do you have your log in set to time out? I have yet to have the problem you speak of.
No, the forum default session length is two hours. One can see this default setting on the "Login" page. Those who login on the home page never realize that there is a two hour limit for the forum. The only way around it is to change the session time-out period or disable it from the Login page. Even if one disables the time-out by checking the "Always stay logged in" checkbox on the Login page, the setting only remains in effect until one clears cookies, which I do every time I close my browser (a practice that everyone who does not wish to be tracked should do because sites do not have to honor the "do not track" setting in one's browser).
Most forums implement a session time-out period for security reasons. Whether the time-out is from the second one logs into the forum or is based on a period of inactivity, most forums do not trash a post that was in the process of being composed when the session times out when the user is prompted to re-authenticate upon clicking the Post or Preview buttons.