Old messages appear for a short time
Old messages momentarily appear and fade away
In tiles mode, whenever a new message appears and an old one, or multiple old ones are pushed out of the list of visible messages, they first disappear (getting replaced by the new message), then after less than a second appear above the new message(s), and gradually fade away. See also:
http://www.adom.de/forums/project.php?issueid=5217, but the issue is more general then is described there.