Feature: Show unread status on threads you're participating in

app/assets/stylesheets/application/utilities.css

@@ -30,6 +30,7 @@
 .txt-tight-lines { line-height: 1.2; }
 .txt-normal { font-weight: 400; font-style: normal; }
 .txt-medium { font-weight: 500; }
+.fw-800 { font-weight: 800; }
 .txt-nowrap { white-space: nowrap; }
 
 .inline { display: inline }

app/models/rooms/thread.rb

@@ -9,4 +9,39 @@ def default_involvement(user: nil)
       "invisible"
     end
   end
+
+  # Check if a user is participating in this thread
+  # A user participates if they:
+  # - Created the parent message (the original message being threaded)
+  # - Created the thread (first commenter)
+  # - Have a visible membership (not "invisible" involvement)
+  # - Were mentioned in the thread (which gives them "mentions" involvement)
+  def participating?(user)
+    return false unless user
+
+    user_id = user.id
+
+    # Parent message creator is always a participant
+    return true if parent_message&.creator_id == user_id
+
+    # Thread creator is always a participant
+    return true if creator_id == user_id
+
+    # Check if user has a visible membership (not "invisible")
+    membership = memberships.active.find_by(user_id: user_id)
+    return false unless membership
+
+    # User is participating if they have a visible involvement (not "invisible")
+    membership.involved_in_invisible? ? false : true
+  end
+
+  # Check if a participating user has unread messages in this thread
+  def unread_for_participating?(user)
+    return false unless user
+
+    membership = memberships.active.find_by(user_id: user.id)
+    return false unless membership
+
+    membership.unread?
+  end
 end

app/views/messages/_threads.html.erb

@@ -11,6 +11,8 @@
     <% participants = User.where(id: participant_users).index_by(&:id) %>
     <% ordered_participants = participant_users.map { |id| participants[id] }.compact %>
     <% reply_count = thread.messages_count || thread.messages.active.count %>
+    <% is_participating = thread.participating?(Current.user) %>
+    <% has_unread = is_participating && thread.unread_for_participating?(Current.user) %>
 
     <%= link_to room_path(thread), class: "thread__link flex-inline align-center gap", data: { turbo_frame: "_top", updated_at: thread.last_active_at.iso8601 } do %>
       <div class="thread__avatars flex-inline gap">
@@ -22,7 +24,8 @@
       </div>
       <%= pluralize(reply_count, 'reply', 'replies') %>
       <% if reply_count > 0 %>
-        <span class="txt-muted">Last reply <%= local_datetime_tag thread.last_active_at, style: :relative %></span>
+        <% timestamp_class = has_unread ? "txt-primary fw-800" : "txt-muted" %>
+        <span class="<%= timestamp_class %>">Last reply <%= local_datetime_tag thread.last_active_at, style: :relative %></span>
       <% end %>
     <% end %>
   <% end %>