This feature currently does exist when you're editing someone else's answer: it takes you to a special "edit" page like
stackoverflow.com/posts/##post-number###/edit. This enables to browser-based crash recovery, since returning to the URL opens the edit-box. This isn't server-side saved drafts, but it's much better than nothing.
(Correction and/or update: you get the separate-page editor on sites where you have low rep, including for your own posts. At high rep you get the inline editor on all posts.)
When Edit doesn't take you to a new URL (which is the normal case for editing your own posts), coming back after a browser restart loses all your work.
This is totally backwards. Your own posts are most likely the ones where you'll come back and make big changes, like to re-order the way you present information if it was clumsy before. Or if you learned something new that means a guess in an old answer was wrong.
I don't think I want to always go to a separate page when editing my own posts, so maybe the lowest-effort thing here would be for that to be an option. I'm having a hard time coming up with the text that should be on this link. Just saying "go to editor page" wouldn't explain why, and calling it "go to draft-saving page" sounds weird.
You can use the separate-page editor any time you want
Middle-click the edit link under a post to edit in a new window, with the
Perhaps a good idea to do this for big edits, but it's a big pain. I'd like some kind of server-side recovery. SO answers can be up to 30k characters and sometimes take significant time to edit.
Sometimes browser recovery doesn't work, e.g. on an upgrade of Chromium I've occasionally had it lose all my half-written answers / edits in textboxes.
Maybe I should just copy stuff to an emacs buffer if I don't post in one sitting and edit offline.