No screenshots or videos because it’s extremely easy to reproduce.
After a few minutes playing the game online, the chat window only displays two messages: The oldest visible message in the chat window and the latest message.
These two will be separated by a large, empty space where other messages should be. Upon pressing enter, the messages between those two will appear. When the chat window fades again, only the two messages are visible.
Screen resolution is 1366 x 768.
I have not sent any messages in the chat. I have changed zones, used attacks and talked to NPCs, and none of those interactions interfered with the bug.