All you've to do is use the Preview button (it's close to the Post button) as you type along. It saves it. It's like a "Control S or Control Save on a PC. Once your comment disappears before you post it, just check your "draft". Draft link is beneath the Preview tab. If you don't look closely you're likely to miss it.
Draft page on Bitcointalk has its limitation and I agree with libert95 that I would like to use Notepad or some note tools to save my text rather than using the forum draft page.
Draft page - Pros & ConsIf you click Preview or Post, your post or your draft will be at
https://bitcointalk.org/index.php?action=draftsCONs- Limited to only last 100 posts and drafts will be automatically deleted after 7 days
- Unable to recover your deleted posts to where it initially posted
- Almost unable to see where you post it (post ordinal number). Exception: You can only get which thread you post it, when you quote posts of others when reply them
- Unable to know when you post it