Based on the emails that are going 'round...."internally?" the issue should be fixed, however this is what is known/suspected about it:
When you close the tab or browser window, the server keeps the session alive for about 2 hours, after which you get logged out (which overrides the "forever" option in the cookie). Chris did something last night that should fix it (neither he nor I experience the issue typically as we keep the forum up in a tab all the time).