While you all may be correct with your suggestions (that I normally also employ, but the post got longer than intended and I concentrated on it, not on dealing with defective forum software): These are workarounds for an issue that should not exist in the first place.
If one could go back in the browser history to retrieve the text it wouldn't be that bad, but since irrecoverably discarding user input being the best equivalent of a middle finger that a computer can express toward a human... it should not happen.
Seth, thank you for your condolences - nevertheless, please fix this.
At least modify the 'Minutes to stay logged in' (default 120 minutes) to be a timeout since the last action (and count previewing a post as an action) instead of determining a fixed time in relation to the moment of login (as it seems to be, or previewing a post dosn't reset the timer).
Technical note: Manual logout while composing, login, then going back in browser history dosn't destroy the contents of the textarea - but timeout-logout does, most likely through redirecting to the compose page with empty content delivered by the server which then wipes the contents from the browser cache. Bug. Fix. Please.