I've noticed recently that the chat notification of a ping does not fade properly after clicking it, nor after sending a new message on many occasions. Either it will look like this, or part of the bottom and/or top will be knocked off and the amount that is fades also varies:
Though normally after sending another message, or someone else posting a message, it disappears properly.
I am running Ubuntu GNOME 16.04 with GNOME 3.20 and Firefox 47. The issue started about a month or so ago.