When sending a new popup message (message type 3, the one in the hot bar) the message that was their disappears before the new one is rendered, this means if you send one message, then send another one half a second later it will go:
message (0.45 seconds)
flash to nothing (0.1 second)
message (0.45 seconds)
(timings inaccurate, just using them as examples)

This did not happen prior to 0.16