired in task-005), and `onAuthSuccess` (to be wired in task-005) — define them as nullable with empty default implementations so the static widget compiles and previews without task-005.
The design token system should provide a `AppColors.surfaceOverlay` or equivalent — if no overlay-specific token exists, use the primary surface token and document the choice for design review. For localization, add keys: `biometricPrompt.title`, `biometricPrompt.subtitle`, `biometricPrompt.fallbackButton` to all ARB files before submitting the task.
Testing Requirements
Widget tests using flutter_test. Test cases: (1) widget renders without overflow on 320px wide screen (minimum supported width); (2) widget renders without overflow on 430px wide screen (large iPhone); (3) fallback button is present and has a non-empty semantic label; (4) tapping fallback button calls the provided onFallbackTapped callback; (5) Android back button press does not pop the route (test with `tester.pageBack()` and verify widget still present); (6) all text uses design token TextStyles (check no raw TextStyle constructors in widget tree via debugDumpApp snapshot). Accessibility test: run flutter accessibility checker on the widget and assert zero violations.
Flutter's AppLifecycleState.resumed event fires in scenarios beyond simple user-initiated app-switching: it also fires when the native biometric dialog itself dismisses and returns control to Flutter, when system alerts (low battery, notifications) temporarily cover the app, and when the app is foregrounded by a deep link. Without careful debouncing and state tracking, SessionResumeManager can trigger multiple overlapping biometric prompts or prompt immediately after a just-completed authentication, creating a confusing loop.
Mitigation & Contingency
Mitigation: Implement a state machine inside SessionResumeManager with explicit states (idle, prompting, authenticated, awaiting-fallback) and guard all prompt triggers with a state check. Add a minimum inter-prompt interval of 3 seconds. Write widget tests that simulate rapid lifecycle event sequences and verify only one prompt is shown.
Contingency: If the state machine approach proves difficult to test or maintain, replace it with a simple boolean isPromptActive flag with a debounce timer, accepting slightly less precise semantics in exchange for simpler reasoning about concurrent lifecycle events.
The BiometricPromptOverlay appears on top of the existing app content when the app resumes. If focus is not correctly transferred to the overlay and returned to the underlying screen after authentication, screen reader users (VoiceOver/TalkBack) will either be unable to interact with the prompt or will lose their navigation position in the app after authentication completes, violating WCAG 2.2 focus management requirements and creating a broken experience for Blindeforbundet users.
Mitigation & Contingency
Mitigation: Use Flutter's FocusScope and FocusTrap utilities to capture focus within the overlay on presentation and restore the previous FocusNode after dismissal. Add a live region announcement (using the accessibility live region announcer component) when the overlay appears. Include dedicated VoiceOver and TalkBack test cases in the acceptance criteria.
Contingency: If FocusTrap behaviour proves unreliable across Flutter versions, implement the overlay as a full Navigator push to a modal route rather than an overlay widget, which gives Flutter's built-in modal semantics and focus management automatic WCAG-compliant behaviour.
The BiometricUnavailableBanner needs to deep-link to biometric enrollment settings on both iOS and Android. iOS uses a single URL scheme (app-settings:) that opens the app's settings page. Android has no universal URL for biometric settings — the correct Intent action (Settings.ACTION_BIOMETRIC_ENROLL) was introduced in API 30, with different fallback actions required for API 23-29. Using the wrong action on older Android devices either crashes or navigates to an unrelated settings screen.
Mitigation & Contingency
Mitigation: Build an Android API version check into LocalAuthIntegration that selects the correct Intent action based on the runtime SDK version. Test against Android API 23, 28, and 30+ in the CI matrix. For iOS, validate that the app-settings: URL scheme is correctly declared.
Contingency: If the Android settings Intent fragmentation cannot be resolved reliably for all target API levels, fall back to navigating to the top-level Settings screen (Settings.ACTION_SETTINGS) with an overlay instruction telling the user to navigate to 'Security > Biometrics' manually, ensuring the user always has a path to resolve the issue even if the deep link is imprecise.