Gray out loaded backlog on starting client or rejoining channel
I would suggest that old, already read messages loaded from the backlog can be assigned a different color via the stylesheet to gray them out, effectively marking all messages that were received during a previous session. Messages that were received since the last disconnect should not be affected. This differs from the marker line as the marker line gets reset once the channel has been viewed once.
The same should also be used for rejoined channels and reopened queries.