That's the system button window. You can see it duplicated on the experience window (upper left corner of your screenshot). What you need to do is go and maximize the experience window, which'll remove the system button window from the experience window and open the stand-alone version. This comes from the default UI revamp several years ago which changed the XP window into a bar across the full width of the screen and integrted the EQII menu button and menu items into the XP bar. Profit handled the change by having two modes for the XP window: minimized attaches the menu button cluster into the XP window, maximized makes it a stand-alone window.
"Hey! I'm not illiterate. I had a book once. ...could've used some ketchup though..."