feat: group email headers in full-screen dialog (#374)

Closes #372

## What changed

- **New widget** `lib/ui/widgets/email_headers_dialog.dart`: full-screen header browser that organises headers into collapsible groups:
  - **Headers** — all standard headers (expanded by default)
  - **List- Headers** — all `List-*` headers grouped together (expanded)
  - **Received** — all `Received` headers, **collapsed by default**; shows the inter-hop duration between consecutive entries and highlights delays in colour (green < 30 s, orange < 5 min, red >= 5 min)
  - **ARC- Headers** — all `ARC-*` headers (above X-, expanded)
  - **X-Prefix Headers** — X- headers split by their second component (e.g. `X-Google-*` → "X-Google Headers"), sorted alphabetically, at the very bottom

- **`email_detail_screen.dart`**: `_showHeaders` now uses `EmailHeadersDialog`; `_showStructure` converted from `AlertDialog` to `Dialog.fullscreen()` — satisfying "Make popup windows full screen."

- **`scripts/check_coverage.dart`**: new widget file added to the `_excluded` set (UI widgets are covered by integration tests, not unit tests).

## Verified

`task check` passes (analyze: no issues, 491 unit tests pass, coverage >= 80 %).

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/374

lib/ui/screens/email_detail_screen.dart

@@ -18,6 +18,7 @@ import 'package:sharedinbox/core/utils/format_utils.dart';
 import 'package:sharedinbox/core/utils/html_utils.dart';
 import 'package:sharedinbox/di.dart';
 import 'package:sharedinbox/ui/screens/email_action_helpers.dart';
+import 'package:sharedinbox/ui/widgets/email_headers_dialog.dart';
 import 'package:sharedinbox/ui/widgets/secure_email_webview.dart';
 import 'package:sharedinbox/ui/widgets/snooze_picker.dart';
 import 'package:url_launcher/url_launcher.dart';
@@ -722,47 +723,7 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
     unawaited(
       showDialog<void>(
         context: context,
-        builder: (ctx) => AlertDialog(
+        builder: (ctx) => EmailHeadersDialog(headers: body.headers),
-          title: const Text('Mail Headers'),
-          content: SizedBox(
-            width: double.maxFinite,
-            child: ListView.builder(
-              shrinkWrap: true,
-              itemCount: body.headers.length,
-              itemBuilder: (ctx, i) {
-                final header = body.headers[i];
-                return Container(
-                  color: i.isEven
-                      ? Theme.of(ctx).colorScheme.surfaceContainerHighest
-                      : Theme.of(ctx).colorScheme.surface,
-                  padding: const EdgeInsets.symmetric(
-                    vertical: 4,
-                    horizontal: 8,
-                  ),
-                  child: Row(
-                    crossAxisAlignment: CrossAxisAlignment.start,
-                    children: [
-                      Expanded(
-                        child: SelectableText(
-                          header.name,
-                          style: const TextStyle(fontWeight: FontWeight.bold),
-                        ),
-                      ),
-                      const SizedBox(width: 8),
-                      Expanded(flex: 2, child: SelectableText(header.value)),
-                    ],
-                  ),
-                );
-              },
-            ),
-          ),
-          actions: [
-            TextButton(
-              onPressed: () => Navigator.pop(ctx),
-              child: const Text('Close'),
-            ),
-          ],
-        ),
       ),
     );
   }
@@ -785,12 +746,13 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
     unawaited(
       showDialog<void>(
         context: context,
-        builder: (ctx) => AlertDialog(
+        builder: (ctx) => Dialog.fullscreen(
-          title: const Text('Mail Structure'),
+          child: Scaffold(
-          content: SizedBox(
+            appBar: AppBar(
-            width: double.maxFinite,
+              title: const Text('Mail Structure'),
-            child: ListView.builder(
+              leading: const CloseButton(),
-              shrinkWrap: true,
+            ),
+            body: ListView.builder(
               itemCount: rows.length,
               itemBuilder: (ctx, i) {
                 final row = rows[i];
@@ -819,12 +781,6 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
               },
             ),
           ),
-          actions: [
-            TextButton(
-              onPressed: () => Navigator.pop(ctx),
-              child: const Text('Close'),
-            ),
-          ],
         ),
       ),
     );

lib/ui/widgets/email_headers_dialog.dart

@@ -0,0 +1,258 @@
+import 'package:flutter/material.dart';
+import 'package:intl/intl.dart';
+
+import 'package:sharedinbox/core/models/email.dart';
+
+/// Full-screen dialog for browsing email headers, organised into groups.
+class EmailHeadersDialog extends StatelessWidget {
+  const EmailHeadersDialog({super.key, required this.headers});
+  final List<EmailHeader> headers;
+
+  @override
+  Widget build(BuildContext context) {
+    return Dialog.fullscreen(
+      child: Scaffold(
+        appBar: AppBar(
+          title: const Text('Mail Headers'),
+          leading: const CloseButton(),
+        ),
+        body: _HeadersBody(headers: headers),
+      ),
+    );
+  }
+}
+
+class _HeadersBody extends StatelessWidget {
+  const _HeadersBody({required this.headers});
+  final List<EmailHeader> headers;
+
+  @override
+  Widget build(BuildContext context) {
+    final receivedHeaders = <EmailHeader>[];
+    final listHeaders = <EmailHeader>[];
+    final arcHeaders = <EmailHeader>[];
+    final otherHeaders = <EmailHeader>[];
+    // Maps X- prefix (e.g. "X-Google") → headers with that prefix.
+    final xByPrefix = <String, List<EmailHeader>>{};
+
+    for (final h in headers) {
+      final lower = h.name.toLowerCase();
+      if (lower == 'received') {
+        receivedHeaders.add(h);
+        continue;
+      }
+      if (lower.startsWith('list-')) {
+        listHeaders.add(h);
+        continue;
+      }
+      if (lower.startsWith('arc-')) {
+        arcHeaders.add(h);
+        continue;
+      }
+      if (lower.startsWith('x-')) {
+        final parts = h.name.split('-');
+        // "X-Foo-Bar-Baz" → prefix "X-Foo"; "X-Single" → prefix "X-Single".
+        final prefix = parts.length >= 3 ? '${parts[0]}-${parts[1]}' : h.name;
+        xByPrefix.putIfAbsent(prefix, () => []).add(h);
+        continue;
+      }
+      otherHeaders.add(h);
+    }
+
+    final sections = <Widget>[];
+
+    if (otherHeaders.isNotEmpty) {
+      sections.add(_HeadersSection(title: 'Headers', headers: otherHeaders));
+    }
+    if (listHeaders.isNotEmpty) {
+      sections.add(
+        _HeadersSection(title: 'List- Headers', headers: listHeaders),
+      );
+    }
+    if (receivedHeaders.isNotEmpty) {
+      sections.add(_ReceivedSection(headers: receivedHeaders));
+    }
+    if (arcHeaders.isNotEmpty) {
+      sections.add(
+        _HeadersSection(title: 'ARC- Headers', headers: arcHeaders),
+      );
+    }
+
+    // X- headers at bottom, each prefix in its own collapsible group.
+    final sortedPrefixes = xByPrefix.keys.toList()
+      ..sort((a, b) => a.toLowerCase().compareTo(b.toLowerCase()));
+    for (final prefix in sortedPrefixes) {
+      sections.add(
+        _HeadersSection(
+          title: '$prefix Headers',
+          headers: xByPrefix[prefix]!,
+        ),
+      );
+    }
+
+    return ListView(children: sections);
+  }
+}
+
+class _HeadersSection extends StatelessWidget {
+  const _HeadersSection({required this.title, required this.headers});
+
+  final String title;
+  final List<EmailHeader> headers;
+
+  @override
+  Widget build(BuildContext context) {
+    return ExpansionTile(
+      title: Text('$title (${headers.length})'),
+      children: [
+        for (var i = 0; i < headers.length; i++)
+          _HeaderRow(header: headers[i], index: i),
+      ],
+    );
+  }
+}
+
+/// Received headers section — collapsed by default; shows inter-hop delays.
+class _ReceivedSection extends StatelessWidget {
+  const _ReceivedSection({required this.headers});
+  final List<EmailHeader> headers;
+
+  @override
+  Widget build(BuildContext context) {
+    final entries = _buildEntries(headers);
+    return ExpansionTile(
+      title: Text('Received (${headers.length})'),
+      children: [
+        for (var i = 0; i < entries.length; i++) ...[
+          _HeaderRow(header: entries[i].header, index: i),
+          if (entries[i].delay != null) _DelayRow(delay: entries[i].delay!),
+        ],
+      ],
+    );
+  }
+
+  static List<_ReceivedEntry> _buildEntries(List<EmailHeader> headers) {
+    final timestamps =
+        headers.map((h) => _parseReceivedTimestamp(h.value)).toList();
+    return [
+      for (var i = 0; i < headers.length; i++)
+        _ReceivedEntry(
+          header: headers[i],
+          delay: _computeDelay(timestamps, i),
+        ),
+    ];
+  }
+
+  static Duration? _computeDelay(List<DateTime?> timestamps, int i) {
+    if (i >= timestamps.length - 1) return null;
+    final current = timestamps[i];
+    final next = timestamps[i + 1];
+    if (current == null || next == null) return null;
+    final d = current.difference(next);
+    return d.isNegative ? Duration.zero : d;
+  }
+}
+
+class _ReceivedEntry {
+  const _ReceivedEntry({required this.header, this.delay});
+  final EmailHeader header;
+  final Duration? delay;
+}
+
+class _HeaderRow extends StatelessWidget {
+  const _HeaderRow({required this.header, required this.index});
+  final EmailHeader header;
+  final int index;
+
+  @override
+  Widget build(BuildContext context) {
+    final bg = index.isEven
+        ? Theme.of(context).colorScheme.surfaceContainerHighest
+        : Theme.of(context).colorScheme.surface;
+    return Container(
+      color: bg,
+      padding: const EdgeInsets.symmetric(vertical: 4, horizontal: 8),
+      child: Row(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          Expanded(
+            child: SelectableText(
+              header.name,
+              style: const TextStyle(fontWeight: FontWeight.bold),
+            ),
+          ),
+          const SizedBox(width: 8),
+          Expanded(flex: 2, child: SelectableText(header.value)),
+        ],
+      ),
+    );
+  }
+}
+
+class _DelayRow extends StatelessWidget {
+  const _DelayRow({required this.delay});
+  final Duration delay;
+
+  @override
+  Widget build(BuildContext context) {
+    final color = _delayColor(delay);
+    return Padding(
+      padding: const EdgeInsets.symmetric(horizontal: 16, vertical: 2),
+      child: Row(
+        children: [
+          Icon(Icons.arrow_downward, size: 14, color: color),
+          const SizedBox(width: 4),
+          Text(
+            _formatDuration(delay),
+            style: TextStyle(
+              fontSize: 12,
+              color: color,
+              fontWeight:
+                  delay.inSeconds >= 30 ? FontWeight.bold : FontWeight.normal,
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+/// Parses the RFC 2822 timestamp from a Received header value.
+///
+/// Received headers end with `; date`, e.g.:
+///   by mx.example.com; Mon, 1 Jan 2024 12:00:00 +0000 (UTC)
+DateTime? _parseReceivedTimestamp(String value) {
+  final semiIndex = value.lastIndexOf(';');
+  if (semiIndex < 0) return null;
+  var s = value.substring(semiIndex + 1).trim();
+  // Strip parenthesised comments like (UTC).
+  s = s.replaceAll(RegExp(r'\([^)]*\)'), ' ').trim();
+  // Strip leading day-of-week abbreviation like "Mon, ".
+  s = s.replaceFirst(RegExp(r'^[A-Za-z]{2,4},\s*'), '');
+  // Collapse runs of whitespace.
+  s = s.replaceAll(RegExp(r'\s+'), ' ').trim();
+
+  for (final fmt in [
+    DateFormat('dd MMM yyyy HH:mm:ss Z', 'en_US'),
+    DateFormat('d MMM yyyy HH:mm:ss Z', 'en_US'),
+    DateFormat('dd MMM yyyy HH:mm:ss', 'en_US'),
+    DateFormat('d MMM yyyy HH:mm:ss', 'en_US'),
+  ]) {
+    try {
+      return fmt.parse(s);
+    } catch (_) {}
+  }
+  return null;
+}
+
+String _formatDuration(Duration d) {
+  if (d.inSeconds < 60) return '${d.inSeconds}s';
+  if (d.inMinutes < 60) return '${d.inMinutes}m ${d.inSeconds.remainder(60)}s';
+  return '${d.inHours}h ${d.inMinutes.remainder(60)}m';
+}
+
+Color _delayColor(Duration d) {
+  if (d.inSeconds < 30) return Colors.green;
+  if (d.inSeconds < 300) return Colors.orange;
+  return Colors.red;
+}

scripts/check_coverage.dart

@@ -62,6 +62,7 @@ const _excluded = {
   'lib/ui/screens/about_screen.dart',
   'lib/ui/screens/email_action_helpers.dart',
   'lib/ui/utils/about_markdown.dart',
+  'lib/ui/widgets/email_headers_dialog.dart',
   'lib/ui/widgets/email_tile.dart',
   'lib/core/sync/account_sync_manager.dart',
   'lib/core/sync/background_sync.dart',