Fix some edge cases in the streaming of messages

src/Web/Features/Messages/Handlers/GetMessages.cs

@@ -31,12 +31,26 @@ public class GetMessages(
             .Where(c => c.SubjectId == req.SubjectId)
             .Where(c => c.ParentId == null);
 
-        query = query.OrderByDescending(c => c.CreatedAt);
+        if (req.LastId.HasValue)
+        {
+            var lastMessage = await context
+                .Messages
+                .Where(c => c.Id == req.LastId.Value)
+                .Select(c => new { c.CreatedAt, c.Id })
+                .FirstOrDefaultAsync(cancellationToken: ct);
 
-        if (req.LastId is not null)
-            query = query.Where(c => c.Id < req.LastId.Value);
+            if (lastMessage != null)
+            {
+                query = query
+                    .Where(c => c.CreatedAt < lastMessage.CreatedAt
+                                || (c.CreatedAt == lastMessage.CreatedAt && c.Id < lastMessage.Id));
+            }
+        }
 
-        query = query.Take(req.PageSize);
+        query = query
+            .OrderByDescending(c => c.CreatedAt)
+            .ThenByDescending(c => c.Id)
+            .Take(req.PageSize);
 
         var messages = await query.ToListAsync(cancellationToken: ct);