content: Handle @-topic mentions

Fixes: #892

lib/model/content.dart

@@ -876,8 +876,9 @@ class _ZulipContentParser {
     final debugHtmlNode = kDebugMode ? element : null;
 
     final classes = element.className.split(' ')..sort();
-    assert(classes.contains('user-mention')
-        || classes.contains('user-group-mention'));
+    assert(classes.contains('topic-mention')
+      || classes.contains('user-mention')
+      || classes.contains('user-group-mention'));
     int i = 0;
 
     if (i >= classes.length) return null;
@@ -895,12 +896,14 @@ class _ZulipContentParser {
     }
 
     if (i >= classes.length) return null;
-    if (classes[i] == 'user-mention'
+    if ((classes[i] == 'topic-mention' && !hasChannelWildcardClass)
+        || classes[i] == 'user-mention'
         || (classes[i] == 'user-group-mention' && !hasChannelWildcardClass)) {
       // The class we already knew we'd find before we called this function.
       // We ignore the distinction between these; see [UserMentionNode].
       // Also, we don't expect "user-group-mention" and "channel-wildcard-mention"
-      // to be in the list at the same time.
+      // to be in the list at the same time and neither we expect "topic-mention"
+      // and "channel-wildcard-mention" to be in the list at the same time.
       i++;
     }
 
@@ -931,9 +934,9 @@ class _ZulipContentParser {
   /// Matches all className values that could be a UserMentionNode,
   /// and no className values that could be any other type of node.
   // Specifically, checks for `user-mention` or `user-group-mention`
-  // as a member of the list.
+  // or `topic-mention` as a member of the list.
   static final _userMentionClassNameRegexp = RegExp(
-    r"(^| )" r"user(?:-group)?-mention" r"( |$)");
+    r"(^| )" r"(?:user(?:-group)?|topic)-mention" r"( |$)");
 
   static final _emojiClassNameRegexp = () {
     const specificEmoji = r"emoji(?:-[0-9a-f]+)+";

test/model/content_test.dart

@@ -181,6 +181,27 @@ class ContentExample {
     '<p><span class="silent user-mention" data-user-id="*">all</span></p>',
     const UserMentionNode(nodes: [TextNode('all')]));
 
+  static final topicMentionPlain = ContentExample.inline(
+    'plain @-topic',
+    "@**topic**",
+    expectedText: '@topic',
+    '<p><span class="topic-mention">@topic</span></p>',
+    const UserMentionNode(nodes: [TextNode('@topic')]));
+
+  static final topicMentionSilent = ContentExample.inline(
+    'silent @-topic',
+    "@_**topic**",
+    expectedText: 'topic',
+    '<p><span class="topic-mention silent">topic</span></p>',
+    const UserMentionNode(nodes: [TextNode('topic')]));
+
+  static final topicMentionSilentClassOrderReversed = ContentExample.inline(
+    'silent @-topic, class order reversed',
+    "@_**topic**", // (hypothetical server variation)
+    expectedText: 'topic',
+    '<p><span class="silent topic-mention">topic</span></p>',
+    const UserMentionNode(nodes: [TextNode('topic')]));
+
   static final emojiUnicode = ContentExample.inline(
     'Unicode emoji, encoded in span element',
     ":thumbs_up:",
@@ -1262,6 +1283,10 @@ void main() {
     testParseExample(ContentExample.legacyChannelWildcardMentionPlain);
     testParseExample(ContentExample.legacyChannelWildcardMentionSilent);
     testParseExample(ContentExample.legacyChannelWildcardMentionSilentClassOrderReversed);
+
+    testParseExample(ContentExample.topicMentionPlain);
+    testParseExample(ContentExample.topicMentionSilent);
+    testParseExample(ContentExample.topicMentionSilentClassOrderReversed);
   });
 
   testParseExample(ContentExample.emojiUnicode);

test/widgets/content_test.dart

@@ -669,6 +669,9 @@ void main() {
     testContentSmoke(ContentExample.legacyChannelWildcardMentionPlain);
     testContentSmoke(ContentExample.legacyChannelWildcardMentionSilent);
     testContentSmoke(ContentExample.legacyChannelWildcardMentionSilentClassOrderReversed);
+    testContentSmoke(ContentExample.topicMentionPlain);
+    testContentSmoke(ContentExample.topicMentionSilent);
+    testContentSmoke(ContentExample.topicMentionSilentClassOrderReversed);
 
     UserMention? findUserMentionInSpan(InlineSpan rootSpan) {
       UserMention? result;