fix(mobile): bound long preview lines and label Buzz links

A single huge line no longer reaches preview preprocessing in full, and
message, repo, pull request and issue links preview with the same short
label as the thread view's chips, via a shared buzzLinkLabel helper.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tom Brow <tomb@block.xyz>

mobile/lib/features/channels/message_content.dart

@@ -37,6 +37,7 @@ import 'message_content/link_normalizer.dart';
 import 'message_content/preview_text.dart';
 import 'message_media.dart';
 import 'message_content/bubble_segments.dart';
+import 'message_content/buzz_link_label.dart';
 import 'message_mention_pill.dart';
 import 'message_gallery.dart';
 import 'message_gallery_frame.dart';
@@ -612,22 +613,21 @@ class MessageContent extends HookConsumerWidget {
         buzzLink is MessageDeepLink ||
         buzzLink is EntityDeepLink;
     final isCanonicalBuzzLabel = isBuzzLink && text == url;
+    String? channelName(String id) =>
+        _channelNameForId(resolvedChannelNames, id);
+    final buzzLabel = buzzLinkLabel(buzzLink, channelName);
     final buzzPresentation = switch (buzzLink) {
-      ChannelDeepLink(:final channelId) => (
+      ChannelDeepLink() => (
         icon: BuzzIcons.hash,
-        label:
-            _channelNameForId(resolvedChannelNames, channelId) ??
-            channelId.substring(0, math.min(8, channelId.length)),
-        semanticLabel:
-            'Open channel ${_channelNameForId(resolvedChannelNames, channelId) ?? channelId.substring(0, math.min(8, channelId.length))}',
+        label: buzzLabel!,
+        semanticLabel: 'Open channel $buzzLabel',
         interactive: true,
       ),
       MessageDeepLink(:final channelId, :final messageId) => (
         icon: BuzzIcons.messageSquare,
-        label:
-            '${_channelNameForId(resolvedChannelNames, channelId) ?? channelId.substring(0, math.min(8, channelId.length))} · ${messageId.substring(0, math.min(8, messageId.length))}',
+        label: buzzLabel!,
         semanticLabel:
-            'Open message ${messageId.substring(0, math.min(8, messageId.length))} in channel ${_channelNameForId(resolvedChannelNames, channelId) ?? channelId.substring(0, math.min(8, channelId.length))}',
+            'Open message ${shortBuzzId(messageId)} in channel ${buzzChannelLabel(channelId, channelName)}',
         interactive: true,
       ),
       EntityDeepLink(:final type, :final repository, :final eventId) => (
@@ -636,14 +636,12 @@ class MessageContent extends HookConsumerWidget {
           'pr' => BuzzIcons.gitPullRequest,
           _ => BuzzIcons.circleDot,
         },
-        label: type == 'repo'
-            ? repository
-            : '$repository · ${eventId!.substring(0, 8)}',
+        label: buzzLabel!,
         semanticLabel: switch (type) {
           'repo' => 'Repository $repository',
           'pr' =>
-            'Pull request ${eventId!.substring(0, 8)} in repository $repository',
-          _ => 'Issue ${eventId!.substring(0, 8)} in repository $repository',
+            'Pull request ${shortBuzzId(eventId ?? '')} in repository $repository',
+          _ => 'Issue ${shortBuzzId(eventId ?? '')} in repository $repository',
         },
         interactive: false,
       ),

mobile/lib/features/channels/message_content/buzz_link_label.dart

@@ -0,0 +1,28 @@
+import 'dart:math' as math;
+
+import '../../../shared/deeplink/deep_link.dart';
+
+/// The short label a Buzz channel, message, repo, pull request or issue link
+/// shows in place of its URL, or null for any other link. [channelName] gives
+/// a channel's display name, or null when it is unknown.
+String? buzzLinkLabel(
+  BuzzDeepLink? link,
+  String? Function(String channelId) channelName,
+) => switch (link) {
+  ChannelDeepLink(:final channelId) => buzzChannelLabel(channelId, channelName),
+  MessageDeepLink(:final channelId, :final messageId) =>
+    '${buzzChannelLabel(channelId, channelName)} · ${shortBuzzId(messageId)}',
+  EntityDeepLink(type: 'repo', :final repository) => repository,
+  EntityDeepLink(:final repository, :final eventId) =>
+    '$repository · ${shortBuzzId(eventId ?? '')}',
+  _ => null,
+};
+
+/// [channelId]'s display name, or its first 8 characters when unknown.
+String buzzChannelLabel(
+  String channelId,
+  String? Function(String channelId) channelName,
+) => channelName(channelId) ?? shortBuzzId(channelId);
+
+/// The first 8 characters of an id, as Buzz links show it.
+String shortBuzzId(String id) => id.substring(0, math.min(8, id.length));

mobile/lib/features/channels/message_content/preview_text.dart

@@ -5,6 +5,7 @@ import 'package:flutter/foundation.dart';
 import '../../../shared/deeplink/deep_link.dart';
 import '../../../shared/mentions/mention_bindings.dart';
 import '../message_media.dart';
+import 'buzz_link_label.dart';
 import 'link_normalizer.dart';
 
 /// One piece of a message preview: plain text or an inline token that renders
@@ -159,13 +160,19 @@ Iterable<({String text, bool code, int end})> _keptLines(
   while (start <= content.length &&
       start <= _maxScanLength &&
       kept <= maxLines) {
-    var end = content.indexOf('\n', start);
-    if (end < 0) end = content.length;
-    // A long line wraps past the visible lines well before this length.
-    var line = content
-        .substring(start, math.min(end, start + 2 * _maxLineLength))
-        .trim();
-    start = end + 1;
+    // A long line wraps past the visible lines well before this length, so
+    // reading stops there rather than at the line's real end.
+    var cap = math.min(content.length, start + 2 * _maxLineLength);
+    if (cap < content.length && _isHighSurrogate(content.codeUnitAt(cap - 1))) {
+      cap--;
+    }
+    var end = start;
+    while (end < cap && content.codeUnitAt(end) != _newline) {
+      end++;
+    }
+    final cut = end == cap && cap < content.length;
+    var line = content.substring(start, end).trim();
+    start = cut ? content.length + 1 : end + 1;
     // A fence opens at any line starting with backticks and closes only at a
     // line holding nothing else, as in gpt_markdown.
     if (!fenceOpen && line.startsWith('```') || fenceOpen && line == '```') {
@@ -189,8 +196,11 @@ Iterable<({String text, bool code, int end})> _keptLines(
 const _maxLineLength = 2000;
 const _maxScanLength = 8 * _maxLineLength;
 const _backslash = 0x5C;
+const _newline = 0x0A;
 const _backtick = 0x60;
 
+bool _isHighSurrogate(int unit) => unit >= 0xD800 && unit <= 0xDBFF;
+
 final _rule = RegExp(r'^([-*_])(?:\s*\1){2,}$');
 final _tableDivider = RegExp(r'^\|?(?:\s*:?-+:?\s*\|)+\s*:?-*:?\s*\|?$');
 final _quote = RegExp(r'^(?:>\s?)+');
@@ -383,25 +393,30 @@ class _PreviewBuilder {
         continue;
       }
       final uri = Uri.tryParse(url);
-      final deepLink = uri?.scheme == 'buzz' ? parseBuzzDeepLink(uri!) : null;
+      final deepLink = uri?.scheme == 'buzz'
+          ? parseBuzzDeepLink(uri!) ?? parseEntityDeepLink(uri)
+          : null;
       if (deepLink is ChannelDeepLink) {
-        add(PreviewChannel(_channelName(deepLink.channelId)));
+        add(PreviewChannel(buzzChannelLabel(deepLink.channelId, _channelName)));
         continue;
       }
-      _tokens(
-        _unwrapEmphasis(
-          _shield(label.isEmpty ? url : decodeMarkdownLabelSyntax(label)),
-        ),
-      );
+      final shown = label.isEmpty ? url : decodeMarkdownLabelSyntax(label);
+      // As in the thread view, a link showing its own URL shows a short label.
+      final short = shown == url ? buzzLinkLabel(deepLink, _channelName) : null;
+      if (short != null) {
+        this.text(short);
+        continue;
+      }
+      _tokens(_unwrapEmphasis(_shield(shown)));
     }
     _tokens(_unwrapEmphasis(source.substring(start)));
   }
 
-  String _channelName(String channelId) {
+  String? _channelName(String channelId) {
     for (final entry in channelNames.entries) {
       if (entry.value == channelId) return entry.key;
     }
-    return channelId.substring(0, channelId.length < 8 ? channelId.length : 8);
+    return null;
   }
 
   /// [_shield]ed plain text with mention, custom emoji and channel tokens.

mobile/test/features/channels/message_content/preview_text_test.dart

@@ -109,8 +109,12 @@ void main() {
       );
     });
 
-    test('caps a very long line', () {
+    test('caps a very long line, and reads only its start', () {
       expect(_text('x' * 5000).length, 2000);
+      expect(
+        messagePreviewSource('x' * 128000, maxLines: 2).length,
+        lessThanOrEqualTo(4000),
+      );
     });
   });
 
@@ -221,6 +225,18 @@ void main() {
       );
     });
 
+    test('message and repo links that show their URL get a short label', () {
+      final message = 'buzz://message?channel=$_channelId&id=${'ab' * 32}';
+      const repo = 'buzz://repo?owner=$_alice&d=buzz';
+      expect(
+        _pieces(
+          '[$message]($message) [$repo]($repo) [named]($message)',
+          channelNames: {'general': _channelId},
+        ),
+        [const PreviewText('general · abababab buzz named')],
+      );
+    });
+
     test('media links become their kind', () {
       expect(_pieces('look ![](https://x.y/a.png) [clip](https://x.y/b.mp4)'), [
         const PreviewText('look '),