Match mobile reply links to desktop connectors (#8362)

## Summary
Reply summaries now appear below mobile message bubbles as inset
avatar/count links with the same rounded connector used on desktop.
Reaction trays keep their overlap with the bubble; connector clearance
adjusts when reactions are present. Channel and nested reply summaries
share the same 20px avatar stack, capped at three, with a 44px minimum
tap target and accessible thread-opening label.

### Related issue
Follows merged #8359 and now targets `main`. This diff contains only the
reply-link changes. No matching duplicate was found in the open issue/PR
search for reply connectors.

### Testing
- Full mobile formatting, analysis, and tests passed before stacking:
3,348 tests plus 3 push-configuration tests, 4 skips.
- Normal pre-push checks passed on the rebased head against main:
formatting, analysis, full mobile tests, and repository file-size
checks.
- Before stacking, installed and launched the preview on Pixel 10 and
iPhone Air with existing app data. Ten profile/community/draft
preference values remained unchanged on each. Verified both connector
spacing variants and thread opening on Pixel; the owner tried and
approved the preview. iPhone screenshot verification was unavailable.
- The rebased build has not been reinstalled on devices. Device evidence
above is from the approved preview before rebasing.

Before: channel replies were filled pills beside reactions; nested
replies used a separate larger avatar row. After, as verified in the
device preview:

![Pixel preview showing reply connectors with and without
reactions](https://buzz.block.builderlab.xyz/media/119f0fbd358895ef3fe5e6c1bc4f2c399c660b1debded5efff08864ff796792c.png)

Signed-off-by: kenny lopez <klopez4212@gmail.com>
Co-authored-by: Terp <5856f6900e8e0a4e0ea1f4f8d2ee33e4753d2119bc122f94ecafc660b46c0511@buzz.block.builderlab.xyz>

mobile/lib/features/channels/channel_detail_page.dart

@@ -2,6 +2,7 @@ import 'huddle_timeline.dart';
 import '../../shared/widgets/modal_presentation.dart';
 import '../../shared/widgets/ios_scroll_to_top.dart';
 import 'message_footer_layout.dart';
+import 'reply_summary_link.dart';
 import 'dart:async';
 import 'dart:math' show cos, max, min, pi;
 import 'dart:ui' show ImageFilter, lerpDouble;

mobile/lib/features/channels/channel_detail_page/message_bubble.dart

@@ -167,9 +167,7 @@ class _MessageBubble extends HookConsumerWidget {
               ),
               child: MessageFooterLayout(
                 animateSize: true,
-                overlap: message.reactions.isNotEmpty || replyFooter != null
-                    ? Grid.xs
-                    : 0,
+                overlap: message.reactions.isNotEmpty ? Grid.xs : 0,
                 footer: message.reactions.isNotEmpty || replyFooter != null
                     ? Padding(
                         padding: const EdgeInsets.only(
@@ -178,10 +176,9 @@ class _MessageBubble extends HookConsumerWidget {
                               messageAvatarContentGap +
                               Grid.xxs,
                         ),
-                        child: Wrap(
-                          spacing: Grid.xxs,
-                          runSpacing: Grid.half,
-                          crossAxisAlignment: WrapCrossAlignment.center,
+                        child: Column(
+                          mainAxisSize: MainAxisSize.min,
+                          crossAxisAlignment: CrossAxisAlignment.start,
                           children: [
                             if (message.reactions.isNotEmpty)
                               ReactionRow(

mobile/lib/features/channels/channel_detail_page/system_rows.dart

@@ -611,9 +611,10 @@ class _ThreadSummaryRow extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
-    final participants = summary.participantPubkeys.take(3).toList();
-    final userCache = ref.watch(userCacheProvider);
-    return GestureDetector(
+    return ReplySummaryLink(
+      key: ValueKey('thread-summary-${message.id}'),
+      summary: summary,
+      hasReactions: message.reactions.isNotEmpty,
       onTap: () {
         Navigator.of(context).push(
           MaterialPageRoute<void>(
@@ -628,61 +629,6 @@ class _ThreadSummaryRow extends ConsumerWidget {
           ),
         );
       },
-      child: Semantics(
-        button: true,
-        child: Container(
-          key: ValueKey('thread-summary-${message.id}'),
-          padding: const EdgeInsets.symmetric(
-            horizontal: Grid.xxs + Grid.quarter,
-            vertical: Grid.half,
-          ),
-          decoration: BoxDecoration(
-            color: context.colors.secondaryContainer,
-            borderRadius: BorderRadius.circular(Radii.container),
-            border: Border.all(color: context.colors.surface, width: 2),
-          ),
-          child: Row(
-            mainAxisSize: MainAxisSize.min,
-            children: [
-              if (participants.isNotEmpty) ...[
-                ExcludeSemantics(
-                  child: SizedBox(
-                    width:
-                        Grid.gutter + (participants.length - 1) * Grid.twelve,
-                    height: Grid.gutter,
-                    child: Stack(
-                      children: [
-                        for (var i = 0; i < participants.length; i++)
-                          Positioned(
-                            left: i * Grid.twelve,
-                            child: SmallAvatar(
-                              pubkey: participants[i],
-                              userCache: userCache,
-                              size: Grid.gutter,
-                            ),
-                          ),
-                      ],
-                    ),
-                  ),
-                ),
-                const SizedBox(width: Grid.half),
-              ],
-              Flexible(
-                child: Text(
-                  summary.isCountPending
-                      ? 'Replies'
-                      : '${summary.replyCount}${summary.isLowerBound ? '+' : ''} ${summary.replyCount == 1 ? 'reply' : 'replies'}',
-                  maxLines: 2,
-                  overflow: TextOverflow.ellipsis,
-                  style: context.textTheme.labelMedium?.copyWith(
-                    color: context.colors.onSurface,
-                  ),
-                ),
-              ),
-            ],
-          ),
-        ),
-      ),
     );
   }
 }

mobile/lib/features/channels/reply_summary_link.dart

@@ -0,0 +1,149 @@
+import 'package:flutter/material.dart';
+import 'package:hooks_riverpod/hooks_riverpod.dart';
+
+import '../../shared/profile/user_cache_provider.dart';
+import '../../shared/theme/theme.dart';
+import 'small_avatar.dart';
+import 'timeline_message.dart';
+
+/// Desktop-style reply link with a curved branch below the message bubble.
+class ReplySummaryLink extends ConsumerWidget {
+  const ReplySummaryLink({
+    super.key,
+    required this.summary,
+    required this.onTap,
+    this.hasReactions = false,
+  });
+
+  final ThreadSummary summary;
+  final VoidCallback onTap;
+  final bool hasReactions;
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final participants = summary.participantPubkeys.take(3).toList();
+    final userCache = ref.watch(userCacheProvider);
+    final label = summary.isCountPending
+        ? 'Replies'
+        : '${summary.replyCount}${summary.isLowerBound ? '+' : ''} ${summary.replyCount == 1 ? 'reply' : 'replies'}';
+    final connectorTop = hasReactions ? -Grid.half : Grid.half;
+    return Padding(
+      padding: const EdgeInsetsDirectional.only(start: Grid.xxs),
+      child: Stack(
+        clipBehavior: Clip.none,
+        children: [
+          Positioned.fill(
+            top: connectorTop,
+            child: IgnorePointer(
+              child: CustomPaint(
+                painter: _ReplyConnector(
+                  context.colors.outlineVariant,
+                  Directionality.of(context),
+                  connectorTop,
+                ),
+              ),
+            ),
+          ),
+          Padding(
+            padding: const EdgeInsetsDirectional.only(start: Grid.twelve),
+            child: Semantics(
+              button: true,
+              label: 'View thread: $label',
+              onTap: onTap,
+              excludeSemantics: true,
+              child: InkWell(
+                onTap: onTap,
+                borderRadius: BorderRadius.circular(Radii.container),
+                child: ConstrainedBox(
+                  constraints: const BoxConstraints(minHeight: 44),
+                  child: Padding(
+                    padding: const EdgeInsets.all(Grid.half),
+                    child: Row(
+                      mainAxisSize: MainAxisSize.min,
+                      children: [
+                        if (participants.isNotEmpty) ...[
+                          SizedBox(
+                            width:
+                                Grid.gutter +
+                                (participants.length - 1) * Grid.twelve,
+                            height: Grid.gutter,
+                            child: Stack(
+                              children: [
+                                for (var i = 0; i < participants.length; i++)
+                                  PositionedDirectional(
+                                    start: i * Grid.twelve,
+                                    child: SmallAvatar(
+                                      pubkey: participants[i],
+                                      userCache: userCache,
+                                    ),
+                                  ),
+                              ],
+                            ),
+                          ),
+                          const SizedBox(width: Grid.half),
+                        ],
+                        Flexible(
+                          child: Text(
+                            label,
+                            maxLines: 2,
+                            overflow: TextOverflow.ellipsis,
+                            style: context.textTheme.labelMedium?.copyWith(
+                              color: context.colors.onSurfaceVariant,
+                            ),
+                          ),
+                        ),
+                      ],
+                    ),
+                  ),
+                ),
+              ),
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}
+
+class _ReplyConnector extends CustomPainter {
+  const _ReplyConnector(this.color, this.direction, this.topOffset);
+  final Color color;
+  final TextDirection direction;
+  final double topOffset;
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    canvas.save();
+    if (direction == TextDirection.rtl) {
+      canvas.translate(size.width, 0);
+      canvas.scale(-1, 1);
+    }
+    // Keep the endpoint aligned with the unchanged reply avatars.
+    final middle = (size.height - topOffset) / 2;
+    // Desktop uses a 12px outer corner with a 2px border: the stroke's
+    // centerline follows an 11px circular arc, rather than a tight elbow.
+    const radius = Grid.twelve - 1;
+    final path = Path()
+      ..moveTo(1, 0)
+      ..lineTo(1, middle - radius)
+      ..arcToPoint(
+        Offset(Grid.twelve, middle),
+        radius: const Radius.circular(radius),
+        clockwise: false,
+      );
+    canvas.drawPath(
+      path,
+      Paint()
+        ..color = color
+        ..style = PaintingStyle.stroke
+        ..strokeWidth = 2,
+    );
+    canvas.restore();
+  }
+
+  @override
+  bool shouldRepaint(_ReplyConnector oldDelegate) =>
+      color != oldDelegate.color ||
+      direction != oldDelegate.direction ||
+      topOffset != oldDelegate.topOffset;
+}

mobile/lib/features/channels/thread_detail_page.dart

@@ -1,5 +1,6 @@
 import '../../shared/widgets/ios_scroll_to_top.dart';
 import 'message_footer_layout.dart';
+import 'reply_summary_link.dart';
 import 'dart:async';
 import 'dart:math' as math;
 
@@ -56,7 +57,6 @@ import 'message_media_avatar.dart';
 import 'reaction_row.dart';
 import '../../shared/read_state/read_state_provider.dart';
 import 'send_message_provider.dart';
-import 'small_avatar.dart';
 import 'sticky_date_header.dart';
 import 'reading_marks.dart';
 import 'timeline_message.dart';

mobile/lib/features/channels/thread_detail_page/nested_thread_summary_row.dart

@@ -23,91 +23,29 @@ class _NestedThreadSummaryRow extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
-    final userCache = ref.watch(userCacheProvider);
-
-    return GestureDetector(
-      onTap: () {
-        Navigator.of(context).push(
-          MaterialPageRoute<void>(
-            builder: (_) => ThreadDetailPage(
-              threadHead: replyMessage,
-              allMessages: allMessages,
-              channelId: channelId,
-              currentPubkey: currentPubkey,
-              isMember: isMember,
-              isArchived: isArchived,
-            ),
-          ),
-        );
-      },
-      child: Padding(
+    return Padding(
+      padding: const EdgeInsets.only(
+        left: messageAvatarSize + messageAvatarContentGap + Grid.xxs,
+        bottom: Grid.xxs,
+      ),
+      child: ReplySummaryLink(
         key: ValueKey('nested-thread-summary-${replyMessage.id}'),
-        padding: const EdgeInsets.only(
-          left: messageAvatarSize + messageAvatarContentGap,
-          top: Grid.half,
-          bottom: Grid.xs,
-        ),
-        child: Row(
-          mainAxisSize: MainAxisSize.min,
-          children: [
-            // Stacked participant avatars.
-            SizedBox(
-              width:
-                  32.0 +
-                  (summary.participantPubkeys.length - 1).clamp(0, 2) * 20.0,
-              height: 32,
-              child: Stack(
-                children: [
-                  for (var i = 0; i < summary.participantPubkeys.length; i++)
-                    Positioned(
-                      left: i * 20.0,
-                      child: SmallAvatar(
-                        pubkey: summary.participantPubkeys[i],
-                        userCache: userCache,
-                        size: 32,
-                      ),
-                    ),
-                ],
+        summary: summary,
+        hasReactions: replyMessage.reactions.isNotEmpty,
+        onTap: () {
+          Navigator.of(context).push(
+            MaterialPageRoute<void>(
+              builder: (_) => ThreadDetailPage(
+                threadHead: replyMessage,
+                allMessages: allMessages,
+                channelId: channelId,
+                currentPubkey: currentPubkey,
+                isMember: isMember,
+                isArchived: isArchived,
               ),
             ),
-            const SizedBox(width: Grid.xxs),
-            Flexible(
-              child: Text.rich(
-                TextSpan(
-                  children: [
-                    TextSpan(
-                      text: summary.isCountPending
-                          ? 'Replies'
-                          : '${summary.replyCount}${summary.isLowerBound ? '+' : ''} ${summary.replyCount == 1 ? 'reply' : 'replies'}',
-                      style: replyPreviewTextStyle.copyWith(
-                        color: context.colors.primary,
-                      ),
-                    ),
-                    if (summary.lastReplyAt case final lastReplyAt?) ...[
-                      TextSpan(
-                        text: ' · ',
-                        style: replyPreviewTextStyle.copyWith(
-                          color: context.colors.onSurfaceVariant.withValues(
-                            alpha: 0.5,
-                          ),
-                        ),
-                      ),
-                      TextSpan(
-                        text:
-                            'last reply ${formatThreadSummaryLastReplyTime(lastReplyAt)}',
-                        style: replyPreviewTextStyle.copyWith(
-                          color: context.colors.onSurfaceVariant,
-                        ),
-                      ),
-                    ],
-                  ],
-                ),
-                maxLines: 2,
-                overflow: TextOverflow.ellipsis,
-              ),
-            ),
-          ],
-        ),
+          );
+        },
       ),
     );
   }