If you click 'remember me' when you log on to the site you will not be logged out automatically after a period of inactivity.
Currently, if you do not click 'remember me' the site will keep you logged in and marked as active for 15 minutes from your last visible activity. If you do not want to use the 'remember me' function and are composing a long post or reply then all you would need to do is open a second window on the site (using the same browser) and periodically (within 15 minutes) refresh the site within that window.
If you do use the 'remember me' option, then simply manually logging-out will clear your session cookie etc. when you've finished.
However if I'm writing a very long post or story I will, as you've suggested, write it within a text editor and cut and paste it into my browser once I'm happy with the content. I've had a browser close down on me unexpectedly on more than one occasion in the past. Once bitten.......
There's also the CTRL A, CTRL C option, within the editing box, every few minutes if you'd rather.
CT