A button to exit from the menu in the same section as Log Out, and a button to exit on the login screen, for when you're in full screen. Currently the only way to exit is Alt + F4 which is only known to power users. There is also no way to get back into windowed mode from the full screen log in screen. So you are trapped with it open if you don't know to use Alt + F4.
Found another post I made about it almost exactly a year ago that explains more: https://help.sansar.com/hc/en-us/community/posts/360050503251-Exit-Options