On the right side of the client, there is a series of collapsible trees that contain notifications about in game events.

Included are city, battle and diplomacy events.  From what I can tell, the UI allows players to collapse the trees to reduce their size on the User Interface.

I started a new game  For some reason, when I continually collapse and expand the "Event Notification Trees,"  notifications disappearing.  I do not know if some notification are meant to disappear after viewing or not.

Started out like this:

After a few times collapsing the diplomacy and city events trees, ended up like this: