As of r111388 tabs with pages are reloaded when made active, especially if one of the tabs is closed. Passwords for restricted areas including javascript and other means are not detained, so if one switches to a different tab and than back to the tab of the restricted area, he has been logged out and needs to login again.
Ok after a complete reset and cache cleaning it seems to work normally.
I also cannot reproduce this. Closing since this doesn't happen any more for you, too.