Implement AttachmentPreviewModal scaffold
epic-document-attachments-ui-task-003 — Build the full-screen modal shell for AttachmentPreviewModal including routing/navigation setup, close button (minimum 44x44 pt touch target), and a loading state placeholder. Accept a signed URL and MIME type as constructor parameters to determine rendering strategy.
Acceptance Criteria
Technical Requirements
Implementation Notes
Implement as a standard StatefulWidget (or ConsumerStatefulWidget if Riverpod is used). Accept signedUrl and mimeType as final fields. Use a WillPopScope (or PopScope in Flutter 3.16+) to handle Android back button. For the close button, use IconButton with constraints: BoxConstraints(minWidth: 44, minHeight: 44).
Apply design token colours to the Scaffold background — avoid hardcoded hex values. Validate mimeType with a simple allowlist check and set an isUnsupportedType bool that will be used in task-004 to render an error state instead of content. Keep this scaffold free of content-rendering logic — content widgets will be composed in task-004. Use go_router's GoRoute with a path like /attachment-preview if the project uses go_router; otherwise use Navigator.push with a custom route transition.
Testing Requirements
Widget tests: verify modal renders at full screen size; verify close button is present and has semantics label 'Close'; verify tapping close button triggers Navigator.pop; verify loading placeholder is shown when content area is empty; verify signedUrl and mimeType parameters are passed through without mutation. Use WidgetTester.pumpWidget with a MaterialApp and Navigator to simulate modal context. Verify that back gesture (WidgetTester equivalent) also dismisses the modal.
Flutter does not include a first-party PDF renderer. Third-party packages (e.g., flutter_pdfview, syncfusion_flutter_pdf) have inconsistent accessibility support, may not honour dynamic type scaling, and can introduce large binary size increases. Choosing the wrong package late in development risks rework or an inaccessible PDF experience for Blindeforbundet users.
Mitigation & Contingency
Mitigation: Evaluate and spike two PDF rendering packages (flutter_pdfview and pdfx) in the first task of this epic before committing to implementation. Criteria: VoiceOver compatibility, dynamic type support, APK/IPA size delta, and licence. Document the decision in an ADR.
Contingency: If no package meets accessibility requirements, fall back to opening PDFs in the system browser via url_launcher, which inherits the OS's accessible PDF viewer. Display a clear 'Opening in external viewer' message to set user expectation.
Flutter's Semantics API for live region announcements (analogous to aria-live) has known gaps on Android for dynamic content updates. Upload progress announcements required by the accessibility user story may not fire reliably on Android devices used by HLF and NHF members.
Mitigation & Contingency
Mitigation: Use SemanticsService.announce() for imperative announcements at each upload state transition rather than relying on declarative Semantics widget tree updates. Test on a physical Android device with TalkBack enabled during development, not only on iOS with VoiceOver.
Contingency: If SemanticsService.announce() proves unreliable, implement a persistent accessible status banner at the top of the screen that reflects the current upload state as plain text, satisfying the WCAG success criterion through a visual + programmatic mechanism.
iOS and Android file picker behaviour diverges significantly: MIME type filtering works reliably on Android but is advisory on iOS (users can still navigate to and select non-compliant files via the Files app). This could allow unsupported file types to reach the upload service, causing validation failures with a confusing user experience.
Mitigation & Contingency
Mitigation: Rely on the AttachmentUploadService (Epic 2) as the authoritative MIME validation gate, regardless of platform. In the UI, re-validate the picked file's extension/MIME after selection and show an inline plain-language error ('Only PDF, JPEG, and PNG files are supported') before even calling the service.
Contingency: If users consistently hit the error due to iOS file picker limitations, add a user-facing help tooltip on the picker sheet explaining the supported file types, reducing support volume while the underlying OS limitation persists.
WCAG 2.2 AA compliance for three new interactive components (picker, grid, modal) with file system integration, role-gated actions, and live regions is a significant accessibility surface area. An incomplete audit before release could result in the feature being unusable for Blindeforbundet screen reader users at launch.
Mitigation & Contingency
Mitigation: Allocate a dedicated accessibility audit task at the end of the epic using the accessibility-test-harness (619) and wcag-compliance-checker (620) components already in the project. Include at least one manual test session on a physical device with VoiceOver enabled. Fail the epic's definition of done if any WCAG 2.2 AA violation remains open.
Contingency: If critical accessibility issues are found late, gate the feature behind the org-level attachments_enabled feature flag and only enable it for NHF (the requesting org) after a targeted fix cycle, rather than delaying the release for all organisations.