Follia

    public class MessageCollection : ObservableCollection<TLMessageBase>
    {
        public ObservableCollection<MessageGroup> Groups { get; private set; } = new ObservableCollection<MessageGroup>();

        protected override void InsertItem(int index, TLMessageBase item)
        {
            base.InsertItem(index, item);

            var group = GroupForIndex(index);
            if (group == null || group?.FromId != item.FromId)
            {
                group = new MessageGroup(this, item.From, item.FromId);
                Groups.Insert(0, group);
            }

            group.Insert(index, item);
        }

        protected override void RemoveItem(int index)
        {
            base.RemoveItem(index);

            var group = GroupForIndex(index);
            if (group != null)
            {
                group.RemoveAt(index);
            }
        }

        private MessageGroup GroupForIndex(int index)
        {
            if (index == 0)
            {
                return Groups.FirstOrDefault();
            }
            else if (index == Count - 1)
            {
                return Groups.LastOrDefault();
            }
            else
            {
                return Groups.FirstOrDefault(x => x.StartIndex >= index && x.EndIndex <= index);
            }
        }
    }

    public class MessageGroup : ObservableCollection<TLMessageBase>
    {
        private ObservableCollection<MessageGroup> _parent;

        public MessageGroup(MessageCollection parent, TLUser from, int? fromId)
        {
            _parent = parent.Groups;

            From = from;
            FromId = fromId;
        }

        public TLUser From { get; private set; }

        public int? FromId { get; private set; }

        public int StartIndex
        {
            get
            {
                var count = 0;
                var index = _parent.IndexOf(this);
                if (index > 0)
                {
                    count = _parent[index - 1].EndIndex;
                }

                return count;
            }
        }

        public int EndIndex
        {
            get
            {
                return StartIndex + Count - 1;
            }
        }

        protected override void InsertItem(int index, TLMessageBase item)
        {
            index -= StartIndex;
            base.InsertItem(index, item);
        }

        protected override void RemoveItem(int index)
        {
            index -= StartIndex;
            base.RemoveItem(index);
        }
    }