Saved Drafts for New Post Replies
That's right! When you go to compose a reply post, you'll notice some new UI for automatically saving what you're writing. The way this works is that we'll save your contents automatically 3 seconds after you pause typing. You can also initiate a save manually, though you usually shouldn't need to. This will work in the the Rich Text Editor, the BBCode Editor, and on Mobile.
Once a reply draft is saved, all you need to do is come back to the thread where you had drafted a reply, at some point in the future, and you'll find your draft waiting for you in the Reply editor. This will work if you draft on one computer and then return to the thread via another computer. It will also work across Mobile & Desktop. Drafts are saved server-side.
You can draft one post reply at a time for any given thread. There's not currently a way to work on a long-term draft but still fire off a quick reply in the meantime. Until you publish your draft, it will occupy the reply editor's entry field. However, you can have a draft saved for each of as many threads as you like. You're not just limited to one single draft, period.
This is a first stage rollout: there is no draft saving for anything other than new replies. No drafts for new threads, no drafts for private messages or articles, comments, reviews, etc. There is also no draft saving of edits to posts.