For me the entire bar disappeared. Someone sent me a message via chat, until then I didn't notice it was gone. When I got the message the bar flashed for a second and disappeared. When I refresh the page the same thing happens. I found out that the chat bar shows for me if I scroll to the very bottom of the screen. I figured out my problem. I use FFixer so I went to the configuration under Menus/Chat. I noticed that the transparency for the bottom menu was set to 10% so I turned it to 0% and clicked refresh. After that my chat buttons were visible.