Sometimes a new notification will arrive, but it displays only a "show" option that doesn't do anything, and there's no "close" option. I have to hit the main body of the notification to display the referenced item, then close that item. After a restart it works as it should.
MacBook Pro
If a "banner" style notification comes in and you are working on a MacBook or a Mac with a trackpad, you can simply swipe right on the notification and it will disappear.
I would recommend that you check out you notification settings under system preferences in order to see what style of notifications you currently allow from your various installed applications.
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Notifications often don't display fully
